Implement database abstraction, error checking (ref #5907) (#6107)

This PR does two things, because one lead to the other:

- Move the leveldb specific stuff into a small "backend" package that
defines a backend interface and the leveldb implementation. This allows,
potentially, in the future, switching the db implementation so another
KV store should we wish to do so.

- Add proper error handling all along the way. The db and backend
packages are now errcheck clean. However, I drew the line at modifying
the FileSet API in order to keep this manageable and not continue
refactoring all of the rest of Syncthing. As such, the FileSet methods
still panic on database errors, except for the "database is closed"
error which is instead handled by silently returning as quickly as
possible, with the assumption that we're anyway "on the way out".

cmd/stindex/dump.go

@@ -13,11 +13,15 @@ import (
 	"time"
 
 	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/protocol"
 )
 
-func dump(ldb *db.Lowlevel) {
-	it := ldb.NewIterator(nil, nil)
+func dump(ldb backend.Backend) {
+	it, err := ldb.NewPrefixIterator(nil)
+	if err != nil {
+		log.Fatal(err)
+	}
 	for it.Next() {
 		key := it.Key()
 		switch key[0] {

cmd/stindex/dumpsize.go

@@ -10,8 +10,10 @@ import (
 	"container/heap"
 	"encoding/binary"
 	"fmt"
+	"log"
 
 	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
 )
 
 type SizedElement struct {
@@ -37,11 +39,14 @@ func (h *ElementHeap) Pop() interface{} {
 	return x
 }
 
-func dumpsize(ldb *db.Lowlevel) {
+func dumpsize(ldb backend.Backend) {
 	h := &ElementHeap{}
 	heap.Init(h)
 
-	it := ldb.NewIterator(nil, nil)
+	it, err := ldb.NewPrefixIterator(nil)
+	if err != nil {
+		log.Fatal(err)
+	}
 	var ele SizedElement
 	for it.Next() {
 		key := it.Key()

cmd/stindex/idxck.go

@@ -10,8 +10,10 @@ import (
 	"bytes"
 	"encoding/binary"
 	"fmt"
+	"log"
 
 	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/protocol"
 )
 
@@ -31,7 +33,7 @@ type sequenceKey struct {
 	sequence uint64
 }
 
-func idxck(ldb *db.Lowlevel) (success bool) {
+func idxck(ldb backend.Backend) (success bool) {
 	folders := make(map[uint32]string)
 	devices := make(map[uint32]string)
 	deviceToIDs := make(map[string]uint32)
@@ -42,7 +44,10 @@ func idxck(ldb *db.Lowlevel) (success bool) {
 	var localDeviceKey uint32
 	success = true
 
-	it := ldb.NewIterator(nil, nil)
+	it, err := ldb.NewPrefixIterator(nil)
+	if err != nil {
+		log.Fatal(err)
+	}
 	for it.Next() {
 		key := it.Key()
 		switch key[0] {

cmd/stindex/main.go

@@ -13,7 +13,7 @@ import (
 	"os"
 	"path/filepath"
 
-	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
 )
 
 func main() {
@@ -30,7 +30,7 @@ func main() {
 		path = filepath.Join(defaultConfigDir(), "index-v0.14.0.db")
 	}
 
-	ldb, err := db.OpenRO(path)
+	ldb, err := backend.OpenLevelDBRO(path)
 	if err != nil {
 		log.Fatal(err)
 	}

lib/config/tuning_test.go

@@ -10,17 +10,17 @@ import (
 	"testing"
 
 	"github.com/syncthing/syncthing/lib/config"
-	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
 )
 
 func TestTuningMatches(t *testing.T) {
-	if int(config.TuningAuto) != int(db.TuningAuto) {
+	if int(config.TuningAuto) != int(backend.TuningAuto) {
 		t.Error("mismatch for TuningAuto")
 	}
-	if int(config.TuningSmall) != int(db.TuningSmall) {
+	if int(config.TuningSmall) != int(backend.TuningSmall) {
 		t.Error("mismatch for TuningSmall")
 	}
-	if int(config.TuningLarge) != int(db.TuningLarge) {
+	if int(config.TuningLarge) != int(backend.TuningLarge) {
 		t.Error("mismatch for TuningLarge")
 	}
 }

lib/db/backend/backend.go

@@ -0,0 +1,170 @@
+// Copyright (C) 2019 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package backend
+
+import (
+	"sync"
+)
+
+// The Reader interface specifies the read-only operations available on the
+// main database and on read-only transactions (snapshots). Note that when
+// called directly on the database handle these operations may take implicit
+// transactions and performance may suffer.
+type Reader interface {
+	Get(key []byte) ([]byte, error)
+	NewPrefixIterator(prefix []byte) (Iterator, error)
+	NewRangeIterator(first, last []byte) (Iterator, error)
+}
+
+// The Writer interface specifies the mutating operations available on the
+// main database and on writable transactions. Note that when called
+// directly on the database handle these operations may take implicit
+// transactions and performance may suffer.
+type Writer interface {
+	Put(key, val []byte) error
+	Delete(key []byte) error
+}
+
+// The ReadTransaction interface specifies the operations on read-only
+// transactions. Every ReadTransaction must be released when no longer
+// required.
+type ReadTransaction interface {
+	Reader
+	Release()
+}
+
+// The WriteTransaction interface specifies the operations on writable
+// transactions. Every WriteTransaction must be either committed or released
+// (i.e., discarded) when no longer required. No further operations must be
+// performed after release or commit (regardless of whether commit succeeded),
+// with one exception -- it's fine to release an already committed or released
+// transaction.
+//
+// A Checkpoint is a potential partial commit of the transaction so far, for
+// purposes of saving memory when transactions are in-RAM. Note that
+// transactions may be checkpointed *anyway* even if this is not called, due to
+// resource constraints, but this gives you a chance to decide when.
+type WriteTransaction interface {
+	ReadTransaction
+	Writer
+	Checkpoint() error
+	Commit() error
+}
+
+// The Iterator interface specifies the operations available on iterators
+// returned by NewPrefixIterator and NewRangeIterator. The iterator pattern
+// is to loop while Next returns true, then check Error after the loop. Next
+// will return false when iteration is complete (Error() == nil) or when
+// there is an error preventing iteration, which is then returned by
+// Error(). For example:
+//
+//     it, err := db.NewPrefixIterator(nil)
+//     if err != nil {
+//         // problem preventing iteration
+//     }
+//     defer it.Release()
+//     for it.Next() {
+//         // ...
+//     }
+//     if err := it.Error(); err != nil {
+//         // there was a database problem while iterating
+//     }
+//
+// An iterator must be Released when no longer required. The Error method
+// can be called either before or after Release with the same results. If an
+// iterator was created in a transaction (whether read-only or write) it
+// must be released before the transaction is released (or committed).
+type Iterator interface {
+	Next() bool
+	Key() []byte
+	Value() []byte
+	Error() error
+	Release()
+}
+
+// The Backend interface represents the main database handle. It supports
+// both read/write operations and opening read-only or writable
+// transactions. Depending on the actual implementation, individual
+// read/write operations may be implicitly wrapped in transactions, making
+// them perform quite badly when used repeatedly. For bulk operations,
+// consider always using a transaction of the appropriate type. The
+// transaction isolation level is "read committed" - there are no dirty
+// reads.
+type Backend interface {
+	Reader
+	Writer
+	NewReadTransaction() (ReadTransaction, error)
+	NewWriteTransaction() (WriteTransaction, error)
+	Close() error
+}
+
+type Tuning int
+
+const (
+	// N.b. these constants must match those in lib/config.Tuning!
+	TuningAuto Tuning = iota
+	TuningSmall
+	TuningLarge
+)
+
+func Open(path string, tuning Tuning) (Backend, error) {
+	return OpenLevelDB(path, tuning)
+}
+
+func OpenMemory() Backend {
+	return OpenLevelDBMemory()
+}
+
+type errClosed struct{}
+
+func (errClosed) Error() string { return "database is closed" }
+
+type errNotFound struct{}
+
+func (errNotFound) Error() string { return "key not found" }
+
+func IsClosed(err error) bool {
+	if _, ok := err.(errClosed); ok {
+		return true
+	}
+	if _, ok := err.(*errClosed); ok {
+		return true
+	}
+	return false
+}
+
+func IsNotFound(err error) bool {
+	if _, ok := err.(errNotFound); ok {
+		return true
+	}
+	if _, ok := err.(*errNotFound); ok {
+		return true
+	}
+	return false
+}
+
+// releaser manages counting on top of a waitgroup
+type releaser struct {
+	wg   *sync.WaitGroup
+	once *sync.Once
+}
+
+func newReleaser(wg *sync.WaitGroup) *releaser {
+	wg.Add(1)
+	return &releaser{
+		wg:   wg,
+		once: new(sync.Once),
+	}
+}
+
+func (r releaser) Release() {
+	// We use the Once because we may get called multiple times from
+	// Commit() and deferred Release().
+	r.once.Do(func() {
+		r.wg.Done()
+	})
+}

lib/db/backend/backend_test.go

@@ -0,0 +1,53 @@
+// Copyright (C) 2019 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package backend
+
+import "testing"
+
+// testBackendBehavior is the generic test suite that must be fulfilled by
+// every backend implementation. It should be called by each implementation
+// as (part of) their test suite.
+func testBackendBehavior(t *testing.T, open func() Backend) {
+	t.Run("WriteIsolation", func(t *testing.T) { testWriteIsolation(t, open) })
+	t.Run("DeleteNonexisten", func(t *testing.T) { testDeleteNonexistent(t, open) })
+}
+
+func testWriteIsolation(t *testing.T, open func() Backend) {
+	// Values written during a transaction should not be read back, our
+	// updateGlobal depends on this.
+
+	db := open()
+	defer db.Close()
+
+	// Sanity check
+	_ = db.Put([]byte("a"), []byte("a"))
+	v, _ := db.Get([]byte("a"))
+	if string(v) != "a" {
+		t.Fatal("read back should work")
+	}
+
+	// Now in a transaction we should still see the old value
+	tx, _ := db.NewWriteTransaction()
+	defer tx.Release()
+	_ = tx.Put([]byte("a"), []byte("b"))
+	v, _ = tx.Get([]byte("a"))
+	if string(v) != "a" {
+		t.Fatal("read in transaction should read the old value")
+	}
+}
+
+func testDeleteNonexistent(t *testing.T, open func() Backend) {
+	// Deleting a non-existent key is not an error
+
+	db := open()
+	defer db.Close()
+
+	err := db.Delete([]byte("a"))
+	if err != nil {
+		t.Error(err)
+	}
+}

lib/db/backend/debug.go

@@ -0,0 +1,15 @@
+// Copyright (C) 2019 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package backend
+
+import (
+	"github.com/syncthing/syncthing/lib/logger"
+)
+
+var (
+	l = logger.DefaultLogger.NewFacility("backend", "The database backend")
+)

lib/db/backend/leveldb_backend.go

@@ -0,0 +1,173 @@
+// Copyright (C) 2018 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package backend
+
+import (
+	"sync"
+
+	"github.com/syndtr/goleveldb/leveldb"
+	"github.com/syndtr/goleveldb/leveldb/util"
+)
+
+const (
+	// Never flush transactions smaller than this, even on Checkpoint()
+	dbFlushBatchMin = 1 << MiB
+	// Once a transaction reaches this size, flush it unconditionally.
+	dbFlushBatchMax = 128 << MiB
+)
+
+// leveldbBackend implements Backend on top of a leveldb
+type leveldbBackend struct {
+	ldb     *leveldb.DB
+	closeWG sync.WaitGroup
+}
+
+func (b *leveldbBackend) NewReadTransaction() (ReadTransaction, error) {
+	return b.newSnapshot()
+}
+
+func (b *leveldbBackend) newSnapshot() (leveldbSnapshot, error) {
+	snap, err := b.ldb.GetSnapshot()
+	if err != nil {
+		return leveldbSnapshot{}, wrapLeveldbErr(err)
+	}
+	return leveldbSnapshot{
+		snap: snap,
+		rel:  newReleaser(&b.closeWG),
+	}, nil
+}
+
+func (b *leveldbBackend) NewWriteTransaction() (WriteTransaction, error) {
+	snap, err := b.newSnapshot()
+	if err != nil {
+		return nil, err // already wrapped
+	}
+	return &leveldbTransaction{
+		leveldbSnapshot: snap,
+		ldb:             b.ldb,
+		batch:           new(leveldb.Batch),
+		rel:             newReleaser(&b.closeWG),
+	}, nil
+}
+
+func (b *leveldbBackend) Close() error {
+	b.closeWG.Wait()
+	return wrapLeveldbErr(b.ldb.Close())
+}
+
+func (b *leveldbBackend) Get(key []byte) ([]byte, error) {
+	val, err := b.ldb.Get(key, nil)
+	return val, wrapLeveldbErr(err)
+}
+
+func (b *leveldbBackend) NewPrefixIterator(prefix []byte) (Iterator, error) {
+	return b.ldb.NewIterator(util.BytesPrefix(prefix), nil), nil
+}
+
+func (b *leveldbBackend) NewRangeIterator(first, last []byte) (Iterator, error) {
+	return b.ldb.NewIterator(&util.Range{Start: first, Limit: last}, nil), nil
+}
+
+func (b *leveldbBackend) Put(key, val []byte) error {
+	return wrapLeveldbErr(b.ldb.Put(key, val, nil))
+}
+
+func (b *leveldbBackend) Delete(key []byte) error {
+	return wrapLeveldbErr(b.ldb.Delete(key, nil))
+}
+
+// leveldbSnapshot implements backend.ReadTransaction
+type leveldbSnapshot struct {
+	snap *leveldb.Snapshot
+	rel  *releaser
+}
+
+func (l leveldbSnapshot) Get(key []byte) ([]byte, error) {
+	val, err := l.snap.Get(key, nil)
+	return val, wrapLeveldbErr(err)
+}
+
+func (l leveldbSnapshot) NewPrefixIterator(prefix []byte) (Iterator, error) {
+	return l.snap.NewIterator(util.BytesPrefix(prefix), nil), nil
+}
+
+func (l leveldbSnapshot) NewRangeIterator(first, last []byte) (Iterator, error) {
+	return l.snap.NewIterator(&util.Range{Start: first, Limit: last}, nil), nil
+}
+
+func (l leveldbSnapshot) Release() {
+	l.snap.Release()
+	l.rel.Release()
+}
+
+// leveldbTransaction implements backend.WriteTransaction using a batch (not
+// an actual leveldb transaction)
+type leveldbTransaction struct {
+	leveldbSnapshot
+	ldb   *leveldb.DB
+	batch *leveldb.Batch
+	rel   *releaser
+}
+
+func (t *leveldbTransaction) Delete(key []byte) error {
+	t.batch.Delete(key)
+	return t.checkFlush(dbFlushBatchMax)
+}
+
+func (t *leveldbTransaction) Put(key, val []byte) error {
+	t.batch.Put(key, val)
+	return t.checkFlush(dbFlushBatchMax)
+}
+
+func (t *leveldbTransaction) Checkpoint() error {
+	return t.checkFlush(dbFlushBatchMin)
+}
+
+func (t *leveldbTransaction) Commit() error {
+	err := wrapLeveldbErr(t.flush())
+	t.leveldbSnapshot.Release()
+	t.rel.Release()
+	return err
+}
+
+func (t *leveldbTransaction) Release() {
+	t.leveldbSnapshot.Release()
+	t.rel.Release()
+}
+
+// checkFlush flushes and resets the batch if its size exceeds the given size.
+func (t *leveldbTransaction) checkFlush(size int) error {
+	if len(t.batch.Dump()) < size {
+		return nil
+	}
+	return t.flush()
+}
+
+func (t *leveldbTransaction) flush() error {
+	if t.batch.Len() == 0 {
+		return nil
+	}
+	if err := t.ldb.Write(t.batch, nil); err != nil {
+		return wrapLeveldbErr(err)
+	}
+	t.batch.Reset()
+	return nil
+}
+
+// wrapLeveldbErr wraps errors so that the backend package can recognize them
+func wrapLeveldbErr(err error) error {
+	if err == nil {
+		return nil
+	}
+	if err == leveldb.ErrClosed {
+		return errClosed{}
+	}
+	if err == leveldb.ErrNotFound {
+		return errNotFound{}
+	}
+	return err
+}

lib/db/backend/leveldb_open.go

@@ -0,0 +1,226 @@
+// Copyright (C) 2018 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package backend
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+
+	"github.com/syndtr/goleveldb/leveldb"
+	"github.com/syndtr/goleveldb/leveldb/errors"
+	"github.com/syndtr/goleveldb/leveldb/opt"
+	"github.com/syndtr/goleveldb/leveldb/storage"
+	"github.com/syndtr/goleveldb/leveldb/util"
+)
+
+const (
+	dbMaxOpenFiles = 100
+
+	// A large database is > 200 MiB. It's a mostly arbitrary value, but
+	// it's also the case that each file is 2 MiB by default and when we
+	// have dbMaxOpenFiles of them we will need to start thrashing fd:s.
+	// Switching to large database settings causes larger files to be used
+	// when compacting, reducing the number.
+	dbLargeThreshold = dbMaxOpenFiles * (2 << MiB)
+
+	KiB = 10
+	MiB = 20
+)
+
+// Open attempts to open the database at the given location, and runs
+// recovery on it if opening fails. Worst case, if recovery is not possible,
+// the database is erased and created from scratch.
+func OpenLevelDB(location string, tuning Tuning) (Backend, error) {
+	opts := optsFor(location, tuning)
+	ldb, err := open(location, opts)
+	if err != nil {
+		return nil, err
+	}
+	return &leveldbBackend{ldb: ldb}, nil
+}
+
+// OpenRO attempts to open the database at the given location, read only.
+func OpenLevelDBRO(location string) (Backend, error) {
+	opts := &opt.Options{
+		OpenFilesCacheCapacity: dbMaxOpenFiles,
+		ReadOnly:               true,
+	}
+	ldb, err := open(location, opts)
+	if err != nil {
+		return nil, err
+	}
+	return &leveldbBackend{ldb: ldb}, nil
+}
+
+// OpenMemory returns a new Lowlevel referencing an in-memory database.
+func OpenLevelDBMemory() Backend {
+	ldb, _ := leveldb.Open(storage.NewMemStorage(), nil)
+	return &leveldbBackend{ldb: ldb}
+}
+
+// optsFor returns the database options to use when opening a database with
+// the given location and tuning. Settings can be overridden by debug
+// environment variables.
+func optsFor(location string, tuning Tuning) *opt.Options {
+	large := false
+	switch tuning {
+	case TuningLarge:
+		large = true
+	case TuningAuto:
+		large = dbIsLarge(location)
+	}
+
+	var (
+		// Set defaults used for small databases.
+		defaultBlockCacheCapacity            = 0 // 0 means let leveldb use default
+		defaultBlockSize                     = 0
+		defaultCompactionTableSize           = 0
+		defaultCompactionTableSizeMultiplier = 0
+		defaultWriteBuffer                   = 16 << MiB                      // increased from leveldb default of 4 MiB
+		defaultCompactionL0Trigger           = opt.DefaultCompactionL0Trigger // explicit because we use it as base for other stuff
+	)
+
+	if large {
+		// Change the parameters for better throughput at the price of some
+		// RAM and larger files. This results in larger batches of writes
+		// and compaction at a lower frequency.
+		l.Infoln("Using large-database tuning")
+
+		defaultBlockCacheCapacity = 64 << MiB
+		defaultBlockSize = 64 << KiB
+		defaultCompactionTableSize = 16 << MiB
+		defaultCompactionTableSizeMultiplier = 20 // 2.0 after division by ten
+		defaultWriteBuffer = 64 << MiB
+		defaultCompactionL0Trigger = 8 // number of l0 files
+	}
+
+	opts := &opt.Options{
+		BlockCacheCapacity:            debugEnvValue("BlockCacheCapacity", defaultBlockCacheCapacity),
+		BlockCacheEvictRemoved:        debugEnvValue("BlockCacheEvictRemoved", 0) != 0,
+		BlockRestartInterval:          debugEnvValue("BlockRestartInterval", 0),
+		BlockSize:                     debugEnvValue("BlockSize", defaultBlockSize),
+		CompactionExpandLimitFactor:   debugEnvValue("CompactionExpandLimitFactor", 0),
+		CompactionGPOverlapsFactor:    debugEnvValue("CompactionGPOverlapsFactor", 0),
+		CompactionL0Trigger:           debugEnvValue("CompactionL0Trigger", defaultCompactionL0Trigger),
+		CompactionSourceLimitFactor:   debugEnvValue("CompactionSourceLimitFactor", 0),
+		CompactionTableSize:           debugEnvValue("CompactionTableSize", defaultCompactionTableSize),
+		CompactionTableSizeMultiplier: float64(debugEnvValue("CompactionTableSizeMultiplier", defaultCompactionTableSizeMultiplier)) / 10.0,
+		CompactionTotalSize:           debugEnvValue("CompactionTotalSize", 0),
+		CompactionTotalSizeMultiplier: float64(debugEnvValue("CompactionTotalSizeMultiplier", 0)) / 10.0,
+		DisableBufferPool:             debugEnvValue("DisableBufferPool", 0) != 0,
+		DisableBlockCache:             debugEnvValue("DisableBlockCache", 0) != 0,
+		DisableCompactionBackoff:      debugEnvValue("DisableCompactionBackoff", 0) != 0,
+		DisableLargeBatchTransaction:  debugEnvValue("DisableLargeBatchTransaction", 0) != 0,
+		NoSync:                        debugEnvValue("NoSync", 0) != 0,
+		NoWriteMerge:                  debugEnvValue("NoWriteMerge", 0) != 0,
+		OpenFilesCacheCapacity:        debugEnvValue("OpenFilesCacheCapacity", dbMaxOpenFiles),
+		WriteBuffer:                   debugEnvValue("WriteBuffer", defaultWriteBuffer),
+		// The write slowdown and pause can be overridden, but even if they
+		// are not and the compaction trigger is overridden we need to
+		// adjust so that we don't pause writes for L0 compaction before we
+		// even *start* L0 compaction...
+		WriteL0SlowdownTrigger: debugEnvValue("WriteL0SlowdownTrigger", 2*debugEnvValue("CompactionL0Trigger", defaultCompactionL0Trigger)),
+		WriteL0PauseTrigger:    debugEnvValue("WriteL0SlowdownTrigger", 3*debugEnvValue("CompactionL0Trigger", defaultCompactionL0Trigger)),
+	}
+
+	return opts
+}
+
+func open(location string, opts *opt.Options) (*leveldb.DB, error) {
+	db, err := leveldb.OpenFile(location, opts)
+	if leveldbIsCorrupted(err) {
+		db, err = leveldb.RecoverFile(location, opts)
+	}
+	if leveldbIsCorrupted(err) {
+		// The database is corrupted, and we've tried to recover it but it
+		// didn't work. At this point there isn't much to do beyond dropping
+		// the database and reindexing...
+		l.Infoln("Database corruption detected, unable to recover. Reinitializing...")
+		if err := os.RemoveAll(location); err != nil {
+			return nil, errorSuggestion{err, "failed to delete corrupted database"}
+		}
+		db, err = leveldb.OpenFile(location, opts)
+	}
+	if err != nil {
+		return nil, errorSuggestion{err, "is another instance of Syncthing running?"}
+	}
+
+	if debugEnvValue("CompactEverything", 0) != 0 {
+		if err := db.CompactRange(util.Range{}); err != nil {
+			l.Warnln("Compacting database:", err)
+		}
+	}
+
+	return db, nil
+}
+
+func debugEnvValue(key string, def int) int {
+	v, err := strconv.ParseInt(os.Getenv("STDEBUG_"+key), 10, 63)
+	if err != nil {
+		return def
+	}
+	return int(v)
+}
+
+// A "better" version of leveldb's errors.IsCorrupted.
+func leveldbIsCorrupted(err error) bool {
+	switch {
+	case err == nil:
+		return false
+
+	case errors.IsCorrupted(err):
+		return true
+
+	case strings.Contains(err.Error(), "corrupted"):
+		return true
+	}
+
+	return false
+}
+
+// dbIsLarge returns whether the estimated size of the database at location
+// is large enough to warrant optimization for large databases.
+func dbIsLarge(location string) bool {
+	if ^uint(0)>>63 == 0 {
+		// We're compiled for a 32 bit architecture. We've seen trouble with
+		// large settings there.
+		// (https://forum.syncthing.net/t/many-small-ldb-files-with-database-tuning/13842)
+		return false
+	}
+
+	dir, err := os.Open(location)
+	if err != nil {
+		return false
+	}
+
+	fis, err := dir.Readdir(-1)
+	if err != nil {
+		return false
+	}
+
+	var size int64
+	for _, fi := range fis {
+		if fi.Name() == "LOG" {
+			// don't count the size
+			continue
+		}
+		size += fi.Size()
+	}
+
+	return size > dbLargeThreshold
+}
+
+type errorSuggestion struct {
+	inner      error
+	suggestion string
+}
+
+func (e errorSuggestion) Error() string {
+	return fmt.Sprintf("%s (%s)", e.inner.Error(), e.suggestion)
+}

lib/db/backend/leveldb_test.go

@@ -0,0 +1,13 @@
+// Copyright (C) 2019 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package backend
+
+import "testing"
+
+func TestLevelDBBackendBehavior(t *testing.T) {
+	testBackendBehavior(t, OpenLevelDBMemory)
+}

lib/db/benchmark_test.go

@@ -11,6 +11,7 @@ import (
 	"testing"
 
 	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/protocol"
 )
@@ -40,7 +41,7 @@ func lazyInitBenchFiles() {
 func getBenchFileSet() (*db.Lowlevel, *db.FileSet) {
 	lazyInitBenchFiles()
 
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 	benchS := db.NewFileSet("test)", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 	replace(benchS, remoteDevice0, files)
 	replace(benchS, protocol.LocalDeviceID, firstHalf)
@@ -49,7 +50,7 @@ func getBenchFileSet() (*db.Lowlevel, *db.FileSet) {
 }
 
 func BenchmarkReplaceAll(b *testing.B) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 	defer ldb.Close()
 
 	b.ResetTimer()
@@ -157,7 +158,7 @@ func BenchmarkNeedHalf(b *testing.B) {
 }
 
 func BenchmarkNeedHalfRemote(b *testing.B) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 	defer ldb.Close()
 	fset := db.NewFileSet("test)", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 	replace(fset, remoteDevice0, firstHalf)

lib/db/blockmap.go

@@ -11,8 +11,6 @@ import (
 	"fmt"
 
 	"github.com/syncthing/syncthing/lib/osutil"
-
-	"github.com/syndtr/goleveldb/leveldb/util"
 )
 
 var blockFinder *BlockFinder
@@ -41,13 +39,22 @@ func (f *BlockFinder) String() string {
 // reason. The iterator finally returns the result, whether or not a
 // satisfying block was eventually found.
 func (f *BlockFinder) Iterate(folders []string, hash []byte, iterFn func(string, string, int32) bool) bool {
-	t := f.db.newReadOnlyTransaction()
+	t, err := f.db.newReadOnlyTransaction()
+	if err != nil {
+		return false
+	}
 	defer t.close()
 
 	var key []byte
 	for _, folder := range folders {
-		key = f.db.keyer.GenerateBlockMapKey(key, []byte(folder), hash, nil)
-		iter := t.NewIterator(util.BytesPrefix(key), nil)
+		key, err = f.db.keyer.GenerateBlockMapKey(key, []byte(folder), hash, nil)
+		if err != nil {
+			return false
+		}
+		iter, err := t.NewPrefixIterator(key)
+		if err != nil {
+			return false
+		}
 
 		for iter.Next() && iter.Error() == nil {
 			file := string(f.db.keyer.NameFromBlockMapKey(iter.Key()))

lib/db/blockmap_test.go

@@ -10,23 +10,10 @@ import (
 	"encoding/binary"
 	"testing"
 
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/protocol"
-	"github.com/syndtr/goleveldb/leveldb/util"
 )
 
-func genBlocks(n int) []protocol.BlockInfo {
-	b := make([]protocol.BlockInfo, n)
-	for i := range b {
-		h := make([]byte, 32)
-		for j := range h {
-			h[j] = byte(i + j)
-		}
-		b[i].Size = int32(i)
-		b[i].Hash = h
-	}
-	return b
-}
-
 var f1, f2, f3 protocol.FileInfo
 var folders = []string{"folder1", "folder2"}
 
@@ -52,18 +39,24 @@ func init() {
 func setup() (*instance, *BlockFinder) {
 	// Setup
 
-	db := OpenMemory()
+	db := NewLowlevel(backend.OpenMemory())
 	return newInstance(db), NewBlockFinder(db)
 }
 
 func dbEmpty(db *instance) bool {
-	iter := db.NewIterator(util.BytesPrefix([]byte{KeyTypeBlock}), nil)
+	iter, err := db.NewPrefixIterator([]byte{KeyTypeBlock})
+	if err != nil {
+		panic(err)
+	}
 	defer iter.Release()
 	return !iter.Next()
 }
 
-func addToBlockMap(db *instance, folder []byte, fs []protocol.FileInfo) {
-	t := db.newReadWriteTransaction()
+func addToBlockMap(db *instance, folder []byte, fs []protocol.FileInfo) error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
 	var keyBuf []byte
@@ -73,15 +66,24 @@ func addToBlockMap(db *instance, folder []byte, fs []protocol.FileInfo) {
 			name := []byte(f.Name)
 			for i, block := range f.Blocks {
 				binary.BigEndian.PutUint32(blockBuf, uint32(i))
-				keyBuf = t.keyer.GenerateBlockMapKey(keyBuf, folder, block.Hash, name)
-				t.Put(keyBuf, blockBuf)
+				keyBuf, err = t.keyer.GenerateBlockMapKey(keyBuf, folder, block.Hash, name)
+				if err != nil {
+					return err
+				}
+				if err := t.Put(keyBuf, blockBuf); err != nil {
+					return err
 				}
 			}
 		}
 	}
+	return t.commit()
+}
 
-func discardFromBlockMap(db *instance, folder []byte, fs []protocol.FileInfo) {
-	t := db.newReadWriteTransaction()
+func discardFromBlockMap(db *instance, folder []byte, fs []protocol.FileInfo) error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
 	var keyBuf []byte
@@ -89,12 +91,18 @@ func discardFromBlockMap(db *instance, folder []byte, fs []protocol.FileInfo) {
 		if !ef.IsDirectory() && !ef.IsDeleted() && !ef.IsInvalid() {
 			name := []byte(ef.Name)
 			for _, block := range ef.Blocks {
-				keyBuf = t.keyer.GenerateBlockMapKey(keyBuf, folder, block.Hash, name)
-				t.Delete(keyBuf)
+				keyBuf, err = t.keyer.GenerateBlockMapKey(keyBuf, folder, block.Hash, name)
+				if err != nil {
+					return err
+				}
+				if err := t.Delete(keyBuf); err != nil {
+					return err
 				}
 			}
 		}
 	}
+	return t.commit()
+}
 
 func TestBlockMapAddUpdateWipe(t *testing.T) {
 	db, f := setup()
@@ -107,7 +115,9 @@ func TestBlockMapAddUpdateWipe(t *testing.T) {
 
 	f3.Type = protocol.FileInfoTypeDirectory
 
-	addToBlockMap(db, folder, []protocol.FileInfo{f1, f2, f3})
+	if err := addToBlockMap(db, folder, []protocol.FileInfo{f1, f2, f3}); err != nil {
+		t.Fatal(err)
+	}
 
 	f.Iterate(folders, f1.Blocks[0].Hash, func(folder, file string, index int32) bool {
 		if folder != "folder1" || file != "f1" || index != 0 {
@@ -128,12 +138,16 @@ func TestBlockMapAddUpdateWipe(t *testing.T) {
 		return true
 	})
 
-	discardFromBlockMap(db, folder, []protocol.FileInfo{f1, f2, f3})
+	if err := discardFromBlockMap(db, folder, []protocol.FileInfo{f1, f2, f3}); err != nil {
+		t.Fatal(err)
+	}
 
 	f1.Deleted = true
 	f2.LocalFlags = protocol.FlagLocalMustRescan // one of the invalid markers
 
-	addToBlockMap(db, folder, []protocol.FileInfo{f1, f2, f3})
+	if err := addToBlockMap(db, folder, []protocol.FileInfo{f1, f2, f3}); err != nil {
+		t.Fatal(err)
+	}
 
 	f.Iterate(folders, f1.Blocks[0].Hash, func(folder, file string, index int32) bool {
 		t.Fatal("Unexpected block")
@@ -152,14 +166,18 @@ func TestBlockMapAddUpdateWipe(t *testing.T) {
 		return true
 	})
 
-	db.dropFolder(folder)
+	if err := db.dropFolder(folder); err != nil {
+		t.Fatal(err)
+	}
 
 	if !dbEmpty(db) {
 		t.Fatal("db not empty")
 	}
 
 	// Should not add
-	addToBlockMap(db, folder, []protocol.FileInfo{f1, f2})
+	if err := addToBlockMap(db, folder, []protocol.FileInfo{f1, f2}); err != nil {
+		t.Fatal(err)
+	}
 
 	if !dbEmpty(db) {
 		t.Fatal("db not empty")
@@ -179,8 +197,12 @@ func TestBlockFinderLookup(t *testing.T) {
 	folder1 := []byte("folder1")
 	folder2 := []byte("folder2")
 
-	addToBlockMap(db, folder1, []protocol.FileInfo{f1})
-	addToBlockMap(db, folder2, []protocol.FileInfo{f1})
+	if err := addToBlockMap(db, folder1, []protocol.FileInfo{f1}); err != nil {
+		t.Fatal(err)
+	}
+	if err := addToBlockMap(db, folder2, []protocol.FileInfo{f1}); err != nil {
+		t.Fatal(err)
+	}
 
 	counter := 0
 	f.Iterate(folders, f1.Blocks[0].Hash, func(folder, file string, index int32) bool {
@@ -204,11 +226,15 @@ func TestBlockFinderLookup(t *testing.T) {
 		t.Fatal("Incorrect count", counter)
 	}
 
-	discardFromBlockMap(db, folder1, []protocol.FileInfo{f1})
+	if err := discardFromBlockMap(db, folder1, []protocol.FileInfo{f1}); err != nil {
+		t.Fatal(err)
+	}
 
 	f1.Deleted = true
 
-	addToBlockMap(db, folder1, []protocol.FileInfo{f1})
+	if err := addToBlockMap(db, folder1, []protocol.FileInfo{f1}); err != nil {
+		t.Fatal(err)
+	}
 
 	counter = 0
 	f.Iterate(folders, f1.Blocks[0].Hash, func(folder, file string, index int32) bool {

lib/db/concurrency_test.go

@@ -1,237 +0,0 @@
-// Copyright (C) 2014 The Syncthing Authors.
-//
-// This Source Code Form is subject to the terms of the Mozilla Public
-// License, v. 2.0. If a copy of the MPL was not distributed with this file,
-// You can obtain one at https://mozilla.org/MPL/2.0/.
-
-// this  is a really tedious test for an old issue
-// +build ignore
-
-package db_test
-
-import (
-	"crypto/rand"
-	"log"
-	"os"
-	"testing"
-	"time"
-
-	"github.com/syncthing/syncthing/lib/sync"
-
-	"github.com/syndtr/goleveldb/leveldb"
-	"github.com/syndtr/goleveldb/leveldb/opt"
-	"github.com/syndtr/goleveldb/leveldb/util"
-)
-
-var keys [][]byte
-
-func init() {
-	for i := 0; i < nItems; i++ {
-		keys = append(keys, randomData(1))
-	}
-}
-
-const nItems = 10000
-
-func randomData(prefix byte) []byte {
-	data := make([]byte, 1+32+64+32)
-	_, err := rand.Reader.Read(data)
-	if err != nil {
-		panic(err)
-	}
-	return append([]byte{prefix}, data...)
-}
-
-func setItems(db *leveldb.DB) error {
-	batch := new(leveldb.Batch)
-	for _, k1 := range keys {
-		k2 := randomData(2)
-		// k2 -> data
-		batch.Put(k2, randomData(42))
-		// k1 -> k2
-		batch.Put(k1, k2)
-	}
-	if testing.Verbose() {
-		log.Printf("batch write (set) %p", batch)
-	}
-	return db.Write(batch, nil)
-}
-
-func clearItems(db *leveldb.DB) error {
-	snap, err := db.GetSnapshot()
-	if err != nil {
-		return err
-	}
-	defer snap.Release()
-
-	// Iterate over k2
-
-	it := snap.NewIterator(util.BytesPrefix([]byte{1}), nil)
-	defer it.Release()
-
-	batch := new(leveldb.Batch)
-	for it.Next() {
-		k1 := it.Key()
-		k2 := it.Value()
-
-		// k2 should exist
-		_, err := snap.Get(k2, nil)
-		if err != nil {
-			return err
-		}
-
-		// Delete the k1 => k2 mapping first
-		batch.Delete(k1)
-		// Then the k2 => data mapping
-		batch.Delete(k2)
-	}
-	if testing.Verbose() {
-		log.Printf("batch write (clear) %p", batch)
-	}
-	return db.Write(batch, nil)
-}
-
-func scanItems(db *leveldb.DB) error {
-	snap, err := db.GetSnapshot()
-	if testing.Verbose() {
-		log.Printf("snap create %p", snap)
-	}
-	if err != nil {
-		return err
-	}
-	defer func() {
-		if testing.Verbose() {
-			log.Printf("snap release %p", snap)
-		}
-		snap.Release()
-	}()
-
-	// Iterate from the start of k2 space to the end
-	it := snap.NewIterator(util.BytesPrefix([]byte{1}), nil)
-	defer it.Release()
-
-	i := 0
-	for it.Next() {
-		// k2 => k1 => data
-		k1 := it.Key()
-		k2 := it.Value()
-		_, err := snap.Get(k2, nil)
-		if err != nil {
-			log.Printf("k1: %x", k1)
-			log.Printf("k2: %x (missing)", k2)
-			return err
-		}
-		i++
-	}
-	if testing.Verbose() {
-		log.Println("scanned", i)
-	}
-	return nil
-}
-
-func TestConcurrentSetClear(t *testing.T) {
-	if testing.Short() {
-		return
-	}
-
-	dur := 30 * time.Second
-	t0 := time.Now()
-	wg := sync.NewWaitGroup()
-
-	os.RemoveAll("testdata/concurrent-set-clear.db")
-	db, err := leveldb.OpenFile("testdata/concurrent-set-clear.db", &opt.Options{OpenFilesCacheCapacity: 10})
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer os.RemoveAll("testdata/concurrent-set-clear.db")
-
-	errChan := make(chan error, 3)
-
-	wg.Add(1)
-	go func() {
-		defer wg.Done()
-		for time.Since(t0) < dur {
-			if err := setItems(db); err != nil {
-				errChan <- err
-				return
-			}
-			if err := clearItems(db); err != nil {
-				errChan <- err
-				return
-			}
-		}
-	}()
-
-	wg.Add(1)
-	go func() {
-		defer wg.Done()
-		for time.Since(t0) < dur {
-			if err := scanItems(db); err != nil {
-				errChan <- err
-				return
-			}
-		}
-	}()
-
-	go func() {
-		wg.Wait()
-		errChan <- nil
-	}()
-
-	err = <-errChan
-	if err != nil {
-		t.Error(err)
-	}
-	db.Close()
-}
-
-func TestConcurrentSetOnly(t *testing.T) {
-	if testing.Short() {
-		return
-	}
-
-	dur := 30 * time.Second
-	t0 := time.Now()
-	wg := sync.NewWaitGroup()
-
-	os.RemoveAll("testdata/concurrent-set-only.db")
-	db, err := leveldb.OpenFile("testdata/concurrent-set-only.db", &opt.Options{OpenFilesCacheCapacity: 10})
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer os.RemoveAll("testdata/concurrent-set-only.db")
-
-	errChan := make(chan error, 3)
-
-	wg.Add(1)
-	go func() {
-		defer wg.Done()
-		for time.Since(t0) < dur {
-			if err := setItems(db); err != nil {
-				errChan <- err
-				return
-			}
-		}
-	}()
-
-	wg.Add(1)
-	go func() {
-		defer wg.Done()
-		for time.Since(t0) < dur {
-			if err := scanItems(db); err != nil {
-				errChan <- err
-				return
-			}
-		}
-	}()
-
-	go func() {
-		wg.Wait()
-		errChan <- nil
-	}()
-
-	err = <-errChan
-	if err != nil {
-		t.Error(err)
-	}
-}

lib/db/db_test.go

@@ -9,17 +9,33 @@ package db
 import (
 	"testing"
 
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/protocol"
 )
 
+func genBlocks(n int) []protocol.BlockInfo {
+	b := make([]protocol.BlockInfo, n)
+	for i := range b {
+		h := make([]byte, 32)
+		for j := range h {
+			h[j] = byte(i + j)
+		}
+		b[i].Size = int32(i)
+		b[i].Hash = h
+	}
+	return b
+}
+
 func TestIgnoredFiles(t *testing.T) {
 	ldb, err := openJSONS("testdata/v0.14.48-ignoredfiles.db.jsons")
 	if err != nil {
 		t.Fatal(err)
 	}
-	db := NewLowlevel(ldb, "<memory>")
-	UpdateSchema(db)
+	db := NewLowlevel(ldb)
+	if err := UpdateSchema(db); err != nil {
+		t.Fatal(err)
+	}
 
 	fs := NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), db)
 
@@ -142,25 +158,35 @@ func TestUpdate0to3(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	db := newInstance(NewLowlevel(ldb, "<memory>"))
+	db := newInstance(NewLowlevel(ldb))
 	updater := schemaUpdater{db}
 
 	folder := []byte(update0to3Folder)
 
-	updater.updateSchema0to1()
+	if err := updater.updateSchema0to1(); err != nil {
+		t.Fatal(err)
+	}
 
-	if _, ok := db.getFileDirty(folder, protocol.LocalDeviceID[:], []byte(slashPrefixed)); ok {
+	if _, ok, err := db.getFileDirty(folder, protocol.LocalDeviceID[:], []byte(slashPrefixed)); err != nil {
+		t.Fatal(err)
+	} else if ok {
 		t.Error("File prefixed by '/' was not removed during transition to schema 1")
 	}
 
-	if _, err := db.Get(db.keyer.GenerateGlobalVersionKey(nil, folder, []byte(invalid)), nil); err != nil {
+	key, err := db.keyer.GenerateGlobalVersionKey(nil, folder, []byte(invalid))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := db.Get(key); err != nil {
 		t.Error("Invalid file wasn't added to global list")
 	}
 
-	updater.updateSchema1to2()
+	if err := updater.updateSchema1to2(); err != nil {
+		t.Fatal(err)
+	}
 
 	found := false
-	db.withHaveSequence(folder, 0, func(fi FileIntf) bool {
+	_ = db.withHaveSequence(folder, 0, func(fi FileIntf) bool {
 		f := fi.(protocol.FileInfo)
 		l.Infoln(f)
 		if found {
@@ -178,14 +204,16 @@ func TestUpdate0to3(t *testing.T) {
 		t.Error("Local file wasn't added to sequence bucket", err)
 	}
 
-	updater.updateSchema2to3()
+	if err := updater.updateSchema2to3(); err != nil {
+		t.Fatal(err)
+	}
 
 	need := map[string]protocol.FileInfo{
 		haveUpdate0to3[remoteDevice0][0].Name: haveUpdate0to3[remoteDevice0][0],
 		haveUpdate0to3[remoteDevice1][0].Name: haveUpdate0to3[remoteDevice1][0],
 		haveUpdate0to3[remoteDevice0][2].Name: haveUpdate0to3[remoteDevice0][2],
 	}
-	db.withNeed(folder, protocol.LocalDeviceID[:], false, func(fi FileIntf) bool {
+	_ = db.withNeed(folder, protocol.LocalDeviceID[:], false, func(fi FileIntf) bool {
 		e, ok := need[fi.FileName()]
 		if !ok {
 			t.Error("Got unexpected needed file:", fi.FileName())
@@ -203,12 +231,17 @@ func TestUpdate0to3(t *testing.T) {
 }
 
 func TestDowngrade(t *testing.T) {
-	db := OpenMemory()
-	UpdateSchema(db) // sets the min version etc
+	db := NewLowlevel(backend.OpenMemory())
+	// sets the min version etc
+	if err := UpdateSchema(db); err != nil {
+		t.Fatal(err)
+	}
 
 	// Bump the database version to something newer than we actually support
 	miscDB := NewMiscDataNamespace(db)
-	miscDB.PutInt64("dbVersion", dbVersion+1)
+	if err := miscDB.PutInt64("dbVersion", dbVersion+1); err != nil {
+		t.Fatal(err)
+	}
 	l.Infoln(dbVersion)
 
 	// Pretend we just opened the DB and attempt to update it again

lib/db/instance.go

@@ -9,11 +9,9 @@ package db
 import (
 	"bytes"
 	"encoding/binary"
-	"fmt"
 
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/protocol"
-	"github.com/syndtr/goleveldb/leveldb"
-	"github.com/syndtr/goleveldb/leveldb/util"
 )
 
 type instance struct {
@@ -30,17 +28,26 @@ func newInstance(ll *Lowlevel) *instance {
 
 // updateRemoteFiles adds a list of fileinfos to the database and updates the
 // global versionlist and metadata.
-func (db *instance) updateRemoteFiles(folder, device []byte, fs []protocol.FileInfo, meta *metadataTracker) {
-	t := db.newReadWriteTransaction()
+func (db *instance) updateRemoteFiles(folder, device []byte, fs []protocol.FileInfo, meta *metadataTracker) error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
 	var dk, gk, keyBuf []byte
 	devID := protocol.DeviceIDFromBytes(device)
 	for _, f := range fs {
 		name := []byte(f.Name)
-		dk = db.keyer.GenerateDeviceFileKey(dk, folder, device, name)
+		dk, err = db.keyer.GenerateDeviceFileKey(dk, folder, device, name)
+		if err != nil {
+			return err
+		}
 
-		ef, ok := t.getFileTrunc(dk, true)
+		ef, ok, err := t.getFileTrunc(dk, true)
+		if err != nil {
+			return err
+		}
 		if ok && unchanged(f, ef) {
 			continue
 		}
@@ -51,28 +58,49 @@ func (db *instance) updateRemoteFiles(folder, device []byte, fs []protocol.FileI
 		meta.addFile(devID, f)
 
 		l.Debugf("insert; folder=%q device=%v %v", folder, devID, f)
-		t.Put(dk, mustMarshal(&f))
-
-		gk = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
-		keyBuf, _ = t.updateGlobal(gk, keyBuf, folder, device, f, meta)
-
-		t.checkFlush()
+		if err := t.Put(dk, mustMarshal(&f)); err != nil {
+			return err
 		}
+
+		gk, err = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
+		if err != nil {
+			return err
+		}
+		keyBuf, _, err = t.updateGlobal(gk, keyBuf, folder, device, f, meta)
+		if err != nil {
+			return err
+		}
+
+		if err := t.Checkpoint(); err != nil {
+			return err
+		}
+	}
+
+	return t.commit()
 }
 
 // updateLocalFiles adds fileinfos to the db, and updates the global versionlist,
 // metadata, sequence and blockmap buckets.
-func (db *instance) updateLocalFiles(folder []byte, fs []protocol.FileInfo, meta *metadataTracker) {
-	t := db.newReadWriteTransaction()
+func (db *instance) updateLocalFiles(folder []byte, fs []protocol.FileInfo, meta *metadataTracker) error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
 	var dk, gk, keyBuf []byte
 	blockBuf := make([]byte, 4)
 	for _, f := range fs {
 		name := []byte(f.Name)
-		dk = db.keyer.GenerateDeviceFileKey(dk, folder, protocol.LocalDeviceID[:], name)
+		dk, err = db.keyer.GenerateDeviceFileKey(dk, folder, protocol.LocalDeviceID[:], name)
+		if err != nil {
+			return err
+		}
 
-		ef, ok := t.getFileByKey(dk)
+		ef, ok, err := t.getFileByKey(dk)
+		if err != nil {
+			return err
+		}
 		if ok && unchanged(f, ef) {
 			continue
 		}
@@ -80,13 +108,23 @@ func (db *instance) updateLocalFiles(folder []byte, fs []protocol.FileInfo, meta
 		if ok {
 			if !ef.IsDirectory() && !ef.IsDeleted() && !ef.IsInvalid() {
 				for _, block := range ef.Blocks {
-					keyBuf = db.keyer.GenerateBlockMapKey(keyBuf, folder, block.Hash, name)
-					t.Delete(keyBuf)
+					keyBuf, err = db.keyer.GenerateBlockMapKey(keyBuf, folder, block.Hash, name)
+					if err != nil {
+						return err
+					}
+					if err := t.Delete(keyBuf); err != nil {
+						return err
+					}
 				}
 			}
 
-			keyBuf = db.keyer.GenerateSequenceKey(keyBuf, folder, ef.SequenceNo())
-			t.Delete(keyBuf)
+			keyBuf, err = db.keyer.GenerateSequenceKey(keyBuf, folder, ef.SequenceNo())
+			if err != nil {
+				return err
+			}
+			if err := t.Delete(keyBuf); err != nil {
+				return err
+			}
 			l.Debugf("removing sequence; folder=%q sequence=%v %v", folder, ef.SequenceNo(), ef.FileName())
 		}
 
@@ -98,29 +136,54 @@ func (db *instance) updateLocalFiles(folder []byte, fs []protocol.FileInfo, meta
 		meta.addFile(protocol.LocalDeviceID, f)
 
 		l.Debugf("insert (local); folder=%q %v", folder, f)
-		t.Put(dk, mustMarshal(&f))
+		if err := t.Put(dk, mustMarshal(&f)); err != nil {
+			return err
+		}
 
-		gk = db.keyer.GenerateGlobalVersionKey(gk, folder, []byte(f.Name))
-		keyBuf, _ = t.updateGlobal(gk, keyBuf, folder, protocol.LocalDeviceID[:], f, meta)
+		gk, err = db.keyer.GenerateGlobalVersionKey(gk, folder, []byte(f.Name))
+		if err != nil {
+			return err
+		}
+		keyBuf, _, err = t.updateGlobal(gk, keyBuf, folder, protocol.LocalDeviceID[:], f, meta)
+		if err != nil {
+			return err
+		}
 
-		keyBuf = db.keyer.GenerateSequenceKey(keyBuf, folder, f.Sequence)
-		t.Put(keyBuf, dk)
+		keyBuf, err = db.keyer.GenerateSequenceKey(keyBuf, folder, f.Sequence)
+		if err != nil {
+			return err
+		}
+		if err := t.Put(keyBuf, dk); err != nil {
+			return err
+		}
 		l.Debugf("adding sequence; folder=%q sequence=%v %v", folder, f.Sequence, f.Name)
 
 		if !f.IsDirectory() && !f.IsDeleted() && !f.IsInvalid() {
 			for i, block := range f.Blocks {
 				binary.BigEndian.PutUint32(blockBuf, uint32(i))
-				keyBuf = db.keyer.GenerateBlockMapKey(keyBuf, folder, block.Hash, name)
-				t.Put(keyBuf, blockBuf)
+				keyBuf, err = db.keyer.GenerateBlockMapKey(keyBuf, folder, block.Hash, name)
+				if err != nil {
+					return err
+				}
+				if err := t.Put(keyBuf, blockBuf); err != nil {
+					return err
+				}
 			}
 		}
 
-		t.checkFlush()
+		if err := t.Checkpoint(); err != nil {
+			return err
 		}
 	}
 
-func (db *instance) withHave(folder, device, prefix []byte, truncate bool, fn Iterator) {
-	t := db.newReadOnlyTransaction()
+	return t.commit()
+}
+
+func (db *instance) withHave(folder, device, prefix []byte, truncate bool, fn Iterator) error {
+	t, err := db.newReadOnlyTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
 	if len(prefix) > 0 {
@@ -131,18 +194,31 @@ func (db *instance) withHave(folder, device, prefix []byte, truncate bool, fn It
 			prefix = append(prefix, '/')
 		}
 
-		if f, ok := t.getFileTrunc(db.keyer.GenerateDeviceFileKey(nil, folder, device, unslashedPrefix), true); ok && !fn(f) {
-			return
+		key, err := db.keyer.GenerateDeviceFileKey(nil, folder, device, unslashedPrefix)
+		if err != nil {
+			return err
+		}
+		if f, ok, err := t.getFileTrunc(key, true); err != nil {
+			return err
+		} else if ok && !fn(f) {
+			return nil
 		}
 	}
 
-	dbi := t.NewIterator(util.BytesPrefix(db.keyer.GenerateDeviceFileKey(nil, folder, device, prefix)), nil)
+	key, err := db.keyer.GenerateDeviceFileKey(nil, folder, device, prefix)
+	if err != nil {
+		return err
+	}
+	dbi, err := t.NewPrefixIterator(key)
+	if err != nil {
+		return err
+	}
 	defer dbi.Release()
 
 	for dbi.Next() {
 		name := db.keyer.NameFromDeviceFileKey(dbi.Key())
 		if len(prefix) > 0 && !bytes.HasPrefix(name, prefix) {
-			return
+			return nil
 		}
 
 		f, err := unmarshalTrunc(dbi.Value(), truncate)
@@ -151,20 +227,38 @@ func (db *instance) withHave(folder, device, prefix []byte, truncate bool, fn It
 			continue
 		}
 		if !fn(f) {
-			return
+			return nil
 		}
 	}
+	return dbi.Error()
 }
 
-func (db *instance) withHaveSequence(folder []byte, startSeq int64, fn Iterator) {
-	t := db.newReadOnlyTransaction()
+func (db *instance) withHaveSequence(folder []byte, startSeq int64, fn Iterator) error {
+	t, err := db.newReadOnlyTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
-	dbi := t.NewIterator(&util.Range{Start: db.keyer.GenerateSequenceKey(nil, folder, startSeq), Limit: db.keyer.GenerateSequenceKey(nil, folder, maxInt64)}, nil)
+	first, err := db.keyer.GenerateSequenceKey(nil, folder, startSeq)
+	if err != nil {
+		return err
+	}
+	last, err := db.keyer.GenerateSequenceKey(nil, folder, maxInt64)
+	if err != nil {
+		return err
+	}
+	dbi, err := t.NewRangeIterator(first, last)
+	if err != nil {
+		return err
+	}
 	defer dbi.Release()
 
 	for dbi.Next() {
-		f, ok := t.getFileByKey(dbi.Value())
+		f, ok, err := t.getFileByKey(dbi.Value())
+		if err != nil {
+			return err
+		}
 		if !ok {
 			l.Debugln("missing file for sequence number", db.keyer.SequenceFromSequenceKey(dbi.Key()))
 			continue
@@ -177,16 +271,27 @@ func (db *instance) withHaveSequence(folder []byte, startSeq int64, fn Iterator)
 			}
 		}
 		if !fn(f) {
-			return
+			return nil
 		}
 	}
+	return dbi.Error()
 }
 
-func (db *instance) withAllFolderTruncated(folder []byte, fn func(device []byte, f FileInfoTruncated) bool) {
-	t := db.newReadWriteTransaction()
+func (db *instance) withAllFolderTruncated(folder []byte, fn func(device []byte, f FileInfoTruncated) bool) error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
-	dbi := t.NewIterator(util.BytesPrefix(db.keyer.GenerateDeviceFileKey(nil, folder, nil, nil).WithoutNameAndDevice()), nil)
+	key, err := db.keyer.GenerateDeviceFileKey(nil, folder, nil, nil)
+	if err != nil {
+		return err
+	}
+	dbi, err := t.NewPrefixIterator(key.WithoutNameAndDevice())
+	if err != nil {
+		return err
+	}
 	defer dbi.Release()
 
 	var gk, keyBuf []byte
@@ -194,8 +299,9 @@ func (db *instance) withAllFolderTruncated(folder []byte, fn func(device []byte,
 		device, ok := db.keyer.DeviceFromDeviceFileKey(dbi.Key())
 		if !ok {
 			// Not having the device in the index is bad. Clear it.
-			t.Delete(dbi.Key())
-			t.checkFlush()
+			if err := t.Delete(dbi.Key()); err != nil {
+				return err
+			}
 			continue
 		}
 		var f FileInfoTruncated
@@ -205,42 +311,61 @@ func (db *instance) withAllFolderTruncated(folder []byte, fn func(device []byte,
 		// we need to copy it.
 		err := f.Unmarshal(append([]byte{}, dbi.Value()...))
 		if err != nil {
-			l.Debugln("unmarshal error:", err)
-			continue
+			return err
 		}
 
 		switch f.Name {
 		case "", ".", "..", "/": // A few obviously invalid filenames
 			l.Infof("Dropping invalid filename %q from database", f.Name)
 			name := []byte(f.Name)
-			gk = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
-			keyBuf = t.removeFromGlobal(gk, keyBuf, folder, device, name, nil)
-			t.Delete(dbi.Key())
-			t.checkFlush()
+			gk, err = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
+			if err != nil {
+				return err
+			}
+			keyBuf, err = t.removeFromGlobal(gk, keyBuf, folder, device, name, nil)
+			if err != nil {
+				return err
+			}
+			if err := t.Delete(dbi.Key()); err != nil {
+				return err
+			}
 			continue
 		}
 
 		if !fn(device, f) {
-			return
+			return nil
 		}
 	}
+	if err := dbi.Error(); err != nil {
+		return err
+	}
+	return t.commit()
 }
 
-func (db *instance) getFileDirty(folder, device, file []byte) (protocol.FileInfo, bool) {
-	t := db.newReadOnlyTransaction()
+func (db *instance) getFileDirty(folder, device, file []byte) (protocol.FileInfo, bool, error) {
+	t, err := db.newReadOnlyTransaction()
+	if err != nil {
+		return protocol.FileInfo{}, false, err
+	}
 	defer t.close()
 	return t.getFile(folder, device, file)
 }
 
-func (db *instance) getGlobalDirty(folder, file []byte, truncate bool) (FileIntf, bool) {
-	t := db.newReadOnlyTransaction()
+func (db *instance) getGlobalDirty(folder, file []byte, truncate bool) (FileIntf, bool, error) {
+	t, err := db.newReadOnlyTransaction()
+	if err != nil {
+		return nil, false, err
+	}
 	defer t.close()
-	_, f, ok := t.getGlobal(nil, folder, file, truncate)
-	return f, ok
+	_, f, ok, err := t.getGlobal(nil, folder, file, truncate)
+	return f, ok, err
 }
 
-func (db *instance) withGlobal(folder, prefix []byte, truncate bool, fn Iterator) {
-	t := db.newReadOnlyTransaction()
+func (db *instance) withGlobal(folder, prefix []byte, truncate bool, fn Iterator) error {
+	t, err := db.newReadOnlyTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
 	if len(prefix) > 0 {
@@ -251,19 +376,28 @@ func (db *instance) withGlobal(folder, prefix []byte, truncate bool, fn Iterator
 			prefix = append(prefix, '/')
 		}
 
-		if _, f, ok := t.getGlobal(nil, folder, unslashedPrefix, truncate); ok && !fn(f) {
-			return
+		if _, f, ok, err := t.getGlobal(nil, folder, unslashedPrefix, truncate); err != nil {
+			return err
+		} else if ok && !fn(f) {
+			return nil
 		}
 	}
 
-	dbi := t.NewIterator(util.BytesPrefix(db.keyer.GenerateGlobalVersionKey(nil, folder, prefix)), nil)
+	key, err := db.keyer.GenerateGlobalVersionKey(nil, folder, prefix)
+	if err != nil {
+		return err
+	}
+	dbi, err := t.NewPrefixIterator(key)
+	if err != nil {
+		return err
+	}
 	defer dbi.Release()
 
 	var dk []byte
 	for dbi.Next() {
 		name := db.keyer.NameFromGlobalVersionKey(dbi.Key())
 		if len(prefix) > 0 && !bytes.HasPrefix(name, prefix) {
-			return
+			return nil
 		}
 
 		vl, ok := unmarshalVersionList(dbi.Value())
@@ -271,33 +405,45 @@ func (db *instance) withGlobal(folder, prefix []byte, truncate bool, fn Iterator
 			continue
 		}
 
-		dk = db.keyer.GenerateDeviceFileKey(dk, folder, vl.Versions[0].Device, name)
+		dk, err = db.keyer.GenerateDeviceFileKey(dk, folder, vl.Versions[0].Device, name)
+		if err != nil {
+			return err
+		}
 
-		f, ok := t.getFileTrunc(dk, truncate)
+		f, ok, err := t.getFileTrunc(dk, truncate)
+		if err != nil {
+			return err
+		}
 		if !ok {
 			continue
 		}
 
 		if !fn(f) {
-			return
-		}
-	}
-}
-
-func (db *instance) availability(folder, file []byte) []protocol.DeviceID {
-	k := db.keyer.GenerateGlobalVersionKey(nil, folder, file)
-	bs, err := db.Get(k, nil)
-	if err == leveldb.ErrNotFound {
 			return nil
 		}
+	}
 	if err != nil {
-		l.Debugln("surprise error:", err)
-		return nil
+		return err
+	}
+	return dbi.Error()
+}
+
+func (db *instance) availability(folder, file []byte) ([]protocol.DeviceID, error) {
+	k, err := db.keyer.GenerateGlobalVersionKey(nil, folder, file)
+	if err != nil {
+		return nil, err
+	}
+	bs, err := db.Get(k)
+	if backend.IsNotFound(err) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
 	}
 
 	vl, ok := unmarshalVersionList(bs)
 	if !ok {
-		return nil
+		return nil, nil
 	}
 
 	var devices []protocol.DeviceID
@@ -312,19 +458,28 @@ func (db *instance) availability(folder, file []byte) []protocol.DeviceID {
 		devices = append(devices, n)
 	}
 
-	return devices
+	return devices, nil
 }
 
-func (db *instance) withNeed(folder, device []byte, truncate bool, fn Iterator) {
+func (db *instance) withNeed(folder, device []byte, truncate bool, fn Iterator) error {
 	if bytes.Equal(device, protocol.LocalDeviceID[:]) {
-		db.withNeedLocal(folder, truncate, fn)
-		return
+		return db.withNeedLocal(folder, truncate, fn)
 	}
 
-	t := db.newReadOnlyTransaction()
+	t, err := db.newReadOnlyTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
-	dbi := t.NewIterator(util.BytesPrefix(db.keyer.GenerateGlobalVersionKey(nil, folder, nil).WithoutName()), nil)
+	key, err := db.keyer.GenerateGlobalVersionKey(nil, folder, nil)
+	if err != nil {
+		return err
+	}
+	dbi, err := t.NewPrefixIterator(key.WithoutName())
+	if err != nil {
+		return err
+	}
 	defer dbi.Release()
 
 	var dk []byte
@@ -358,8 +513,14 @@ func (db *instance) withNeed(folder, device []byte, truncate bool, fn Iterator)
 				continue
 			}
 
-			dk = db.keyer.GenerateDeviceFileKey(dk, folder, vl.Versions[i].Device, name)
-			gf, ok := t.getFileTrunc(dk, truncate)
+			dk, err = db.keyer.GenerateDeviceFileKey(dk, folder, vl.Versions[i].Device, name)
+			if err != nil {
+				return err
+			}
+			gf, ok, err := t.getFileTrunc(dk, truncate)
+			if err != nil {
+				return err
+			}
 			if !ok {
 				continue
 			}
@@ -372,81 +533,171 @@ func (db *instance) withNeed(folder, device []byte, truncate bool, fn Iterator)
 			l.Debugf("need folder=%q device=%v name=%q have=%v invalid=%v haveV=%v globalV=%v globalDev=%v", folder, devID, name, have, haveFV.Invalid, haveFV.Version, needVersion, needDevice)
 
 			if !fn(gf) {
-				return
+				return nil
 			}
 
 			// This file is handled, no need to look further in the version list
 			break
 		}
 	}
+	return dbi.Error()
 }
 
-func (db *instance) withNeedLocal(folder []byte, truncate bool, fn Iterator) {
-	t := db.newReadOnlyTransaction()
+func (db *instance) withNeedLocal(folder []byte, truncate bool, fn Iterator) error {
+	t, err := db.newReadOnlyTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
-	dbi := t.NewIterator(util.BytesPrefix(db.keyer.GenerateNeedFileKey(nil, folder, nil).WithoutName()), nil)
+	key, err := db.keyer.GenerateNeedFileKey(nil, folder, nil)
+	if err != nil {
+		return err
+	}
+	dbi, err := t.NewPrefixIterator(key.WithoutName())
+	if err != nil {
+		return err
+	}
 	defer dbi.Release()
 
 	var keyBuf []byte
 	var f FileIntf
 	var ok bool
 	for dbi.Next() {
-		keyBuf, f, ok = t.getGlobal(keyBuf, folder, db.keyer.NameFromGlobalVersionKey(dbi.Key()), truncate)
+		keyBuf, f, ok, err = t.getGlobal(keyBuf, folder, db.keyer.NameFromGlobalVersionKey(dbi.Key()), truncate)
+		if err != nil {
+			return err
+		}
 		if !ok {
 			continue
 		}
 		if !fn(f) {
-			return
+			return nil
 		}
 	}
+	return dbi.Error()
 }
 
-func (db *instance) dropFolder(folder []byte) {
-	t := db.newReadWriteTransaction()
+func (db *instance) dropFolder(folder []byte) error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
-	for _, key := range [][]byte{
 	// Remove all items related to the given folder from the device->file bucket
-		db.keyer.GenerateDeviceFileKey(nil, folder, nil, nil).WithoutNameAndDevice(),
-		// Remove all sequences related to the folder
-		db.keyer.GenerateSequenceKey(nil, []byte(folder), 0).WithoutSequence(),
-		// Remove all items related to the given folder from the global bucket
-		db.keyer.GenerateGlobalVersionKey(nil, folder, nil).WithoutName(),
-		// Remove all needs related to the folder
-		db.keyer.GenerateNeedFileKey(nil, folder, nil).WithoutName(),
-		// Remove the blockmap of the folder
-		db.keyer.GenerateBlockMapKey(nil, folder, nil, nil).WithoutHashAndName(),
-	} {
-		t.deleteKeyPrefix(key)
+	k0, err := db.keyer.GenerateDeviceFileKey(nil, folder, nil, nil)
+	if err != nil {
+		return err
 	}
+	if err := t.deleteKeyPrefix(k0.WithoutNameAndDevice()); err != nil {
+		return err
 	}
 
-func (db *instance) dropDeviceFolder(device, folder []byte, meta *metadataTracker) {
-	t := db.newReadWriteTransaction()
+	// Remove all sequences related to the folder
+	k1, err := db.keyer.GenerateSequenceKey(nil, folder, 0)
+	if err != nil {
+		return err
+	}
+	if err := t.deleteKeyPrefix(k1.WithoutSequence()); err != nil {
+		return err
+	}
+
+	// Remove all items related to the given folder from the global bucket
+	k2, err := db.keyer.GenerateGlobalVersionKey(nil, folder, nil)
+	if err != nil {
+		return err
+	}
+	if err := t.deleteKeyPrefix(k2.WithoutName()); err != nil {
+		return err
+	}
+
+	// Remove all needs related to the folder
+	k3, err := db.keyer.GenerateNeedFileKey(nil, folder, nil)
+	if err != nil {
+		return err
+	}
+	if err := t.deleteKeyPrefix(k3.WithoutName()); err != nil {
+		return err
+	}
+
+	// Remove the blockmap of the folder
+	k4, err := db.keyer.GenerateBlockMapKey(nil, folder, nil, nil)
+	if err != nil {
+		return err
+	}
+	if err := t.deleteKeyPrefix(k4.WithoutHashAndName()); err != nil {
+		return err
+	}
+
+	return t.commit()
+}
+
+func (db *instance) dropDeviceFolder(device, folder []byte, meta *metadataTracker) error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
-	dbi := t.NewIterator(util.BytesPrefix(db.keyer.GenerateDeviceFileKey(nil, folder, device, nil)), nil)
-	defer dbi.Release()
-
+	key, err := db.keyer.GenerateDeviceFileKey(nil, folder, device, nil)
+	if err != nil {
+		return err
+	}
+	dbi, err := t.NewPrefixIterator(key)
+	if err != nil {
+		return err
+	}
 	var gk, keyBuf []byte
 	for dbi.Next() {
 		name := db.keyer.NameFromDeviceFileKey(dbi.Key())
-		gk = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
-		keyBuf = t.removeFromGlobal(gk, keyBuf, folder, device, name, meta)
-		t.Delete(dbi.Key())
-		t.checkFlush()
+		gk, err = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
+		if err != nil {
+			return err
 		}
+		keyBuf, err = t.removeFromGlobal(gk, keyBuf, folder, device, name, meta)
+		if err != nil {
+			return err
+		}
+		if err := t.Delete(dbi.Key()); err != nil {
+			return err
+		}
+		if err := t.Checkpoint(); err != nil {
+			return err
+		}
+	}
+	if err := dbi.Error(); err != nil {
+		return err
+	}
+	dbi.Release()
+
 	if bytes.Equal(device, protocol.LocalDeviceID[:]) {
-		t.deleteKeyPrefix(db.keyer.GenerateBlockMapKey(nil, folder, nil, nil).WithoutHashAndName())
+		key, err := db.keyer.GenerateBlockMapKey(nil, folder, nil, nil)
+		if err != nil {
+			return err
 		}
+		if err := t.deleteKeyPrefix(key.WithoutHashAndName()); err != nil {
+			return err
+		}
+	}
+	return t.commit()
 }
 
-func (db *instance) checkGlobals(folder []byte, meta *metadataTracker) {
-	t := db.newReadWriteTransaction()
+func (db *instance) checkGlobals(folder []byte, meta *metadataTracker) error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
-	dbi := t.NewIterator(util.BytesPrefix(db.keyer.GenerateGlobalVersionKey(nil, folder, nil).WithoutName()), nil)
+	key, err := db.keyer.GenerateGlobalVersionKey(nil, folder, nil)
+	if err != nil {
+		return err
+	}
+	dbi, err := t.NewPrefixIterator(key.WithoutName())
+	if err != nil {
+		return err
+	}
 	defer dbi.Release()
 
 	var dk []byte
@@ -464,66 +715,98 @@ func (db *instance) checkGlobals(folder []byte, meta *metadataTracker) {
 		name := db.keyer.NameFromGlobalVersionKey(dbi.Key())
 		var newVL VersionList
 		for i, version := range vl.Versions {
-			dk = db.keyer.GenerateDeviceFileKey(dk, folder, version.Device, name)
-			_, err := t.Get(dk, nil)
-			if err == leveldb.ErrNotFound {
+			dk, err = db.keyer.GenerateDeviceFileKey(dk, folder, version.Device, name)
+			if err != nil {
+				return err
+			}
+			_, err := t.Get(dk)
+			if backend.IsNotFound(err) {
 				continue
 			}
 			if err != nil {
-				l.Debugln("surprise error:", err)
-				return
+				return err
 			}
 			newVL.Versions = append(newVL.Versions, version)
 
 			if i == 0 {
-				if fi, ok := t.getFileByKey(dk); ok {
+				if fi, ok, err := t.getFileByKey(dk); err != nil {
+					return err
+				} else if ok {
 					meta.addFile(protocol.GlobalDeviceID, fi)
 				}
 			}
 		}
 
 		if len(newVL.Versions) != len(vl.Versions) {
-			t.Put(dbi.Key(), mustMarshal(&newVL))
-			t.checkFlush()
+			if err := t.Put(dbi.Key(), mustMarshal(&newVL)); err != nil {
+				return err
 			}
 		}
-	l.Debugf("db check completed for %q", folder)
+	}
+	if err := dbi.Error(); err != nil {
+		return err
 	}
 
-func (db *instance) getIndexID(device, folder []byte) protocol.IndexID {
-	cur, err := db.Get(db.keyer.GenerateIndexIDKey(nil, device, folder), nil)
+	l.Debugf("db check completed for %q", folder)
+	return t.commit()
+}
+
+func (db *instance) getIndexID(device, folder []byte) (protocol.IndexID, error) {
+	key, err := db.keyer.GenerateIndexIDKey(nil, device, folder)
 	if err != nil {
-		return 0
+		return 0, err
+	}
+	cur, err := db.Get(key)
+	if backend.IsNotFound(err) {
+		return 0, nil
+	} else if err != nil {
+		return 0, err
 	}
 
 	var id protocol.IndexID
 	if err := id.Unmarshal(cur); err != nil {
-		return 0
+		return 0, nil
 	}
 
-	return id
+	return id, nil
 }
 
-func (db *instance) setIndexID(device, folder []byte, id protocol.IndexID) {
+func (db *instance) setIndexID(device, folder []byte, id protocol.IndexID) error {
 	bs, _ := id.Marshal() // marshalling can't fail
-	if err := db.Put(db.keyer.GenerateIndexIDKey(nil, device, folder), bs, nil); err != nil && err != leveldb.ErrClosed {
-		panic("storing index ID: " + err.Error())
+	key, err := db.keyer.GenerateIndexIDKey(nil, device, folder)
+	if err != nil {
+		return err
 	}
+	return db.Put(key, bs)
 }
 
-func (db *instance) dropMtimes(folder []byte) {
-	db.dropPrefix(db.keyer.GenerateMtimesKey(nil, folder))
+func (db *instance) dropMtimes(folder []byte) error {
+	key, err := db.keyer.GenerateMtimesKey(nil, folder)
+	if err != nil {
+		return err
+	}
+	return db.dropPrefix(key)
 }
 
-func (db *instance) dropFolderMeta(folder []byte) {
-	db.dropPrefix(db.keyer.GenerateFolderMetaKey(nil, folder))
+func (db *instance) dropFolderMeta(folder []byte) error {
+	key, err := db.keyer.GenerateFolderMetaKey(nil, folder)
+	if err != nil {
+		return err
+	}
+	return db.dropPrefix(key)
 }
 
-func (db *instance) dropPrefix(prefix []byte) {
-	t := db.newReadWriteTransaction()
+func (db *instance) dropPrefix(prefix []byte) error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
-	t.deleteKeyPrefix(prefix)
+	if err := t.deleteKeyPrefix(prefix); err != nil {
+		return err
+	}
+	return t.commit()
 }
 
 func unmarshalTrunc(bs []byte, truncate bool) (FileIntf, error) {
@@ -551,15 +834,6 @@ func unmarshalVersionList(data []byte) (VersionList, bool) {
 	return vl, true
 }
 
-type errorSuggestion struct {
-	inner      error
-	suggestion string
-}
-
-func (e errorSuggestion) Error() string {
-	return fmt.Sprintf("%s (%s)", e.inner.Error(), e.suggestion)
-}
-
 // unchanged checks if two files are the same and thus don't need to be updated.
 // Local flags or the invalid bit might change without the version
 // being bumped.

lib/db/keyer.go

@@ -63,36 +63,36 @@ const (
 
 type keyer interface {
 	// device file key stuff
-	GenerateDeviceFileKey(key, folder, device, name []byte) deviceFileKey
+	GenerateDeviceFileKey(key, folder, device, name []byte) (deviceFileKey, error)
 	NameFromDeviceFileKey(key []byte) []byte
 	DeviceFromDeviceFileKey(key []byte) ([]byte, bool)
 	FolderFromDeviceFileKey(key []byte) ([]byte, bool)
 
 	// global version key stuff
-	GenerateGlobalVersionKey(key, folder, name []byte) globalVersionKey
+	GenerateGlobalVersionKey(key, folder, name []byte) (globalVersionKey, error)
 	NameFromGlobalVersionKey(key []byte) []byte
 	FolderFromGlobalVersionKey(key []byte) ([]byte, bool)
 
 	// block map key stuff (former BlockMap)
-	GenerateBlockMapKey(key, folder, hash, name []byte) blockMapKey
+	GenerateBlockMapKey(key, folder, hash, name []byte) (blockMapKey, error)
 	NameFromBlockMapKey(key []byte) []byte
 
 	// file need index
-	GenerateNeedFileKey(key, folder, name []byte) needFileKey
+	GenerateNeedFileKey(key, folder, name []byte) (needFileKey, error)
 
 	// file sequence index
-	GenerateSequenceKey(key, folder []byte, seq int64) sequenceKey
+	GenerateSequenceKey(key, folder []byte, seq int64) (sequenceKey, error)
 	SequenceFromSequenceKey(key []byte) int64
 
 	// index IDs
-	GenerateIndexIDKey(key, device, folder []byte) indexIDKey
+	GenerateIndexIDKey(key, device, folder []byte) (indexIDKey, error)
 	DeviceFromIndexIDKey(key []byte) ([]byte, bool)
 
 	// Mtimes
-	GenerateMtimesKey(key, folder []byte) mtimesKey
+	GenerateMtimesKey(key, folder []byte) (mtimesKey, error)
 
 	// Folder metadata
-	GenerateFolderMetaKey(key, folder []byte) folderMetaKey
+	GenerateFolderMetaKey(key, folder []byte) (folderMetaKey, error)
 }
 
 // defaultKeyer implements our key scheme. It needs folder and device
@@ -115,13 +115,21 @@ func (k deviceFileKey) WithoutNameAndDevice() []byte {
 	return k[:keyPrefixLen+keyFolderLen]
 }
 
-func (k defaultKeyer) GenerateDeviceFileKey(key, folder, device, name []byte) deviceFileKey {
+func (k defaultKeyer) GenerateDeviceFileKey(key, folder, device, name []byte) (deviceFileKey, error) {
+	folderID, err := k.folderIdx.ID(folder)
+	if err != nil {
+		return nil, err
+	}
+	deviceID, err := k.deviceIdx.ID(device)
+	if err != nil {
+		return nil, err
+	}
 	key = resize(key, keyPrefixLen+keyFolderLen+keyDeviceLen+len(name))
 	key[0] = KeyTypeDevice
-	binary.BigEndian.PutUint32(key[keyPrefixLen:], k.folderIdx.ID(folder))
-	binary.BigEndian.PutUint32(key[keyPrefixLen+keyFolderLen:], k.deviceIdx.ID(device))
+	binary.BigEndian.PutUint32(key[keyPrefixLen:], folderID)
+	binary.BigEndian.PutUint32(key[keyPrefixLen+keyFolderLen:], deviceID)
 	copy(key[keyPrefixLen+keyFolderLen+keyDeviceLen:], name)
-	return key
+	return key, nil
 }
 
 func (k defaultKeyer) NameFromDeviceFileKey(key []byte) []byte {
@@ -142,12 +150,16 @@ func (k globalVersionKey) WithoutName() []byte {
 	return k[:keyPrefixLen+keyFolderLen]
 }
 
-func (k defaultKeyer) GenerateGlobalVersionKey(key, folder, name []byte) globalVersionKey {
+func (k defaultKeyer) GenerateGlobalVersionKey(key, folder, name []byte) (globalVersionKey, error) {
+	folderID, err := k.folderIdx.ID(folder)
+	if err != nil {
+		return nil, err
+	}
 	key = resize(key, keyPrefixLen+keyFolderLen+len(name))
 	key[0] = KeyTypeGlobal
-	binary.BigEndian.PutUint32(key[keyPrefixLen:], k.folderIdx.ID(folder))
+	binary.BigEndian.PutUint32(key[keyPrefixLen:], folderID)
 	copy(key[keyPrefixLen+keyFolderLen:], name)
-	return key
+	return key, nil
 }
 
 func (k defaultKeyer) NameFromGlobalVersionKey(key []byte) []byte {
@@ -160,13 +172,17 @@ func (k defaultKeyer) FolderFromGlobalVersionKey(key []byte) ([]byte, bool) {
 
 type blockMapKey []byte
 
-func (k defaultKeyer) GenerateBlockMapKey(key, folder, hash, name []byte) blockMapKey {
+func (k defaultKeyer) GenerateBlockMapKey(key, folder, hash, name []byte) (blockMapKey, error) {
+	folderID, err := k.folderIdx.ID(folder)
+	if err != nil {
+		return nil, err
+	}
 	key = resize(key, keyPrefixLen+keyFolderLen+keyHashLen+len(name))
 	key[0] = KeyTypeBlock
-	binary.BigEndian.PutUint32(key[keyPrefixLen:], k.folderIdx.ID(folder))
+	binary.BigEndian.PutUint32(key[keyPrefixLen:], folderID)
 	copy(key[keyPrefixLen+keyFolderLen:], hash)
 	copy(key[keyPrefixLen+keyFolderLen+keyHashLen:], name)
-	return key
+	return key, nil
 }
 
 func (k defaultKeyer) NameFromBlockMapKey(key []byte) []byte {
@@ -183,12 +199,16 @@ func (k needFileKey) WithoutName() []byte {
 	return k[:keyPrefixLen+keyFolderLen]
 }
 
-func (k defaultKeyer) GenerateNeedFileKey(key, folder, name []byte) needFileKey {
+func (k defaultKeyer) GenerateNeedFileKey(key, folder, name []byte) (needFileKey, error) {
+	folderID, err := k.folderIdx.ID(folder)
+	if err != nil {
+		return nil, err
+	}
 	key = resize(key, keyPrefixLen+keyFolderLen+len(name))
 	key[0] = KeyTypeNeed
-	binary.BigEndian.PutUint32(key[keyPrefixLen:], k.folderIdx.ID(folder))
+	binary.BigEndian.PutUint32(key[keyPrefixLen:], folderID)
 	copy(key[keyPrefixLen+keyFolderLen:], name)
-	return key
+	return key, nil
 }
 
 type sequenceKey []byte
@@ -197,12 +217,16 @@ func (k sequenceKey) WithoutSequence() []byte {
 	return k[:keyPrefixLen+keyFolderLen]
 }
 
-func (k defaultKeyer) GenerateSequenceKey(key, folder []byte, seq int64) sequenceKey {
+func (k defaultKeyer) GenerateSequenceKey(key, folder []byte, seq int64) (sequenceKey, error) {
+	folderID, err := k.folderIdx.ID(folder)
+	if err != nil {
+		return nil, err
+	}
 	key = resize(key, keyPrefixLen+keyFolderLen+keySequenceLen)
 	key[0] = KeyTypeSequence
-	binary.BigEndian.PutUint32(key[keyPrefixLen:], k.folderIdx.ID(folder))
+	binary.BigEndian.PutUint32(key[keyPrefixLen:], folderID)
 	binary.BigEndian.PutUint64(key[keyPrefixLen+keyFolderLen:], uint64(seq))
-	return key
+	return key, nil
 }
 
 func (k defaultKeyer) SequenceFromSequenceKey(key []byte) int64 {
@@ -211,12 +235,20 @@ func (k defaultKeyer) SequenceFromSequenceKey(key []byte) int64 {
 
 type indexIDKey []byte
 
-func (k defaultKeyer) GenerateIndexIDKey(key, device, folder []byte) indexIDKey {
+func (k defaultKeyer) GenerateIndexIDKey(key, device, folder []byte) (indexIDKey, error) {
+	deviceID, err := k.deviceIdx.ID(device)
+	if err != nil {
+		return nil, err
+	}
+	folderID, err := k.folderIdx.ID(folder)
+	if err != nil {
+		return nil, err
+	}
 	key = resize(key, keyPrefixLen+keyDeviceLen+keyFolderLen)
 	key[0] = KeyTypeIndexID
-	binary.BigEndian.PutUint32(key[keyPrefixLen:], k.deviceIdx.ID(device))
-	binary.BigEndian.PutUint32(key[keyPrefixLen+keyDeviceLen:], k.folderIdx.ID(folder))
-	return key
+	binary.BigEndian.PutUint32(key[keyPrefixLen:], deviceID)
+	binary.BigEndian.PutUint32(key[keyPrefixLen+keyDeviceLen:], folderID)
+	return key, nil
 }
 
 func (k defaultKeyer) DeviceFromIndexIDKey(key []byte) ([]byte, bool) {
@@ -225,20 +257,28 @@ func (k defaultKeyer) DeviceFromIndexIDKey(key []byte) ([]byte, bool) {
 
 type mtimesKey []byte
 
-func (k defaultKeyer) GenerateMtimesKey(key, folder []byte) mtimesKey {
+func (k defaultKeyer) GenerateMtimesKey(key, folder []byte) (mtimesKey, error) {
+	folderID, err := k.folderIdx.ID(folder)
+	if err != nil {
+		return nil, err
+	}
 	key = resize(key, keyPrefixLen+keyFolderLen)
 	key[0] = KeyTypeVirtualMtime
-	binary.BigEndian.PutUint32(key[keyPrefixLen:], k.folderIdx.ID(folder))
-	return key
+	binary.BigEndian.PutUint32(key[keyPrefixLen:], folderID)
+	return key, nil
 }
 
 type folderMetaKey []byte
 
-func (k defaultKeyer) GenerateFolderMetaKey(key, folder []byte) folderMetaKey {
+func (k defaultKeyer) GenerateFolderMetaKey(key, folder []byte) (folderMetaKey, error) {
+	folderID, err := k.folderIdx.ID(folder)
+	if err != nil {
+		return nil, err
+	}
 	key = resize(key, keyPrefixLen+keyFolderLen)
 	key[0] = KeyTypeFolderMeta
-	binary.BigEndian.PutUint32(key[keyPrefixLen:], k.folderIdx.ID(folder))
-	return key
+	binary.BigEndian.PutUint32(key[keyPrefixLen:], folderID)
+	return key, nil
 }
 
 // resize returns a byte slice of the specified size, reusing bs if possible

lib/db/keyer_test.go

@@ -9,6 +9,8 @@ package db
 import (
 	"bytes"
 	"testing"
+
+	"github.com/syncthing/syncthing/lib/db/backend"
 )
 
 func TestDeviceKey(t *testing.T) {
@@ -16,9 +18,12 @@ func TestDeviceKey(t *testing.T) {
 	dev := []byte("device67890123456789012345678901")
 	name := []byte("name")
 
-	db := newInstance(OpenMemory())
+	db := newInstance(NewLowlevel(backend.OpenMemory()))
 
-	key := db.keyer.GenerateDeviceFileKey(nil, fld, dev, name)
+	key, err := db.keyer.GenerateDeviceFileKey(nil, fld, dev, name)
+	if err != nil {
+		t.Fatal(err)
+	}
 
 	fld2, ok := db.keyer.FolderFromDeviceFileKey(key)
 	if !ok {
@@ -44,9 +49,12 @@ func TestGlobalKey(t *testing.T) {
 	fld := []byte("folder6789012345678901234567890123456789012345678901234567890123")
 	name := []byte("name")
 
-	db := newInstance(OpenMemory())
+	db := newInstance(NewLowlevel(backend.OpenMemory()))
 
-	key := db.keyer.GenerateGlobalVersionKey(nil, fld, name)
+	key, err := db.keyer.GenerateGlobalVersionKey(nil, fld, name)
+	if err != nil {
+		t.Fatal(err)
+	}
 
 	fld2, ok := db.keyer.FolderFromGlobalVersionKey(key)
 	if !ok {
@@ -69,10 +77,13 @@ func TestGlobalKey(t *testing.T) {
 func TestSequenceKey(t *testing.T) {
 	fld := []byte("folder6789012345678901234567890123456789012345678901234567890123")
 
-	db := newInstance(OpenMemory())
+	db := newInstance(NewLowlevel(backend.OpenMemory()))
 
 	const seq = 1234567890
-	key := db.keyer.GenerateSequenceKey(nil, fld, seq)
+	key, err := db.keyer.GenerateSequenceKey(nil, fld, seq)
+	if err != nil {
+		t.Fatal(err)
+	}
 	outSeq := db.keyer.SequenceFromSequenceKey(key)
 	if outSeq != seq {
 		t.Errorf("sequence number mangled, %d != %d", outSeq, seq)

lib/db/lowlevel.go

@@ -7,431 +7,30 @@
 package db
 
 import (
-	"os"
-	"strconv"
-	"strings"
-	"sync"
-	"sync/atomic"
-
-	"github.com/syndtr/goleveldb/leveldb"
-	"github.com/syndtr/goleveldb/leveldb/errors"
-	"github.com/syndtr/goleveldb/leveldb/iterator"
-	"github.com/syndtr/goleveldb/leveldb/opt"
-	"github.com/syndtr/goleveldb/leveldb/storage"
-	"github.com/syndtr/goleveldb/leveldb/util"
-)
-
-const (
-	dbMaxOpenFiles = 100
-	dbFlushBatch   = 4 << MiB
-
-	// A large database is > 200 MiB. It's a mostly arbitrary value, but
-	// it's also the case that each file is 2 MiB by default and when we
-	// have dbMaxOpenFiles of them we will need to start thrashing fd:s.
-	// Switching to large database settings causes larger files to be used
-	// when compacting, reducing the number.
-	dbLargeThreshold = dbMaxOpenFiles * (2 << MiB)
-
-	KiB = 10
-	MiB = 20
-)
-
-type Tuning int
-
-const (
-	// N.b. these constants must match those in lib/config.Tuning!
-	TuningAuto Tuning = iota
-	TuningSmall
-	TuningLarge
+	"github.com/syncthing/syncthing/lib/db/backend"
 )
 
 // Lowlevel is the lowest level database interface. It has a very simple
-// purpose: hold the actual *leveldb.DB database, and the in-memory state
+// purpose: hold the actual backend database, and the in-memory state
 // that belong to that database. In the same way that a single on disk
 // database can only be opened once, there should be only one Lowlevel for
-// any given *leveldb.DB.
+// any given backend.
 type Lowlevel struct {
-	committed int64 // atomic, must come first
-	*leveldb.DB
-	location  string
+	backend.Backend
 	folderIdx *smallIndex
 	deviceIdx *smallIndex
-	closed    bool
-	closeMut  *sync.RWMutex
-	iterWG    sync.WaitGroup
 }
 
-// Open attempts to open the database at the given location, and runs
-// recovery on it if opening fails. Worst case, if recovery is not possible,
-// the database is erased and created from scratch.
-func Open(location string, tuning Tuning) (*Lowlevel, error) {
-	opts := optsFor(location, tuning)
-	return open(location, opts)
+// NewLowlevel wraps the given *leveldb.DB into a *lowlevel
+func NewLowlevel(db backend.Backend) *Lowlevel {
+	return &Lowlevel{
+		Backend:   db,
+		folderIdx: newSmallIndex(db, []byte{KeyTypeFolderIdx}),
+		deviceIdx: newSmallIndex(db, []byte{KeyTypeDeviceIdx}),
 	}
-
-// optsFor returns the database options to use when opening a database with
-// the given location and tuning. Settings can be overridden by debug
-// environment variables.
-func optsFor(location string, tuning Tuning) *opt.Options {
-	large := false
-	switch tuning {
-	case TuningLarge:
-		large = true
-	case TuningAuto:
-		large = dbIsLarge(location)
-	}
-
-	var (
-		// Set defaults used for small databases.
-		defaultBlockCacheCapacity            = 0 // 0 means let leveldb use default
-		defaultBlockSize                     = 0
-		defaultCompactionTableSize           = 0
-		defaultCompactionTableSizeMultiplier = 0
-		defaultWriteBuffer                   = 16 << MiB                      // increased from leveldb default of 4 MiB
-		defaultCompactionL0Trigger           = opt.DefaultCompactionL0Trigger // explicit because we use it as base for other stuff
-	)
-
-	if large {
-		// Change the parameters for better throughput at the price of some
-		// RAM and larger files. This results in larger batches of writes
-		// and compaction at a lower frequency.
-		l.Infoln("Using large-database tuning")
-
-		defaultBlockCacheCapacity = 64 << MiB
-		defaultBlockSize = 64 << KiB
-		defaultCompactionTableSize = 16 << MiB
-		defaultCompactionTableSizeMultiplier = 20 // 2.0 after division by ten
-		defaultWriteBuffer = 64 << MiB
-		defaultCompactionL0Trigger = 8 // number of l0 files
-	}
-
-	opts := &opt.Options{
-		BlockCacheCapacity:            debugEnvValue("BlockCacheCapacity", defaultBlockCacheCapacity),
-		BlockCacheEvictRemoved:        debugEnvValue("BlockCacheEvictRemoved", 0) != 0,
-		BlockRestartInterval:          debugEnvValue("BlockRestartInterval", 0),
-		BlockSize:                     debugEnvValue("BlockSize", defaultBlockSize),
-		CompactionExpandLimitFactor:   debugEnvValue("CompactionExpandLimitFactor", 0),
-		CompactionGPOverlapsFactor:    debugEnvValue("CompactionGPOverlapsFactor", 0),
-		CompactionL0Trigger:           debugEnvValue("CompactionL0Trigger", defaultCompactionL0Trigger),
-		CompactionSourceLimitFactor:   debugEnvValue("CompactionSourceLimitFactor", 0),
-		CompactionTableSize:           debugEnvValue("CompactionTableSize", defaultCompactionTableSize),
-		CompactionTableSizeMultiplier: float64(debugEnvValue("CompactionTableSizeMultiplier", defaultCompactionTableSizeMultiplier)) / 10.0,
-		CompactionTotalSize:           debugEnvValue("CompactionTotalSize", 0),
-		CompactionTotalSizeMultiplier: float64(debugEnvValue("CompactionTotalSizeMultiplier", 0)) / 10.0,
-		DisableBufferPool:             debugEnvValue("DisableBufferPool", 0) != 0,
-		DisableBlockCache:             debugEnvValue("DisableBlockCache", 0) != 0,
-		DisableCompactionBackoff:      debugEnvValue("DisableCompactionBackoff", 0) != 0,
-		DisableLargeBatchTransaction:  debugEnvValue("DisableLargeBatchTransaction", 0) != 0,
-		NoSync:                        debugEnvValue("NoSync", 0) != 0,
-		NoWriteMerge:                  debugEnvValue("NoWriteMerge", 0) != 0,
-		OpenFilesCacheCapacity:        debugEnvValue("OpenFilesCacheCapacity", dbMaxOpenFiles),
-		WriteBuffer:                   debugEnvValue("WriteBuffer", defaultWriteBuffer),
-		// The write slowdown and pause can be overridden, but even if they
-		// are not and the compaction trigger is overridden we need to
-		// adjust so that we don't pause writes for L0 compaction before we
-		// even *start* L0 compaction...
-		WriteL0SlowdownTrigger: debugEnvValue("WriteL0SlowdownTrigger", 2*debugEnvValue("CompactionL0Trigger", defaultCompactionL0Trigger)),
-		WriteL0PauseTrigger:    debugEnvValue("WriteL0SlowdownTrigger", 3*debugEnvValue("CompactionL0Trigger", defaultCompactionL0Trigger)),
-	}
-
-	return opts
-}
-
-// OpenRO attempts to open the database at the given location, read only.
-func OpenRO(location string) (*Lowlevel, error) {
-	opts := &opt.Options{
-		OpenFilesCacheCapacity: dbMaxOpenFiles,
-		ReadOnly:               true,
-	}
-	return open(location, opts)
-}
-
-func open(location string, opts *opt.Options) (*Lowlevel, error) {
-	db, err := leveldb.OpenFile(location, opts)
-	if leveldbIsCorrupted(err) {
-		db, err = leveldb.RecoverFile(location, opts)
-	}
-	if leveldbIsCorrupted(err) {
-		// The database is corrupted, and we've tried to recover it but it
-		// didn't work. At this point there isn't much to do beyond dropping
-		// the database and reindexing...
-		l.Infoln("Database corruption detected, unable to recover. Reinitializing...")
-		if err := os.RemoveAll(location); err != nil {
-			return nil, errorSuggestion{err, "failed to delete corrupted database"}
-		}
-		db, err = leveldb.OpenFile(location, opts)
-	}
-	if err != nil {
-		return nil, errorSuggestion{err, "is another instance of Syncthing running?"}
-	}
-
-	if debugEnvValue("CompactEverything", 0) != 0 {
-		if err := db.CompactRange(util.Range{}); err != nil {
-			l.Warnln("Compacting database:", err)
-		}
-	}
-
-	return NewLowlevel(db, location), nil
-}
-
-// OpenMemory returns a new Lowlevel referencing an in-memory database.
-func OpenMemory() *Lowlevel {
-	db, _ := leveldb.Open(storage.NewMemStorage(), nil)
-	return NewLowlevel(db, "<memory>")
 }
 
 // ListFolders returns the list of folders currently in the database
 func (db *Lowlevel) ListFolders() []string {
 	return db.folderIdx.Values()
 }
-
-// Committed returns the number of items committed to the database since startup
-func (db *Lowlevel) Committed() int64 {
-	return atomic.LoadInt64(&db.committed)
-}
-
-func (db *Lowlevel) Put(key, val []byte, wo *opt.WriteOptions) error {
-	db.closeMut.RLock()
-	defer db.closeMut.RUnlock()
-	if db.closed {
-		return leveldb.ErrClosed
-	}
-	atomic.AddInt64(&db.committed, 1)
-	return db.DB.Put(key, val, wo)
-}
-
-func (db *Lowlevel) Write(batch *leveldb.Batch, wo *opt.WriteOptions) error {
-	db.closeMut.RLock()
-	defer db.closeMut.RUnlock()
-	if db.closed {
-		return leveldb.ErrClosed
-	}
-	return db.DB.Write(batch, wo)
-}
-
-func (db *Lowlevel) Delete(key []byte, wo *opt.WriteOptions) error {
-	db.closeMut.RLock()
-	defer db.closeMut.RUnlock()
-	if db.closed {
-		return leveldb.ErrClosed
-	}
-	atomic.AddInt64(&db.committed, 1)
-	return db.DB.Delete(key, wo)
-}
-
-func (db *Lowlevel) NewIterator(slice *util.Range, ro *opt.ReadOptions) iterator.Iterator {
-	return db.newIterator(func() iterator.Iterator { return db.DB.NewIterator(slice, ro) })
-}
-
-// newIterator returns an iterator created with the given constructor only if db
-// is not yet closed. If it is closed, a closedIter is returned instead.
-func (db *Lowlevel) newIterator(constr func() iterator.Iterator) iterator.Iterator {
-	db.closeMut.RLock()
-	defer db.closeMut.RUnlock()
-	if db.closed {
-		return &closedIter{}
-	}
-	db.iterWG.Add(1)
-	return &iter{
-		Iterator: constr(),
-		db:       db,
-	}
-}
-
-func (db *Lowlevel) GetSnapshot() snapshot {
-	s, err := db.DB.GetSnapshot()
-	if err != nil {
-		if err == leveldb.ErrClosed {
-			return &closedSnap{}
-		}
-		panic(err)
-	}
-	return &snap{
-		Snapshot: s,
-		db:       db,
-	}
-}
-
-func (db *Lowlevel) Close() {
-	db.closeMut.Lock()
-	if db.closed {
-		db.closeMut.Unlock()
-		return
-	}
-	db.closed = true
-	db.closeMut.Unlock()
-	db.iterWG.Wait()
-	db.DB.Close()
-}
-
-// dbIsLarge returns whether the estimated size of the database at location
-// is large enough to warrant optimization for large databases.
-func dbIsLarge(location string) bool {
-	if ^uint(0)>>63 == 0 {
-		// We're compiled for a 32 bit architecture. We've seen trouble with
-		// large settings there.
-		// (https://forum.syncthing.net/t/many-small-ldb-files-with-database-tuning/13842)
-		return false
-	}
-
-	dir, err := os.Open(location)
-	if err != nil {
-		return false
-	}
-
-	fis, err := dir.Readdir(-1)
-	if err != nil {
-		return false
-	}
-
-	var size int64
-	for _, fi := range fis {
-		if fi.Name() == "LOG" {
-			// don't count the size
-			continue
-		}
-		size += fi.Size()
-	}
-
-	return size > dbLargeThreshold
-}
-
-// NewLowlevel wraps the given *leveldb.DB into a *lowlevel
-func NewLowlevel(db *leveldb.DB, location string) *Lowlevel {
-	return &Lowlevel{
-		DB:        db,
-		location:  location,
-		folderIdx: newSmallIndex(db, []byte{KeyTypeFolderIdx}),
-		deviceIdx: newSmallIndex(db, []byte{KeyTypeDeviceIdx}),
-		closeMut:  &sync.RWMutex{},
-		iterWG:    sync.WaitGroup{},
-	}
-}
-
-// A "better" version of leveldb's errors.IsCorrupted.
-func leveldbIsCorrupted(err error) bool {
-	switch {
-	case err == nil:
-		return false
-
-	case errors.IsCorrupted(err):
-		return true
-
-	case strings.Contains(err.Error(), "corrupted"):
-		return true
-	}
-
-	return false
-}
-
-type batch struct {
-	*leveldb.Batch
-	db *Lowlevel
-}
-
-func (db *Lowlevel) newBatch() *batch {
-	return &batch{
-		Batch: new(leveldb.Batch),
-		db:    db,
-	}
-}
-
-// checkFlush flushes and resets the batch if its size exceeds dbFlushBatch.
-func (b *batch) checkFlush() {
-	if len(b.Dump()) > dbFlushBatch {
-		b.flush()
-		b.Reset()
-	}
-}
-
-func (b *batch) flush() {
-	if err := b.db.Write(b.Batch, nil); err != nil && err != leveldb.ErrClosed {
-		panic(err)
-	}
-}
-
-type closedIter struct{}
-
-func (it *closedIter) Release()                           {}
-func (it *closedIter) Key() []byte                        { return nil }
-func (it *closedIter) Value() []byte                      { return nil }
-func (it *closedIter) Next() bool                         { return false }
-func (it *closedIter) Prev() bool                         { return false }
-func (it *closedIter) First() bool                        { return false }
-func (it *closedIter) Last() bool                         { return false }
-func (it *closedIter) Seek(key []byte) bool               { return false }
-func (it *closedIter) Valid() bool                        { return false }
-func (it *closedIter) Error() error                       { return leveldb.ErrClosed }
-func (it *closedIter) SetReleaser(releaser util.Releaser) {}
-
-type snapshot interface {
-	Get([]byte, *opt.ReadOptions) ([]byte, error)
-	Has([]byte, *opt.ReadOptions) (bool, error)
-	NewIterator(*util.Range, *opt.ReadOptions) iterator.Iterator
-	Release()
-}
-
-type closedSnap struct{}
-
-func (s *closedSnap) Get([]byte, *opt.ReadOptions) ([]byte, error) { return nil, leveldb.ErrClosed }
-func (s *closedSnap) Has([]byte, *opt.ReadOptions) (bool, error)   { return false, leveldb.ErrClosed }
-func (s *closedSnap) NewIterator(*util.Range, *opt.ReadOptions) iterator.Iterator {
-	return &closedIter{}
-}
-func (s *closedSnap) Release() {}
-
-type snap struct {
-	*leveldb.Snapshot
-	db *Lowlevel
-}
-
-func (s *snap) NewIterator(slice *util.Range, ro *opt.ReadOptions) iterator.Iterator {
-	return s.db.newIterator(func() iterator.Iterator { return s.Snapshot.NewIterator(slice, ro) })
-}
-
-// iter implements iterator.Iterator which allows tracking active iterators
-// and aborts if the underlying database is being closed.
-type iter struct {
-	iterator.Iterator
-	db *Lowlevel
-}
-
-func (it *iter) Release() {
-	it.db.iterWG.Done()
-	it.Iterator.Release()
-}
-
-func (it *iter) Next() bool {
-	return it.execIfNotClosed(it.Iterator.Next)
-}
-func (it *iter) Prev() bool {
-	return it.execIfNotClosed(it.Iterator.Prev)
-}
-func (it *iter) First() bool {
-	return it.execIfNotClosed(it.Iterator.First)
-}
-func (it *iter) Last() bool {
-	return it.execIfNotClosed(it.Iterator.Last)
-}
-func (it *iter) Seek(key []byte) bool {
-	return it.execIfNotClosed(func() bool {
-		return it.Iterator.Seek(key)
-	})
-}
-
-func (it *iter) execIfNotClosed(fn func() bool) bool {
-	it.db.closeMut.RLock()
-	defer it.db.closeMut.RUnlock()
-	if it.db.closed {
-		return false
-	}
-	return fn()
-}
-
-func debugEnvValue(key string, def int) int {
-	v, err := strconv.ParseInt(os.Getenv("STDEBUG_"+key), 10, 63)
-	if err != nil {
-		return def
-	}
-	return int(v)
-}

lib/db/meta.go

@@ -57,7 +57,10 @@ func (m *metadataTracker) Marshal() ([]byte, error) {
 // toDB saves the marshalled metadataTracker to the given db, under the key
 // corresponding to the given folder
 func (m *metadataTracker) toDB(db *instance, folder []byte) error {
-	key := db.keyer.GenerateFolderMetaKey(nil, folder)
+	key, err := db.keyer.GenerateFolderMetaKey(nil, folder)
+	if err != nil {
+		return err
+	}
 
 	m.mut.RLock()
 	defer m.mut.RUnlock()
@@ -70,7 +73,7 @@ func (m *metadataTracker) toDB(db *instance, folder []byte) error {
 	if err != nil {
 		return err
 	}
-	err = db.Put(key, bs, nil)
+	err = db.Put(key, bs)
 	if err == nil {
 		m.dirty = false
 	}
@@ -81,8 +84,11 @@ func (m *metadataTracker) toDB(db *instance, folder []byte) error {
 // fromDB initializes the metadataTracker from the marshalled data found in
 // the database under the key corresponding to the given folder
 func (m *metadataTracker) fromDB(db *instance, folder []byte) error {
-	key := db.keyer.GenerateFolderMetaKey(nil, folder)
-	bs, err := db.Get(key, nil)
+	key, err := db.keyer.GenerateFolderMetaKey(nil, folder)
+	if err != nil {
+		return err
+	}
+	bs, err := db.Get(key)
 	if err != nil {
 		return err
 	}

lib/db/namespaced.go

@@ -9,8 +9,6 @@ package db
 import (
 	"encoding/binary"
 	"time"
-
-	"github.com/syndtr/goleveldb/leveldb/util"
 )
 
 // NamespacedKV is a simple key-value store using a specific namespace within
@@ -34,30 +32,18 @@ func NewNamespacedKV(db *Lowlevel, prefix string) *NamespacedKV {
 	}
 }
 
-// Reset removes all entries in this namespace.
-func (n *NamespacedKV) Reset() {
-	it := n.db.NewIterator(util.BytesPrefix(n.prefix), nil)
-	defer it.Release()
-	batch := n.db.newBatch()
-	for it.Next() {
-		batch.Delete(it.Key())
-		batch.checkFlush()
-	}
-	batch.flush()
-}
-
 // PutInt64 stores a new int64. Any existing value (even if of another type)
 // is overwritten.
-func (n *NamespacedKV) PutInt64(key string, val int64) {
+func (n *NamespacedKV) PutInt64(key string, val int64) error {
 	var valBs [8]byte
 	binary.BigEndian.PutUint64(valBs[:], uint64(val))
-	n.db.Put(n.prefixedKey(key), valBs[:], nil)
+	return n.db.Put(n.prefixedKey(key), valBs[:])
 }
 
 // Int64 returns the stored value interpreted as an int64 and a boolean that
 // is false if no value was stored at the key.
 func (n *NamespacedKV) Int64(key string) (int64, bool) {
-	valBs, err := n.db.Get(n.prefixedKey(key), nil)
+	valBs, err := n.db.Get(n.prefixedKey(key))
 	if err != nil {
 		return 0, false
 	}
@@ -67,16 +53,16 @@ func (n *NamespacedKV) Int64(key string) (int64, bool) {
 
 // PutTime stores a new time.Time. Any existing value (even if of another
 // type) is overwritten.
-func (n *NamespacedKV) PutTime(key string, val time.Time) {
+func (n *NamespacedKV) PutTime(key string, val time.Time) error {
 	valBs, _ := val.MarshalBinary() // never returns an error
-	n.db.Put(n.prefixedKey(key), valBs, nil)
+	return n.db.Put(n.prefixedKey(key), valBs)
 }
 
 // Time returns the stored value interpreted as a time.Time and a boolean
 // that is false if no value was stored at the key.
 func (n NamespacedKV) Time(key string) (time.Time, bool) {
 	var t time.Time
-	valBs, err := n.db.Get(n.prefixedKey(key), nil)
+	valBs, err := n.db.Get(n.prefixedKey(key))
 	if err != nil {
 		return t, false
 	}
@@ -86,14 +72,14 @@ func (n NamespacedKV) Time(key string) (time.Time, bool) {
 
 // PutString stores a new string. Any existing value (even if of another type)
 // is overwritten.
-func (n *NamespacedKV) PutString(key, val string) {
-	n.db.Put(n.prefixedKey(key), []byte(val), nil)
+func (n *NamespacedKV) PutString(key, val string) error {
+	return n.db.Put(n.prefixedKey(key), []byte(val))
 }
 
 // String returns the stored value interpreted as a string and a boolean that
 // is false if no value was stored at the key.
 func (n NamespacedKV) String(key string) (string, bool) {
-	valBs, err := n.db.Get(n.prefixedKey(key), nil)
+	valBs, err := n.db.Get(n.prefixedKey(key))
 	if err != nil {
 		return "", false
 	}
@@ -102,14 +88,14 @@ func (n NamespacedKV) String(key string) (string, bool) {
 
 // PutBytes stores a new byte slice. Any existing value (even if of another type)
 // is overwritten.
-func (n *NamespacedKV) PutBytes(key string, val []byte) {
-	n.db.Put(n.prefixedKey(key), val, nil)
+func (n *NamespacedKV) PutBytes(key string, val []byte) error {
+	return n.db.Put(n.prefixedKey(key), val)
 }
 
 // Bytes returns the stored value as a raw byte slice and a boolean that
 // is false if no value was stored at the key.
 func (n NamespacedKV) Bytes(key string) ([]byte, bool) {
-	valBs, err := n.db.Get(n.prefixedKey(key), nil)
+	valBs, err := n.db.Get(n.prefixedKey(key))
 	if err != nil {
 		return nil, false
 	}
@@ -118,18 +104,17 @@ func (n NamespacedKV) Bytes(key string) ([]byte, bool) {
 
 // PutBool stores a new boolean. Any existing value (even if of another type)
 // is overwritten.
-func (n *NamespacedKV) PutBool(key string, val bool) {
+func (n *NamespacedKV) PutBool(key string, val bool) error {
 	if val {
-		n.db.Put(n.prefixedKey(key), []byte{0x0}, nil)
-	} else {
-		n.db.Put(n.prefixedKey(key), []byte{0x1}, nil)
+		return n.db.Put(n.prefixedKey(key), []byte{0x0})
 	}
+	return n.db.Put(n.prefixedKey(key), []byte{0x1})
 }
 
 // Bool returns the stored value as a boolean and a boolean that
 // is false if no value was stored at the key.
 func (n NamespacedKV) Bool(key string) (bool, bool) {
-	valBs, err := n.db.Get(n.prefixedKey(key), nil)
+	valBs, err := n.db.Get(n.prefixedKey(key))
 	if err != nil {
 		return false, false
 	}
@@ -138,8 +123,8 @@ func (n NamespacedKV) Bool(key string) (bool, bool) {
 
 // Delete deletes the specified key. It is allowed to delete a nonexistent
 // key.
-func (n NamespacedKV) Delete(key string) {
-	n.db.Delete(n.prefixedKey(key), nil)
+func (n NamespacedKV) Delete(key string) error {
+	return n.db.Delete(n.prefixedKey(key))
 }
 
 func (n NamespacedKV) prefixedKey(key string) []byte {

lib/db/namespaced_test.go

@@ -9,10 +9,12 @@ package db
 import (
 	"testing"
 	"time"
+
+	"github.com/syncthing/syncthing/lib/db/backend"
 )
 
 func TestNamespacedInt(t *testing.T) {
-	ldb := OpenMemory()
+	ldb := NewLowlevel(backend.OpenMemory())
 
 	n1 := NewNamespacedKV(ldb, "foo")
 	n2 := NewNamespacedKV(ldb, "bar")
@@ -23,7 +25,9 @@ func TestNamespacedInt(t *testing.T) {
 		t.Errorf("Incorrect return v %v != 0 || ok %v != false", v, ok)
 	}
 
-	n1.PutInt64("test", 42)
+	if err := n1.PutInt64("test", 42); err != nil {
+		t.Fatal(err)
+	}
 
 	// It should now exist in n1
 
@@ -37,7 +41,9 @@ func TestNamespacedInt(t *testing.T) {
 		t.Errorf("Incorrect return v %v != 0 || ok %v != false", v, ok)
 	}
 
-	n1.Delete("test")
+	if err := n1.Delete("test"); err != nil {
+		t.Fatal(err)
+	}
 
 	// It should no longer exist
 
@@ -47,7 +53,7 @@ func TestNamespacedInt(t *testing.T) {
 }
 
 func TestNamespacedTime(t *testing.T) {
-	ldb := OpenMemory()
+	ldb := NewLowlevel(backend.OpenMemory())
 
 	n1 := NewNamespacedKV(ldb, "foo")
 
@@ -56,7 +62,9 @@ func TestNamespacedTime(t *testing.T) {
 	}
 
 	now := time.Now()
-	n1.PutTime("test", now)
+	if err := n1.PutTime("test", now); err != nil {
+		t.Fatal(err)
+	}
 
 	if v, ok := n1.Time("test"); !v.Equal(now) || !ok {
 		t.Errorf("Incorrect return v %v != %v || ok %v != true", v, now, ok)
@@ -64,7 +72,7 @@ func TestNamespacedTime(t *testing.T) {
 }
 
 func TestNamespacedString(t *testing.T) {
-	ldb := OpenMemory()
+	ldb := NewLowlevel(backend.OpenMemory())
 
 	n1 := NewNamespacedKV(ldb, "foo")
 
@@ -72,7 +80,9 @@ func TestNamespacedString(t *testing.T) {
 		t.Errorf("Incorrect return v %q != \"\" || ok %v != false", v, ok)
 	}
 
-	n1.PutString("test", "yo")
+	if err := n1.PutString("test", "yo"); err != nil {
+		t.Fatal(err)
+	}
 
 	if v, ok := n1.String("test"); v != "yo" || !ok {
 		t.Errorf("Incorrect return v %q != \"yo\" || ok %v != true", v, ok)
@@ -80,13 +90,19 @@ func TestNamespacedString(t *testing.T) {
 }
 
 func TestNamespacedReset(t *testing.T) {
-	ldb := OpenMemory()
+	ldb := NewLowlevel(backend.OpenMemory())
 
 	n1 := NewNamespacedKV(ldb, "foo")
 
-	n1.PutString("test1", "yo1")
-	n1.PutString("test2", "yo2")
-	n1.PutString("test3", "yo3")
+	if err := n1.PutString("test1", "yo1"); err != nil {
+		t.Fatal(err)
+	}
+	if err := n1.PutString("test2", "yo2"); err != nil {
+		t.Fatal(err)
+	}
+	if err := n1.PutString("test3", "yo3"); err != nil {
+		t.Fatal(err)
+	}
 
 	if v, ok := n1.String("test1"); v != "yo1" || !ok {
 		t.Errorf("Incorrect return v %q != \"yo1\" || ok %v != true", v, ok)
@@ -98,7 +114,7 @@ func TestNamespacedReset(t *testing.T) {
 		t.Errorf("Incorrect return v %q != \"yo3\" || ok %v != true", v, ok)
 	}
 
-	n1.Reset()
+	reset(n1)
 
 	if v, ok := n1.String("test1"); v != "" || ok {
 		t.Errorf("Incorrect return v %q != \"\" || ok %v != false", v, ok)
@@ -110,3 +126,22 @@ func TestNamespacedReset(t *testing.T) {
 		t.Errorf("Incorrect return v %q != \"\" || ok %v != false", v, ok)
 	}
 }
+
+// reset removes all entries in this namespace.
+func reset(n *NamespacedKV) {
+	tr, err := n.db.NewWriteTransaction()
+	if err != nil {
+		return
+	}
+	defer tr.Release()
+
+	it, err := tr.NewPrefixIterator(n.prefix)
+	if err != nil {
+		return
+	}
+	for it.Next() {
+		_ = tr.Delete(it.Key())
+	}
+	it.Release()
+	_ = tr.Commit()
+}

lib/db/schemaupdater.go

@@ -11,7 +11,6 @@ import (
 	"strings"
 
 	"github.com/syncthing/syncthing/lib/protocol"
-	"github.com/syndtr/goleveldb/leveldb/util"
 )
 
 // List of all dbVersion to dbMinSyncthingVersion pairs for convenience
@@ -65,36 +64,58 @@ func (db *schemaUpdater) updateSchema() error {
 	}
 
 	if prevVersion < 1 {
-		db.updateSchema0to1()
+		if err := db.updateSchema0to1(); err != nil {
+			return err
+		}
 	}
 	if prevVersion < 2 {
-		db.updateSchema1to2()
+		if err := db.updateSchema1to2(); err != nil {
+			return err
+		}
 	}
 	if prevVersion < 3 {
-		db.updateSchema2to3()
+		if err := db.updateSchema2to3(); err != nil {
+			return err
+		}
 	}
 	// This update fixes problems existing in versions 3 and 4
 	if prevVersion == 3 || prevVersion == 4 {
-		db.updateSchemaTo5()
+		if err := db.updateSchemaTo5(); err != nil {
+			return err
+		}
 	}
 	if prevVersion < 6 {
-		db.updateSchema5to6()
+		if err := db.updateSchema5to6(); err != nil {
+			return err
+		}
 	}
 	if prevVersion < 7 {
-		db.updateSchema6to7()
+		if err := db.updateSchema6to7(); err != nil {
+			return err
+		}
 	}
 
-	miscDB.PutInt64("dbVersion", dbVersion)
-	miscDB.PutString("dbMinSyncthingVersion", dbMinSyncthingVersion)
+	if err := miscDB.PutInt64("dbVersion", dbVersion); err != nil {
+		return err
+	}
+	if err := miscDB.PutString("dbMinSyncthingVersion", dbMinSyncthingVersion); err != nil {
+		return err
+	}
 
 	return nil
 }
 
-func (db *schemaUpdater) updateSchema0to1() {
-	t := db.newReadWriteTransaction()
+func (db *schemaUpdater) updateSchema0to1() error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
-	dbi := t.NewIterator(util.BytesPrefix([]byte{KeyTypeDevice}), nil)
+	dbi, err := t.NewPrefixIterator([]byte{KeyTypeDevice})
+	if err != nil {
+		return err
+	}
 	defer dbi.Release()
 
 	symlinkConv := 0
@@ -104,18 +125,20 @@ func (db *schemaUpdater) updateSchema0to1() {
 	var gk, buf []byte
 
 	for dbi.Next() {
-		t.checkFlush()
-
 		folder, ok := db.keyer.FolderFromDeviceFileKey(dbi.Key())
 		if !ok {
 			// not having the folder in the index is bad; delete and continue
-			t.Delete(dbi.Key())
+			if err := t.Delete(dbi.Key()); err != nil {
+				return err
+			}
 			continue
 		}
 		device, ok := db.keyer.DeviceFromDeviceFileKey(dbi.Key())
 		if !ok {
 			// not having the device in the index is bad; delete and continue
-			t.Delete(dbi.Key())
+			if err := t.Delete(dbi.Key()); err != nil {
+				return err
+			}
 			continue
 		}
 		name := db.keyer.NameFromDeviceFileKey(dbi.Key())
@@ -125,9 +148,17 @@ func (db *schemaUpdater) updateSchema0to1() {
 			if _, ok := changedFolders[string(folder)]; !ok {
 				changedFolders[string(folder)] = struct{}{}
 			}
-			gk = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
-			buf = t.removeFromGlobal(gk, buf, folder, device, nil, nil)
-			t.Delete(dbi.Key())
+			gk, err = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
+			if err != nil {
+				return err
+			}
+			buf, err = t.removeFromGlobal(gk, buf, folder, device, nil, nil)
+			if err != nil {
+				return err
+			}
+			if err := t.Delete(dbi.Key()); err != nil {
+				return err
+			}
 			continue
 		}
 
@@ -147,14 +178,21 @@ func (db *schemaUpdater) updateSchema0to1() {
 			if err != nil {
 				panic("can't happen: " + err.Error())
 			}
-			t.Put(dbi.Key(), bs)
+			if err := t.Put(dbi.Key(), bs); err != nil {
+				return err
+			}
 			symlinkConv++
 		}
 
 		// Add invalid files to global list
 		if f.IsInvalid() {
-			gk = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
-			if buf, ok = t.updateGlobal(gk, buf, folder, device, f, meta); ok {
+			gk, err = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
+			if err != nil {
+				return err
+			}
+			if buf, ok, err = t.updateGlobal(gk, buf, folder, device, f, meta); err != nil {
+				return err
+			} else if ok {
 				if _, ok = changedFolders[string(folder)]; !ok {
 					changedFolders[string(folder)] = struct{}{}
 				}
@@ -164,86 +202,139 @@ func (db *schemaUpdater) updateSchema0to1() {
 	}
 
 	for folder := range changedFolders {
-		db.dropFolderMeta([]byte(folder))
+		if err := db.dropFolderMeta([]byte(folder)); err != nil {
+			return err
 		}
 	}
+	return t.commit()
+}
 
 // updateSchema1to2 introduces a sequenceKey->deviceKey bucket for local items
 // to allow iteration in sequence order (simplifies sending indexes).
-func (db *schemaUpdater) updateSchema1to2() {
-	t := db.newReadWriteTransaction()
+func (db *schemaUpdater) updateSchema1to2() error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
 	var sk []byte
 	var dk []byte
 	for _, folderStr := range db.ListFolders() {
 		folder := []byte(folderStr)
-		db.withHave(folder, protocol.LocalDeviceID[:], nil, true, func(f FileIntf) bool {
-			sk = db.keyer.GenerateSequenceKey(sk, folder, f.SequenceNo())
-			dk = db.keyer.GenerateDeviceFileKey(dk, folder, protocol.LocalDeviceID[:], []byte(f.FileName()))
-			t.Put(sk, dk)
-			t.checkFlush()
-			return true
-		})
+		var putErr error
+		err := db.withHave(folder, protocol.LocalDeviceID[:], nil, true, func(f FileIntf) bool {
+			sk, putErr = db.keyer.GenerateSequenceKey(sk, folder, f.SequenceNo())
+			if putErr != nil {
+				return false
 			}
+			dk, putErr = db.keyer.GenerateDeviceFileKey(dk, folder, protocol.LocalDeviceID[:], []byte(f.FileName()))
+			if putErr != nil {
+				return false
+			}
+			putErr = t.Put(sk, dk)
+			return putErr == nil
+		})
+		if putErr != nil {
+			return putErr
+		}
+		if err != nil {
+			return err
+		}
+	}
+	return t.commit()
 }
 
 // updateSchema2to3 introduces a needKey->nil bucket for locally needed files.
-func (db *schemaUpdater) updateSchema2to3() {
-	t := db.newReadWriteTransaction()
+func (db *schemaUpdater) updateSchema2to3() error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
 	var nk []byte
 	var dk []byte
 	for _, folderStr := range db.ListFolders() {
 		folder := []byte(folderStr)
-		db.withGlobal(folder, nil, true, func(f FileIntf) bool {
+		var putErr error
+		err := db.withGlobal(folder, nil, true, func(f FileIntf) bool {
 			name := []byte(f.FileName())
-			dk = db.keyer.GenerateDeviceFileKey(dk, folder, protocol.LocalDeviceID[:], name)
+			dk, putErr = db.keyer.GenerateDeviceFileKey(dk, folder, protocol.LocalDeviceID[:], name)
+			if putErr != nil {
+				return false
+			}
 			var v protocol.Vector
-			haveFile, ok := t.getFileTrunc(dk, true)
+			haveFile, ok, err := t.getFileTrunc(dk, true)
+			if err != nil {
+				putErr = err
+				return false
+			}
 			if ok {
 				v = haveFile.FileVersion()
 			}
 			if !need(f, ok, v) {
 				return true
 			}
-			nk = t.keyer.GenerateNeedFileKey(nk, folder, []byte(f.FileName()))
-			t.Put(nk, nil)
-			t.checkFlush()
-			return true
-		})
+			nk, putErr = t.keyer.GenerateNeedFileKey(nk, folder, []byte(f.FileName()))
+			if putErr != nil {
+				return false
 			}
+			putErr = t.Put(nk, nil)
+			return putErr == nil
+		})
+		if putErr != nil {
+			return putErr
+		}
+		if err != nil {
+			return err
+		}
+	}
+	return t.commit()
 }
 
 // updateSchemaTo5 resets the need bucket due to bugs existing in the v0.14.49
 // release candidates (dbVersion 3 and 4)
 // https://github.com/syncthing/syncthing/issues/5007
 // https://github.com/syncthing/syncthing/issues/5053
-func (db *schemaUpdater) updateSchemaTo5() {
-	t := db.newReadWriteTransaction()
+func (db *schemaUpdater) updateSchemaTo5() error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	var nk []byte
 	for _, folderStr := range db.ListFolders() {
-		nk = db.keyer.GenerateNeedFileKey(nk, []byte(folderStr), nil)
-		t.deleteKeyPrefix(nk[:keyPrefixLen+keyFolderLen])
+		nk, err = db.keyer.GenerateNeedFileKey(nk, []byte(folderStr), nil)
+		if err != nil {
+			return err
 		}
-	t.close()
-
-	db.updateSchema2to3()
+		if err := t.deleteKeyPrefix(nk[:keyPrefixLen+keyFolderLen]); err != nil {
+			return err
+		}
+	}
+	if err := t.commit(); err != nil {
+		return err
 	}
 
-func (db *schemaUpdater) updateSchema5to6() {
+	return db.updateSchema2to3()
+}
+
+func (db *schemaUpdater) updateSchema5to6() error {
 	// For every local file with the Invalid bit set, clear the Invalid bit and
 	// set LocalFlags = FlagLocalIgnored.
 
-	t := db.newReadWriteTransaction()
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
 	var dk []byte
 
 	for _, folderStr := range db.ListFolders() {
 		folder := []byte(folderStr)
-		db.withHave(folder, protocol.LocalDeviceID[:], nil, false, func(f FileIntf) bool {
+		var putErr error
+		err := db.withHave(folder, protocol.LocalDeviceID[:], nil, false, func(f FileIntf) bool {
 			if !f.IsInvalid() {
 				return true
 			}
@@ -253,19 +344,31 @@ func (db *schemaUpdater) updateSchema5to6() {
 			fi.LocalFlags = protocol.FlagLocalIgnored
 			bs, _ := fi.Marshal()
 
-			dk = db.keyer.GenerateDeviceFileKey(dk, folder, protocol.LocalDeviceID[:], []byte(fi.Name))
-			t.Put(dk, bs)
-
-			t.checkFlush()
-			return true
-		})
+			dk, putErr = db.keyer.GenerateDeviceFileKey(dk, folder, protocol.LocalDeviceID[:], []byte(fi.Name))
+			if putErr != nil {
+				return false
 			}
+			putErr = t.Put(dk, bs)
+
+			return putErr == nil
+		})
+		if putErr != nil {
+			return putErr
+		}
+		if err != nil {
+			return err
+		}
+	}
+	return t.commit()
 }
 
 // updateSchema6to7 checks whether all currently locally needed files are really
 // needed and removes them if not.
-func (db *schemaUpdater) updateSchema6to7() {
-	t := db.newReadWriteTransaction()
+func (db *schemaUpdater) updateSchema6to7() error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
 	var gk []byte
@@ -273,15 +376,24 @@ func (db *schemaUpdater) updateSchema6to7() {
 
 	for _, folderStr := range db.ListFolders() {
 		folder := []byte(folderStr)
-		db.withNeedLocal(folder, false, func(f FileIntf) bool {
+		var delErr error
+		err := db.withNeedLocal(folder, false, func(f FileIntf) bool {
 			name := []byte(f.FileName())
 			global := f.(protocol.FileInfo)
-			gk = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
-			svl, err := t.Get(gk, nil)
+			gk, delErr = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
+			if delErr != nil {
+				return false
+			}
+			svl, err := t.Get(gk)
 			if err != nil {
 				// If there is no global list, we hardly need it.
-				t.Delete(t.keyer.GenerateNeedFileKey(nk, folder, name))
-				return true
+				key, err := t.keyer.GenerateNeedFileKey(nk, folder, name)
+				if err != nil {
+					delErr = err
+					return false
+				}
+				delErr = t.Delete(key)
+				return delErr == nil
 			}
 			var fl VersionList
 			err = fl.Unmarshal(svl)
@@ -291,9 +403,18 @@ func (db *schemaUpdater) updateSchema6to7() {
 				return true
 			}
 			if localFV, haveLocalFV := fl.Get(protocol.LocalDeviceID[:]); !need(global, haveLocalFV, localFV.Version) {
-				t.Delete(t.keyer.GenerateNeedFileKey(nk, folder, name))
+				key, err := t.keyer.GenerateNeedFileKey(nk, folder, name)
+				if err != nil {
+					delErr = err
+					return false
 				}
-			return true
+				delErr = t.Delete(key)
+			}
+			return delErr == nil
 		})
+		if err != nil {
+			return err
 		}
 	}
+	return t.commit()
+}

lib/db/set.go

@@ -16,11 +16,11 @@ import (
 	"os"
 	"time"
 
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/osutil"
 	"github.com/syncthing/syncthing/lib/protocol"
 	"github.com/syncthing/syncthing/lib/sync"
-	"github.com/syndtr/goleveldb/leveldb/util"
 )
 
 type FileSet struct {
@@ -83,29 +83,42 @@ func NewFileSet(folder string, fs fs.Filesystem, ll *Lowlevel) *FileSet {
 
 	if err := s.meta.fromDB(db, []byte(folder)); err != nil {
 		l.Infof("No stored folder metadata for %q: recalculating", folder)
-		s.recalcCounts()
+		if err := s.recalcCounts(); backend.IsClosed(err) {
+			return nil
+		} else if err != nil {
+			panic(err)
+		}
 	} else if age := time.Since(s.meta.Created()); age > databaseRecheckInterval {
 		l.Infof("Stored folder metadata for %q is %v old; recalculating", folder, age)
-		s.recalcCounts()
+		if err := s.recalcCounts(); backend.IsClosed(err) {
+			return nil
+		} else if err != nil {
+			panic(err)
+		}
 	}
 
 	return &s
 }
 
-func (s *FileSet) recalcCounts() {
+func (s *FileSet) recalcCounts() error {
 	s.meta = newMetadataTracker()
 
-	s.db.checkGlobals([]byte(s.folder), s.meta)
+	if err := s.db.checkGlobals([]byte(s.folder), s.meta); err != nil {
+		return err
+	}
 
 	var deviceID protocol.DeviceID
-	s.db.withAllFolderTruncated([]byte(s.folder), func(device []byte, f FileInfoTruncated) bool {
+	err := s.db.withAllFolderTruncated([]byte(s.folder), func(device []byte, f FileInfoTruncated) bool {
 		copy(deviceID[:], device)
 		s.meta.addFile(deviceID, f)
 		return true
 	})
+	if err != nil {
+		return err
+	}
 
 	s.meta.SetCreated()
-	s.meta.toDB(s.db, []byte(s.folder))
+	return s.meta.toDB(s.db, []byte(s.folder))
 }
 
 func (s *FileSet) Drop(device protocol.DeviceID) {
@@ -114,7 +127,11 @@ func (s *FileSet) Drop(device protocol.DeviceID) {
 	s.updateMutex.Lock()
 	defer s.updateMutex.Unlock()
 
-	s.db.dropDeviceFolder(device[:], []byte(s.folder), s.meta)
+	if err := s.db.dropDeviceFolder(device[:], []byte(s.folder), s.meta); backend.IsClosed(err) {
+		return
+	} else if err != nil {
+		panic(err)
+	}
 
 	if device == protocol.LocalDeviceID {
 		s.meta.resetCounts(device)
@@ -131,7 +148,11 @@ func (s *FileSet) Drop(device protocol.DeviceID) {
 		s.meta.resetAll(device)
 	}
 
-	s.meta.toDB(s.db, []byte(s.folder))
+	if err := s.meta.toDB(s.db, []byte(s.folder)); backend.IsClosed(err) {
+		return
+	} else if err != nil {
+		panic(err)
+	}
 }
 
 func (s *FileSet) Update(device protocol.DeviceID, fs []protocol.FileInfo) {
@@ -145,73 +166,110 @@ func (s *FileSet) Update(device protocol.DeviceID, fs []protocol.FileInfo) {
 	s.updateMutex.Lock()
 	defer s.updateMutex.Unlock()
 
-	defer s.meta.toDB(s.db, []byte(s.folder))
+	defer func() {
+		if err := s.meta.toDB(s.db, []byte(s.folder)); err != nil && !backend.IsClosed(err) {
+			panic(err)
+		}
+	}()
 
 	if device == protocol.LocalDeviceID {
 		// For the local device we have a bunch of metadata to track.
-		s.db.updateLocalFiles([]byte(s.folder), fs, s.meta)
+		if err := s.db.updateLocalFiles([]byte(s.folder), fs, s.meta); err != nil && !backend.IsClosed(err) {
+			panic(err)
+		}
 		return
 	}
 	// Easy case, just update the files and we're done.
-	s.db.updateRemoteFiles([]byte(s.folder), device[:], fs, s.meta)
+	if err := s.db.updateRemoteFiles([]byte(s.folder), device[:], fs, s.meta); err != nil && !backend.IsClosed(err) {
+		panic(err)
+	}
 }
 
 func (s *FileSet) WithNeed(device protocol.DeviceID, fn Iterator) {
 	l.Debugf("%s WithNeed(%v)", s.folder, device)
-	s.db.withNeed([]byte(s.folder), device[:], false, nativeFileIterator(fn))
+	if err := s.db.withNeed([]byte(s.folder), device[:], false, nativeFileIterator(fn)); err != nil && !backend.IsClosed(err) {
+		panic(err)
+	}
 }
 
 func (s *FileSet) WithNeedTruncated(device protocol.DeviceID, fn Iterator) {
 	l.Debugf("%s WithNeedTruncated(%v)", s.folder, device)
-	s.db.withNeed([]byte(s.folder), device[:], true, nativeFileIterator(fn))
+	if err := s.db.withNeed([]byte(s.folder), device[:], true, nativeFileIterator(fn)); err != nil && !backend.IsClosed(err) {
+		panic(err)
+	}
 }
 
 func (s *FileSet) WithHave(device protocol.DeviceID, fn Iterator) {
 	l.Debugf("%s WithHave(%v)", s.folder, device)
-	s.db.withHave([]byte(s.folder), device[:], nil, false, nativeFileIterator(fn))
+	if err := s.db.withHave([]byte(s.folder), device[:], nil, false, nativeFileIterator(fn)); err != nil && !backend.IsClosed(err) {
+		panic(err)
+	}
 }
 
 func (s *FileSet) WithHaveTruncated(device protocol.DeviceID, fn Iterator) {
 	l.Debugf("%s WithHaveTruncated(%v)", s.folder, device)
-	s.db.withHave([]byte(s.folder), device[:], nil, true, nativeFileIterator(fn))
+	if err := s.db.withHave([]byte(s.folder), device[:], nil, true, nativeFileIterator(fn)); err != nil && !backend.IsClosed(err) {
+		panic(err)
+	}
 }
 
 func (s *FileSet) WithHaveSequence(startSeq int64, fn Iterator) {
 	l.Debugf("%s WithHaveSequence(%v)", s.folder, startSeq)
-	s.db.withHaveSequence([]byte(s.folder), startSeq, nativeFileIterator(fn))
+	if err := s.db.withHaveSequence([]byte(s.folder), startSeq, nativeFileIterator(fn)); err != nil && !backend.IsClosed(err) {
+		panic(err)
+	}
 }
 
 // Except for an item with a path equal to prefix, only children of prefix are iterated.
 // E.g. for prefix "dir", "dir/file" is iterated, but "dir.file" is not.
 func (s *FileSet) WithPrefixedHaveTruncated(device protocol.DeviceID, prefix string, fn Iterator) {
 	l.Debugf(`%s WithPrefixedHaveTruncated(%v, "%v")`, s.folder, device, prefix)
-	s.db.withHave([]byte(s.folder), device[:], []byte(osutil.NormalizedFilename(prefix)), true, nativeFileIterator(fn))
+	if err := s.db.withHave([]byte(s.folder), device[:], []byte(osutil.NormalizedFilename(prefix)), true, nativeFileIterator(fn)); err != nil && !backend.IsClosed(err) {
+		panic(err)
 	}
+}
+
 func (s *FileSet) WithGlobal(fn Iterator) {
 	l.Debugf("%s WithGlobal()", s.folder)
-	s.db.withGlobal([]byte(s.folder), nil, false, nativeFileIterator(fn))
+	if err := s.db.withGlobal([]byte(s.folder), nil, false, nativeFileIterator(fn)); err != nil && !backend.IsClosed(err) {
+		panic(err)
+	}
 }
 
 func (s *FileSet) WithGlobalTruncated(fn Iterator) {
 	l.Debugf("%s WithGlobalTruncated()", s.folder)
-	s.db.withGlobal([]byte(s.folder), nil, true, nativeFileIterator(fn))
+	if err := s.db.withGlobal([]byte(s.folder), nil, true, nativeFileIterator(fn)); err != nil && !backend.IsClosed(err) {
+		panic(err)
+	}
 }
 
 // Except for an item with a path equal to prefix, only children of prefix are iterated.
 // E.g. for prefix "dir", "dir/file" is iterated, but "dir.file" is not.
 func (s *FileSet) WithPrefixedGlobalTruncated(prefix string, fn Iterator) {
 	l.Debugf(`%s WithPrefixedGlobalTruncated("%v")`, s.folder, prefix)
-	s.db.withGlobal([]byte(s.folder), []byte(osutil.NormalizedFilename(prefix)), true, nativeFileIterator(fn))
+	if err := s.db.withGlobal([]byte(s.folder), []byte(osutil.NormalizedFilename(prefix)), true, nativeFileIterator(fn)); err != nil && !backend.IsClosed(err) {
+		panic(err)
+	}
 }
 
 func (s *FileSet) Get(device protocol.DeviceID, file string) (protocol.FileInfo, bool) {
-	f, ok := s.db.getFileDirty([]byte(s.folder), device[:], []byte(osutil.NormalizedFilename(file)))
+	f, ok, err := s.db.getFileDirty([]byte(s.folder), device[:], []byte(osutil.NormalizedFilename(file)))
+	if backend.IsClosed(err) {
+		return protocol.FileInfo{}, false
+	} else if err != nil {
+		panic(err)
+	}
 	f.Name = osutil.NativeFilename(f.Name)
 	return f, ok
 }
 
 func (s *FileSet) GetGlobal(file string) (protocol.FileInfo, bool) {
-	fi, ok := s.db.getGlobalDirty([]byte(s.folder), []byte(osutil.NormalizedFilename(file)), false)
+	fi, ok, err := s.db.getGlobalDirty([]byte(s.folder), []byte(osutil.NormalizedFilename(file)), false)
+	if backend.IsClosed(err) {
+		return protocol.FileInfo{}, false
+	} else if err != nil {
+		panic(err)
+	}
 	if !ok {
 		return protocol.FileInfo{}, false
 	}
@@ -221,7 +279,12 @@ func (s *FileSet) GetGlobal(file string) (protocol.FileInfo, bool) {
 }
 
 func (s *FileSet) GetGlobalTruncated(file string) (FileInfoTruncated, bool) {
-	fi, ok := s.db.getGlobalDirty([]byte(s.folder), []byte(osutil.NormalizedFilename(file)), true)
+	fi, ok, err := s.db.getGlobalDirty([]byte(s.folder), []byte(osutil.NormalizedFilename(file)), true)
+	if backend.IsClosed(err) {
+		return FileInfoTruncated{}, false
+	} else if err != nil {
+		panic(err)
+	}
 	if !ok {
 		return FileInfoTruncated{}, false
 	}
@@ -231,7 +294,13 @@ func (s *FileSet) GetGlobalTruncated(file string) (FileInfoTruncated, bool) {
 }
 
 func (s *FileSet) Availability(file string) []protocol.DeviceID {
-	return s.db.availability([]byte(s.folder), []byte(osutil.NormalizedFilename(file)))
+	av, err := s.db.availability([]byte(s.folder), []byte(osutil.NormalizedFilename(file)))
+	if backend.IsClosed(err) {
+		return nil
+	} else if err != nil {
+		panic(err)
+	}
+	return av
 }
 
 func (s *FileSet) Sequence(device protocol.DeviceID) int64 {
@@ -255,11 +324,21 @@ func (s *FileSet) GlobalSize() Counts {
 }
 
 func (s *FileSet) IndexID(device protocol.DeviceID) protocol.IndexID {
-	id := s.db.getIndexID(device[:], []byte(s.folder))
+	id, err := s.db.getIndexID(device[:], []byte(s.folder))
+	if backend.IsClosed(err) {
+		return 0
+	} else if err != nil {
+		panic(err)
+	}
 	if id == 0 && device == protocol.LocalDeviceID {
 		// No index ID set yet. We create one now.
 		id = protocol.NewIndexID()
-		s.db.setIndexID(device[:], []byte(s.folder), id)
+		err := s.db.setIndexID(device[:], []byte(s.folder), id)
+		if backend.IsClosed(err) {
+			return 0
+		} else if err != nil {
+			panic(err)
+		}
 	}
 	return id
 }
@@ -268,11 +347,18 @@ func (s *FileSet) SetIndexID(device protocol.DeviceID, id protocol.IndexID) {
 	if device == protocol.LocalDeviceID {
 		panic("do not explicitly set index ID for local device")
 	}
-	s.db.setIndexID(device[:], []byte(s.folder), id)
+	if err := s.db.setIndexID(device[:], []byte(s.folder), id); err != nil && !backend.IsClosed(err) {
+		panic(err)
+	}
 }
 
 func (s *FileSet) MtimeFS() *fs.MtimeFS {
-	prefix := s.db.keyer.GenerateMtimesKey(nil, []byte(s.folder))
+	prefix, err := s.db.keyer.GenerateMtimesKey(nil, []byte(s.folder))
+	if backend.IsClosed(err) {
+		return nil
+	} else if err != nil {
+		panic(err)
+	}
 	kv := NewNamespacedKV(s.db.Lowlevel, string(prefix))
 	return fs.NewMtimeFS(s.fs, kv)
 }
@@ -285,21 +371,39 @@ func (s *FileSet) ListDevices() []protocol.DeviceID {
 // database.
 func DropFolder(ll *Lowlevel, folder string) {
 	db := newInstance(ll)
-	db.dropFolder([]byte(folder))
-	db.dropMtimes([]byte(folder))
-	db.dropFolderMeta([]byte(folder))
 
-	// Also clean out the folder ID mapping.
-	db.folderIdx.Delete([]byte(folder))
+	droppers := []func([]byte) error{
+		db.dropFolder,
+		db.dropMtimes,
+		db.dropFolderMeta,
+		db.folderIdx.Delete,
+	}
+	for _, drop := range droppers {
+		if err := drop([]byte(folder)); backend.IsClosed(err) {
+			return
+		} else if err != nil {
+			panic(err)
+		}
+	}
 }
 
 // DropDeltaIndexIDs removes all delta index IDs from the database.
 // This will cause a full index transmission on the next connection.
 func DropDeltaIndexIDs(db *Lowlevel) {
-	dbi := db.NewIterator(util.BytesPrefix([]byte{KeyTypeIndexID}), nil)
+	dbi, err := db.NewPrefixIterator([]byte{KeyTypeIndexID})
+	if backend.IsClosed(err) {
+		return
+	} else if err != nil {
+		panic(err)
+	}
 	defer dbi.Release()
 	for dbi.Next() {
-		db.Delete(dbi.Key(), nil)
+		if err := db.Delete(dbi.Key()); err != nil && !backend.IsClosed(err) {
+			panic(err)
+		}
+	}
+	if err := dbi.Error(); err != nil && !backend.IsClosed(err) {
+		panic(err)
 	}
 }
 

lib/db/set_test.go

@@ -17,6 +17,7 @@ import (
 
 	"github.com/d4l3k/messagediff"
 	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/protocol"
 )
@@ -117,7 +118,7 @@ func (l fileList) String() string {
 }
 
 func TestGlobalSet(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	m := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 
@@ -332,7 +333,7 @@ func TestGlobalSet(t *testing.T) {
 }
 
 func TestNeedWithInvalid(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	s := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 
@@ -369,7 +370,7 @@ func TestNeedWithInvalid(t *testing.T) {
 }
 
 func TestUpdateToInvalid(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	folder := "test"
 	s := db.NewFileSet(folder, fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
@@ -425,7 +426,7 @@ func TestUpdateToInvalid(t *testing.T) {
 }
 
 func TestInvalidAvailability(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	s := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 
@@ -463,7 +464,7 @@ func TestInvalidAvailability(t *testing.T) {
 }
 
 func TestGlobalReset(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	m := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 
@@ -501,7 +502,7 @@ func TestGlobalReset(t *testing.T) {
 }
 
 func TestNeed(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	m := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 
@@ -539,7 +540,7 @@ func TestNeed(t *testing.T) {
 }
 
 func TestSequence(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	m := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 
@@ -569,7 +570,7 @@ func TestSequence(t *testing.T) {
 }
 
 func TestListDropFolder(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	s0 := db.NewFileSet("test0", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 	local1 := []protocol.FileInfo{
@@ -619,7 +620,7 @@ func TestListDropFolder(t *testing.T) {
 }
 
 func TestGlobalNeedWithInvalid(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	s := db.NewFileSet("test1", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 
@@ -660,7 +661,7 @@ func TestGlobalNeedWithInvalid(t *testing.T) {
 }
 
 func TestLongPath(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	s := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 
@@ -671,7 +672,7 @@ func TestLongPath(t *testing.T) {
 	name := b.String() // 5000 characters
 
 	local := []protocol.FileInfo{
-		{Name: string(name), Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}},
+		{Name: name, Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}},
 	}
 
 	replace(s, protocol.LocalDeviceID, local)
@@ -686,39 +687,6 @@ func TestLongPath(t *testing.T) {
 	}
 }
 
-func TestCommitted(t *testing.T) {
-	// Verify that the Committed counter increases when we change things and
-	// doesn't increase when we don't.
-
-	ldb := db.OpenMemory()
-
-	s := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
-
-	local := []protocol.FileInfo{
-		{Name: string("file"), Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}},
-	}
-
-	// Adding a file should increase the counter
-
-	c0 := ldb.Committed()
-
-	replace(s, protocol.LocalDeviceID, local)
-
-	c1 := ldb.Committed()
-	if c1 <= c0 {
-		t.Errorf("committed data didn't increase; %d <= %d", c1, c0)
-	}
-
-	// Updating with something identical should not do anything
-
-	s.Update(protocol.LocalDeviceID, local)
-
-	c2 := ldb.Committed()
-	if c2 > c1 {
-		t.Errorf("replace with same contents should do nothing but %d > %d", c2, c1)
-	}
-}
-
 func BenchmarkUpdateOneFile(b *testing.B) {
 	local0 := fileList{
 		protocol.FileInfo{Name: "a", Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}, Blocks: genBlocks(1)},
@@ -729,10 +697,11 @@ func BenchmarkUpdateOneFile(b *testing.B) {
 		protocol.FileInfo{Name: "zajksdhaskjdh/askjdhaskjdashkajshd/kasjdhaskjdhaskdjhaskdjash/dkjashdaksjdhaskdjahskdjh", Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}, Blocks: genBlocks(8)},
 	}
 
-	ldb, err := db.Open("testdata/benchmarkupdate.db", db.TuningAuto)
+	be, err := backend.Open("testdata/benchmarkupdate.db", backend.TuningAuto)
 	if err != nil {
 		b.Fatal(err)
 	}
+	ldb := db.NewLowlevel(be)
 	defer func() {
 		ldb.Close()
 		os.RemoveAll("testdata/benchmarkupdate.db")
@@ -751,7 +720,7 @@ func BenchmarkUpdateOneFile(b *testing.B) {
 }
 
 func TestIndexID(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	s := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 
@@ -783,7 +752,7 @@ func TestIndexID(t *testing.T) {
 }
 
 func TestDropFiles(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	m := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 
@@ -846,7 +815,7 @@ func TestDropFiles(t *testing.T) {
 }
 
 func TestIssue4701(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	s := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 
@@ -887,7 +856,7 @@ func TestIssue4701(t *testing.T) {
 }
 
 func TestWithHaveSequence(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	folder := "test"
 	s := db.NewFileSet(folder, fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
@@ -915,14 +884,14 @@ func TestWithHaveSequence(t *testing.T) {
 
 func TestStressWithHaveSequence(t *testing.T) {
 	// This races two loops against each other: one that contiously does
-	// updates, and one that continously does sequence walks. The test fails
+	// updates, and one that continuously does sequence walks. The test fails
 	// if the sequence walker sees a discontinuity.
 
 	if testing.Short() {
 		t.Skip("Takes a long time")
 	}
 
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	folder := "test"
 	s := db.NewFileSet(folder, fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
@@ -945,7 +914,7 @@ func TestStressWithHaveSequence(t *testing.T) {
 		close(done)
 	}()
 
-	var prevSeq int64 = 0
+	var prevSeq int64
 loop:
 	for {
 		select {
@@ -964,7 +933,7 @@ loop:
 }
 
 func TestIssue4925(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	folder := "test"
 	s := db.NewFileSet(folder, fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
@@ -990,7 +959,7 @@ func TestIssue4925(t *testing.T) {
 }
 
 func TestMoveGlobalBack(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	folder := "test"
 	file := "foo"
@@ -1054,7 +1023,7 @@ func TestMoveGlobalBack(t *testing.T) {
 // needed files.
 // https://github.com/syncthing/syncthing/issues/5007
 func TestIssue5007(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	folder := "test"
 	file := "foo"
@@ -1081,7 +1050,7 @@ func TestIssue5007(t *testing.T) {
 // TestNeedDeleted checks that a file that doesn't exist locally isn't needed
 // when the global file is deleted.
 func TestNeedDeleted(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	folder := "test"
 	file := "foo"
@@ -1115,7 +1084,7 @@ func TestNeedDeleted(t *testing.T) {
 }
 
 func TestReceiveOnlyAccounting(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	folder := "test"
 	s := db.NewFileSet(folder, fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
@@ -1219,7 +1188,7 @@ func TestReceiveOnlyAccounting(t *testing.T) {
 }
 
 func TestNeedAfterUnignore(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	folder := "test"
 	file := "foo"
@@ -1251,7 +1220,7 @@ func TestNeedAfterUnignore(t *testing.T) {
 func TestRemoteInvalidNotAccounted(t *testing.T) {
 	// Remote files with the invalid bit should not count.
 
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 	s := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 
 	files := []protocol.FileInfo{
@@ -1270,7 +1239,7 @@ func TestRemoteInvalidNotAccounted(t *testing.T) {
 }
 
 func TestNeedWithNewerInvalid(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	s := db.NewFileSet("default", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 
@@ -1308,7 +1277,7 @@ func TestNeedWithNewerInvalid(t *testing.T) {
 }
 
 func TestNeedAfterDeviceRemove(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	file := "foo"
 	s := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
@@ -1335,7 +1304,7 @@ func TestNeedAfterDeviceRemove(t *testing.T) {
 func TestCaseSensitive(t *testing.T) {
 	// Normal case sensitive lookup should work
 
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 	s := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 
 	local := []protocol.FileInfo{
@@ -1372,7 +1341,7 @@ func TestSequenceIndex(t *testing.T) {
 
 	// Set up a db and a few files that we will manipulate.
 
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 	s := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
 
 	local := []protocol.FileInfo{
@@ -1463,7 +1432,7 @@ func TestSequenceIndex(t *testing.T) {
 }
 
 func TestIgnoreAfterReceiveOnly(t *testing.T) {
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 
 	file := "foo"
 	s := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)

lib/db/smallindex.go

@@ -10,16 +10,15 @@ import (
 	"encoding/binary"
 	"sort"
 
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/sync"
-	"github.com/syndtr/goleveldb/leveldb"
-	"github.com/syndtr/goleveldb/leveldb/util"
 )
 
 // A smallIndex is an in memory bidirectional []byte to uint32 map. It gives
 // fast lookups in both directions and persists to the database. Don't use for
 // storing more items than fit comfortably in RAM.
 type smallIndex struct {
-	db     *leveldb.DB
+	db     backend.Backend
 	prefix []byte
 	id2val map[uint32]string
 	val2id map[string]uint32
@@ -27,7 +26,7 @@ type smallIndex struct {
 	mut    sync.Mutex
 }
 
-func newSmallIndex(db *leveldb.DB, prefix []byte) *smallIndex {
+func newSmallIndex(db backend.Backend, prefix []byte) *smallIndex {
 	idx := &smallIndex{
 		db:     db,
 		prefix: prefix,
@@ -42,7 +41,10 @@ func newSmallIndex(db *leveldb.DB, prefix []byte) *smallIndex {
 // load iterates over the prefix space in the database and populates the in
 // memory maps.
 func (i *smallIndex) load() {
-	it := i.db.NewIterator(util.BytesPrefix(i.prefix), nil)
+	it, err := i.db.NewPrefixIterator(i.prefix)
+	if err != nil {
+		panic("loading small index: " + err.Error())
+	}
 	defer it.Release()
 	for it.Next() {
 		val := string(it.Value())
@@ -60,7 +62,7 @@ func (i *smallIndex) load() {
 
 // ID returns the index number for the given byte slice, allocating a new one
 // and persisting this to the database if necessary.
-func (i *smallIndex) ID(val []byte) uint32 {
+func (i *smallIndex) ID(val []byte) (uint32, error) {
 	i.mut.Lock()
 	// intentionally avoiding defer here as we want this call to be as fast as
 	// possible in the general case (folder ID already exists). The map lookup
@@ -69,7 +71,7 @@ func (i *smallIndex) ID(val []byte) uint32 {
 	// here.
 	if id, ok := i.val2id[string(val)]; ok {
 		i.mut.Unlock()
-		return id
+		return id, nil
 	}
 
 	id := i.nextID
@@ -82,10 +84,13 @@ func (i *smallIndex) ID(val []byte) uint32 {
 	key := make([]byte, len(i.prefix)+8) // prefix plus uint32 id
 	copy(key, i.prefix)
 	binary.BigEndian.PutUint32(key[len(i.prefix):], id)
-	i.db.Put(key, val, nil)
+	if err := i.db.Put(key, val); err != nil {
+		i.mut.Unlock()
+		return 0, err
+	}
 
 	i.mut.Unlock()
-	return id
+	return id, nil
 }
 
 // Val returns the value for the given index number, or (nil, false) if there
@@ -101,7 +106,7 @@ func (i *smallIndex) Val(id uint32) ([]byte, bool) {
 	return []byte(val), true
 }
 
-func (i *smallIndex) Delete(val []byte) {
+func (i *smallIndex) Delete(val []byte) error {
 	i.mut.Lock()
 	defer i.mut.Unlock()
 
@@ -115,7 +120,9 @@ func (i *smallIndex) Delete(val []byte) {
 		// Put an empty value into the database. This indicates that the
 		// entry does not exist any more and prevents the ID from being
 		// reused in the future.
-		i.db.Put(key, []byte{}, nil)
+		if err := i.db.Put(key, []byte{}); err != nil {
+			return err
+		}
 
 		// Delete reverse mapping.
 		delete(i.id2val, id)
@@ -123,6 +130,7 @@ func (i *smallIndex) Delete(val []byte) {
 
 	// Delete forward mapping.
 	delete(i.val2id, string(val))
+	return nil
 }
 
 // Values returns the set of values in the index

lib/db/smallindex_test.go

@@ -6,11 +6,15 @@
 
 package db
 
-import "testing"
+import (
+	"testing"
+
+	"github.com/syncthing/syncthing/lib/db/backend"
+)
 
 func TestSmallIndex(t *testing.T) {
-	db := OpenMemory()
-	idx := newSmallIndex(db.DB, []byte{12, 34})
+	db := NewLowlevel(backend.OpenMemory())
+	idx := newSmallIndex(db, []byte{12, 34})
 
 	// ID zero should be unallocated
 	if val, ok := idx.Val(0); ok || val != nil {
@@ -18,7 +22,9 @@ func TestSmallIndex(t *testing.T) {
 	}
 
 	// A new key should get ID zero
-	if id := idx.ID([]byte("hello")); id != 0 {
+	if id, err := idx.ID([]byte("hello")); err != nil {
+		t.Fatal(err)
+	} else if id != 0 {
 		t.Fatal("Expected 0, not", id)
 	}
 	// Looking up ID zero should work
@@ -30,23 +36,29 @@ func TestSmallIndex(t *testing.T) {
 	idx.Delete([]byte("hello"))
 
 	// Next ID should be one
-	if id := idx.ID([]byte("key2")); id != 1 {
+	if id, err := idx.ID([]byte("key2")); err != nil {
+		t.Fatal(err)
+	} else if id != 1 {
 		t.Fatal("Expected 1, not", id)
 	}
 
 	// Now lets create a new index instance based on what's actually serialized to the database.
-	idx = newSmallIndex(db.DB, []byte{12, 34})
+	idx = newSmallIndex(db, []byte{12, 34})
 
 	// Status should be about the same as before.
 	if val, ok := idx.Val(0); ok || val != nil {
 		t.Fatal("Unexpected return for deleted ID 0")
 	}
-	if id := idx.ID([]byte("key2")); id != 1 {
+	if id, err := idx.ID([]byte("key2")); err != nil {
+		t.Fatal(err)
+	} else if id != 1 {
 		t.Fatal("Expected 1, not", id)
 	}
 
 	// Setting "hello" again should get us ID 2, not 0 as it was originally.
-	if id := idx.ID([]byte("hello")); id != 2 {
+	if id, err := idx.ID([]byte("hello")); err != nil {
+		t.Fatal(err)
+	} else if id != 2 {
 		t.Fatal("Expected 2, not", id)
 	}
 }

lib/db/structs.go

@@ -175,7 +175,7 @@ func (vl VersionList) String() string {
 // update brings the VersionList up to date with file. It returns the updated
 // VersionList, a potentially removed old FileVersion and its index, as well as
 // the index where the new FileVersion was inserted.
-func (vl VersionList) update(folder, device []byte, file protocol.FileInfo, t readOnlyTransaction) (_ VersionList, removedFV FileVersion, removedAt int, insertedAt int) {
+func (vl VersionList) update(folder, device []byte, file protocol.FileInfo, t readOnlyTransaction) (_ VersionList, removedFV FileVersion, removedAt int, insertedAt int, err error) {
 	vl, removedFV, removedAt = vl.pop(device)
 
 	nv := FileVersion{
@@ -198,7 +198,7 @@ func (vl VersionList) update(folder, device []byte, file protocol.FileInfo, t re
 			// The version at this point in the list is equal to or lesser
 			// ("older") than us. We insert ourselves in front of it.
 			vl = vl.insertAt(i, nv)
-			return vl, removedFV, removedAt, i
+			return vl, removedFV, removedAt, i, nil
 
 		case protocol.ConcurrentLesser, protocol.ConcurrentGreater:
 			// The version at this point is in conflict with us. We must pull
@@ -209,9 +209,11 @@ func (vl VersionList) update(folder, device []byte, file protocol.FileInfo, t re
 			// to determine the winner.)
 			//
 			// A surprise missing file entry here is counted as a win for us.
-			if of, ok := t.getFile(folder, vl.Versions[i].Device, []byte(file.Name)); !ok || file.WinsConflict(of) {
+			if of, ok, err := t.getFile(folder, vl.Versions[i].Device, []byte(file.Name)); err != nil {
+				return vl, removedFV, removedAt, i, err
+			} else if !ok || file.WinsConflict(of) {
 				vl = vl.insertAt(i, nv)
-				return vl, removedFV, removedAt, i
+				return vl, removedFV, removedAt, i, nil
 			}
 		}
 	}
@@ -219,7 +221,7 @@ func (vl VersionList) update(folder, device []byte, file protocol.FileInfo, t re
 	// We didn't find a position for an insert above, so append to the end.
 	vl.Versions = append(vl.Versions, nv)
 
-	return vl, removedFV, removedAt, len(vl.Versions) - 1
+	return vl, removedFV, removedAt, len(vl.Versions) - 1, nil
 }
 
 func (vl VersionList) insertAt(i int, v FileVersion) VersionList {

lib/db/transactions.go

@@ -7,111 +7,146 @@
 package db
 
 import (
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/protocol"
-	"github.com/syndtr/goleveldb/leveldb"
-	"github.com/syndtr/goleveldb/leveldb/util"
 )
 
 // A readOnlyTransaction represents a database snapshot.
 type readOnlyTransaction struct {
-	snapshot
+	backend.ReadTransaction
 	keyer keyer
 }
 
-func (db *instance) newReadOnlyTransaction() readOnlyTransaction {
-	return readOnlyTransaction{
-		snapshot: db.GetSnapshot(),
-		keyer:    db.keyer,
+func (db *instance) newReadOnlyTransaction() (readOnlyTransaction, error) {
+	tran, err := db.NewReadTransaction()
+	if err != nil {
+		return readOnlyTransaction{}, err
 	}
+	return readOnlyTransaction{
+		ReadTransaction: tran,
+		keyer:           db.keyer,
+	}, nil
 }
 
 func (t readOnlyTransaction) close() {
 	t.Release()
 }
 
-func (t readOnlyTransaction) getFile(folder, device, file []byte) (protocol.FileInfo, bool) {
-	return t.getFileByKey(t.keyer.GenerateDeviceFileKey(nil, folder, device, file))
+func (t readOnlyTransaction) getFile(folder, device, file []byte) (protocol.FileInfo, bool, error) {
+	key, err := t.keyer.GenerateDeviceFileKey(nil, folder, device, file)
+	if err != nil {
+		return protocol.FileInfo{}, false, err
+	}
+	return t.getFileByKey(key)
 }
 
-func (t readOnlyTransaction) getFileByKey(key []byte) (protocol.FileInfo, bool) {
-	if f, ok := t.getFileTrunc(key, false); ok {
-		return f.(protocol.FileInfo), true
+func (t readOnlyTransaction) getFileByKey(key []byte) (protocol.FileInfo, bool, error) {
+	f, ok, err := t.getFileTrunc(key, false)
+	if err != nil || !ok {
+		return protocol.FileInfo{}, false, err
 	}
-	return protocol.FileInfo{}, false
+	return f.(protocol.FileInfo), true, nil
 }
 
-func (t readOnlyTransaction) getFileTrunc(key []byte, trunc bool) (FileIntf, bool) {
-	bs, err := t.Get(key, nil)
-	if err == leveldb.ErrNotFound {
-		return nil, false
+func (t readOnlyTransaction) getFileTrunc(key []byte, trunc bool) (FileIntf, bool, error) {
+	bs, err := t.Get(key)
+	if backend.IsNotFound(err) {
+		return nil, false, nil
 	}
 	if err != nil {
-		l.Debugln("surprise error:", err)
-		return nil, false
+		return nil, false, err
 	}
 	f, err := unmarshalTrunc(bs, trunc)
 	if err != nil {
-		l.Debugln("unmarshal error:", err)
-		return nil, false
+		return nil, false, err
 	}
-	return f, true
+	return f, true, nil
 }
 
-func (t readOnlyTransaction) getGlobal(keyBuf, folder, file []byte, truncate bool) ([]byte, FileIntf, bool) {
-	keyBuf = t.keyer.GenerateGlobalVersionKey(keyBuf, folder, file)
-
-	bs, err := t.Get(keyBuf, nil)
+func (t readOnlyTransaction) getGlobal(keyBuf, folder, file []byte, truncate bool) ([]byte, FileIntf, bool, error) {
+	var err error
+	keyBuf, err = t.keyer.GenerateGlobalVersionKey(keyBuf, folder, file)
 	if err != nil {
-		return keyBuf, nil, false
+		return nil, nil, false, err
+	}
+
+	bs, err := t.Get(keyBuf)
+	if backend.IsNotFound(err) {
+		return keyBuf, nil, false, nil
+	}
+	if err != nil {
+		return nil, nil, false, err
 	}
 
 	vl, ok := unmarshalVersionList(bs)
 	if !ok {
-		return keyBuf, nil, false
+		return keyBuf, nil, false, nil
 	}
 
-	keyBuf = t.keyer.GenerateDeviceFileKey(keyBuf, folder, vl.Versions[0].Device, file)
-	if fi, ok := t.getFileTrunc(keyBuf, truncate); ok {
-		return keyBuf, fi, true
+	keyBuf, err = t.keyer.GenerateDeviceFileKey(keyBuf, folder, vl.Versions[0].Device, file)
+	if err != nil {
+		return nil, nil, false, err
 	}
-
-	return keyBuf, nil, false
+	fi, ok, err := t.getFileTrunc(keyBuf, truncate)
+	if err != nil || !ok {
+		return keyBuf, nil, false, err
+	}
+	return keyBuf, fi, true, nil
 }
 
 // A readWriteTransaction is a readOnlyTransaction plus a batch for writes.
 // The batch will be committed on close() or by checkFlush() if it exceeds the
 // batch size.
 type readWriteTransaction struct {
+	backend.WriteTransaction
 	readOnlyTransaction
-	*batch
 }
 
-func (db *instance) newReadWriteTransaction() readWriteTransaction {
-	return readWriteTransaction{
-		readOnlyTransaction: db.newReadOnlyTransaction(),
-		batch:               db.newBatch(),
+func (db *instance) newReadWriteTransaction() (readWriteTransaction, error) {
+	tran, err := db.NewWriteTransaction()
+	if err != nil {
+		return readWriteTransaction{}, err
 	}
+	return readWriteTransaction{
+		WriteTransaction: tran,
+		readOnlyTransaction: readOnlyTransaction{
+			ReadTransaction: tran,
+			keyer:           db.keyer,
+		},
+	}, nil
+}
+
+func (t readWriteTransaction) commit() error {
+	t.readOnlyTransaction.close()
+	return t.WriteTransaction.Commit()
 }
 
 func (t readWriteTransaction) close() {
-	t.flush()
 	t.readOnlyTransaction.close()
+	t.WriteTransaction.Release()
 }
 
 // updateGlobal adds this device+version to the version list for the given
 // file. If the device is already present in the list, the version is updated.
 // If the file does not have an entry in the global list, it is created.
-func (t readWriteTransaction) updateGlobal(gk, keyBuf, folder, device []byte, file protocol.FileInfo, meta *metadataTracker) ([]byte, bool) {
+func (t readWriteTransaction) updateGlobal(gk, keyBuf, folder, device []byte, file protocol.FileInfo, meta *metadataTracker) ([]byte, bool, error) {
 	l.Debugf("update global; folder=%q device=%v file=%q version=%v invalid=%v", folder, protocol.DeviceIDFromBytes(device), file.Name, file.Version, file.IsInvalid())
 
 	var fl VersionList
-	if svl, err := t.Get(gk, nil); err == nil {
-		fl.Unmarshal(svl) // Ignore error, continue with empty fl
+	svl, err := t.Get(gk)
+	if err == nil {
+		_ = fl.Unmarshal(svl) // Ignore error, continue with empty fl
+	} else if !backend.IsNotFound(err) {
+		return nil, false, err
+	}
+
+	fl, removedFV, removedAt, insertedAt, err := fl.update(folder, device, file, t.readOnlyTransaction)
+	if err != nil {
+		return nil, false, err
 	}
-	fl, removedFV, removedAt, insertedAt := fl.update(folder, device, file, t.readOnlyTransaction)
 	if insertedAt == -1 {
 		l.Debugln("update global; same version, global unchanged")
-		return keyBuf, false
+		return keyBuf, false, nil
 	}
 
 	name := []byte(file.Name)
@@ -121,24 +156,29 @@ func (t readWriteTransaction) updateGlobal(gk, keyBuf, folder, device []byte, fi
 		// Inserted a new newest version
 		global = file
 	} else {
-		keyBuf = t.keyer.GenerateDeviceFileKey(keyBuf, folder, fl.Versions[0].Device, name)
-		if new, ok := t.getFileByKey(keyBuf); ok {
-			global = new
-		} else {
-			// This file must exist in the db, so this must be caused
-			// by the db being closed - bail out.
-			l.Debugln("File should exist:", name)
-			return keyBuf, false
+		keyBuf, err = t.keyer.GenerateDeviceFileKey(keyBuf, folder, fl.Versions[0].Device, name)
+		if err != nil {
+			return nil, false, err
 		}
+		new, ok, err := t.getFileByKey(keyBuf)
+		if err != nil || !ok {
+			return keyBuf, false, err
+		}
+		global = new
 	}
 
 	// Fixup the list of files we need.
-	keyBuf = t.updateLocalNeed(keyBuf, folder, name, fl, global)
+	keyBuf, err = t.updateLocalNeed(keyBuf, folder, name, fl, global)
+	if err != nil {
+		return nil, false, err
+	}
 
 	if removedAt != 0 && insertedAt != 0 {
 		l.Debugf(`new global for "%v" after update: %v`, file.Name, fl)
-		t.Put(gk, mustMarshal(&fl))
-		return keyBuf, true
+		if err := t.Put(gk, mustMarshal(&fl)); err != nil {
+			return nil, false, err
+		}
+		return keyBuf, true, nil
 	}
 
 	// Remove the old global from the global size counter
@@ -149,8 +189,15 @@ func (t readWriteTransaction) updateGlobal(gk, keyBuf, folder, device []byte, fi
 		// The previous newest version is now at index 1
 		oldGlobalFV = fl.Versions[1]
 	}
-	keyBuf = t.keyer.GenerateDeviceFileKey(keyBuf, folder, oldGlobalFV.Device, name)
-	if oldFile, ok := t.getFileByKey(keyBuf); ok {
+	keyBuf, err = t.keyer.GenerateDeviceFileKey(keyBuf, folder, oldGlobalFV.Device, name)
+	if err != nil {
+		return nil, false, err
+	}
+	oldFile, ok, err := t.getFileByKey(keyBuf)
+	if err != nil {
+		return nil, false, err
+	}
+	if ok {
 		// A failure to get the file here is surprising and our
 		// global size data will be incorrect until a restart...
 		meta.removeFile(protocol.GlobalDeviceID, oldFile)
@@ -160,27 +207,41 @@ func (t readWriteTransaction) updateGlobal(gk, keyBuf, folder, device []byte, fi
 	meta.addFile(protocol.GlobalDeviceID, global)
 
 	l.Debugf(`new global for "%v" after update: %v`, file.Name, fl)
-	t.Put(gk, mustMarshal(&fl))
+	if err := t.Put(gk, mustMarshal(&fl)); err != nil {
+		return nil, false, err
+	}
 
-	return keyBuf, true
+	return keyBuf, true, nil
 }
 
 // updateLocalNeed checks whether the given file is still needed on the local
 // device according to the version list and global FileInfo given and updates
 // the db accordingly.
-func (t readWriteTransaction) updateLocalNeed(keyBuf, folder, name []byte, fl VersionList, global protocol.FileInfo) []byte {
-	keyBuf = t.keyer.GenerateNeedFileKey(keyBuf, folder, name)
-	hasNeeded, _ := t.Has(keyBuf, nil)
+func (t readWriteTransaction) updateLocalNeed(keyBuf, folder, name []byte, fl VersionList, global protocol.FileInfo) ([]byte, error) {
+	var err error
+	keyBuf, err = t.keyer.GenerateNeedFileKey(keyBuf, folder, name)
+	if err != nil {
+		return nil, err
+	}
+	_, err = t.Get(keyBuf)
+	if err != nil && !backend.IsNotFound(err) {
+		return nil, err
+	}
+	hasNeeded := err == nil
 	if localFV, haveLocalFV := fl.Get(protocol.LocalDeviceID[:]); need(global, haveLocalFV, localFV.Version) {
 		if !hasNeeded {
 			l.Debugf("local need insert; folder=%q, name=%q", folder, name)
-			t.Put(keyBuf, nil)
+			if err := t.Put(keyBuf, nil); err != nil {
+				return nil, err
+			}
 		}
 	} else if hasNeeded {
 		l.Debugf("local need delete; folder=%q, name=%q", folder, name)
-		t.Delete(keyBuf)
+		if err := t.Delete(keyBuf); err != nil {
+			return nil, err
 		}
-	return keyBuf
+	}
+	return keyBuf, nil
 }
 
 func need(global FileIntf, haveLocal bool, localVersion protocol.Vector) bool {
@@ -202,71 +263,94 @@ func need(global FileIntf, haveLocal bool, localVersion protocol.Vector) bool {
 // removeFromGlobal removes the device from the global version list for the
 // given file. If the version list is empty after this, the file entry is
 // removed entirely.
-func (t readWriteTransaction) removeFromGlobal(gk, keyBuf, folder, device []byte, file []byte, meta *metadataTracker) []byte {
+func (t readWriteTransaction) removeFromGlobal(gk, keyBuf, folder, device []byte, file []byte, meta *metadataTracker) ([]byte, error) {
 	l.Debugf("remove from global; folder=%q device=%v file=%q", folder, protocol.DeviceIDFromBytes(device), file)
 
-	svl, err := t.Get(gk, nil)
-	if err != nil {
+	svl, err := t.Get(gk)
+	if backend.IsNotFound(err) {
 		// We might be called to "remove" a global version that doesn't exist
 		// if the first update for the file is already marked invalid.
-		return keyBuf
+		return keyBuf, nil
+	} else if err != nil {
+		return nil, err
 	}
 
 	var fl VersionList
 	err = fl.Unmarshal(svl)
 	if err != nil {
-		l.Debugln("unmarshal error:", err)
-		return keyBuf
+		return nil, err
 	}
 
 	fl, _, removedAt := fl.pop(device)
 	if removedAt == -1 {
 		// There is no version for the given device
-		return keyBuf
+		return keyBuf, nil
 	}
 
 	if removedAt == 0 {
 		// A failure to get the file here is surprising and our
 		// global size data will be incorrect until a restart...
-		keyBuf = t.keyer.GenerateDeviceFileKey(keyBuf, folder, device, file)
-		if f, ok := t.getFileByKey(keyBuf); ok {
+		keyBuf, err = t.keyer.GenerateDeviceFileKey(keyBuf, folder, device, file)
+		if err != nil {
+			return nil, err
+		}
+		if f, ok, err := t.getFileByKey(keyBuf); err != nil {
+			return keyBuf, nil
+		} else if ok {
 			meta.removeFile(protocol.GlobalDeviceID, f)
 		}
 	}
 
 	if len(fl.Versions) == 0 {
-		keyBuf = t.keyer.GenerateNeedFileKey(keyBuf, folder, file)
-		t.Delete(keyBuf)
-		t.Delete(gk)
-		return keyBuf
+		keyBuf, err = t.keyer.GenerateNeedFileKey(keyBuf, folder, file)
+		if err != nil {
+			return nil, err
+		}
+		if err := t.Delete(keyBuf); err != nil {
+			return nil, err
+		}
+		if err := t.Delete(gk); err != nil {
+			return nil, err
+		}
+		return keyBuf, nil
 	}
 
 	if removedAt == 0 {
-		keyBuf = t.keyer.GenerateDeviceFileKey(keyBuf, folder, fl.Versions[0].Device, file)
-		global, ok := t.getFileByKey(keyBuf)
-		if !ok {
-			// This file must exist in the db, so this must be caused
-			// by the db being closed - bail out.
-			l.Debugln("File should exist:", file)
-			return keyBuf
+		keyBuf, err = t.keyer.GenerateDeviceFileKey(keyBuf, folder, fl.Versions[0].Device, file)
+		if err != nil {
+			return nil, err
+		}
+		global, ok, err := t.getFileByKey(keyBuf)
+		if err != nil || !ok {
+			return keyBuf, err
+		}
+		keyBuf, err = t.updateLocalNeed(keyBuf, folder, file, fl, global)
+		if err != nil {
+			return nil, err
 		}
-		keyBuf = t.updateLocalNeed(keyBuf, folder, file, fl, global)
 		meta.addFile(protocol.GlobalDeviceID, global)
 	}
 
 	l.Debugf("new global after remove: %v", fl)
-	t.Put(gk, mustMarshal(&fl))
-
-	return keyBuf
+	if err := t.Put(gk, mustMarshal(&fl)); err != nil {
+		return nil, err
 	}
 
-func (t readWriteTransaction) deleteKeyPrefix(prefix []byte) {
-	dbi := t.NewIterator(util.BytesPrefix(prefix), nil)
+	return keyBuf, nil
+}
+
+func (t readWriteTransaction) deleteKeyPrefix(prefix []byte) error {
+	dbi, err := t.NewPrefixIterator(prefix)
+	if err != nil {
+		return err
+	}
+	defer dbi.Release()
 	for dbi.Next() {
-		t.Delete(dbi.Key())
-		t.checkFlush()
+		if err := t.Delete(dbi.Key()); err != nil {
+			return err
 		}
-	dbi.Release()
+	}
+	return dbi.Error()
 }
 
 type marshaller interface {

lib/db/util_test.go

@@ -11,22 +11,26 @@ import (
 	"io"
 	"os"
 
-	"github.com/syndtr/goleveldb/leveldb"
-	"github.com/syndtr/goleveldb/leveldb/storage"
-	"github.com/syndtr/goleveldb/leveldb/util"
+	"github.com/syncthing/syncthing/lib/db/backend"
 )
 
 // writeJSONS serializes the database to a JSON stream that can be checked
 // in to the repo and used for tests.
-func writeJSONS(w io.Writer, db *leveldb.DB) {
-	it := db.NewIterator(&util.Range{}, nil)
+func writeJSONS(w io.Writer, db backend.Backend) {
+	it, err := db.NewPrefixIterator(nil)
+	if err != nil {
+		panic(err)
+	}
 	defer it.Release()
 	enc := json.NewEncoder(w)
 	for it.Next() {
-		enc.Encode(map[string][]byte{
+		err := enc.Encode(map[string][]byte{
 			"k": it.Key(),
 			"v": it.Value(),
 		})
+		if err != nil {
+			panic(err)
+		}
 	}
 }
 
@@ -34,15 +38,15 @@ func writeJSONS(w io.Writer, db *leveldb.DB) {
 // here and the linter to not complain.
 var _ = writeJSONS
 
-// openJSONS reads a JSON stream file into a leveldb.DB
-func openJSONS(file string) (*leveldb.DB, error) {
+// openJSONS reads a JSON stream file into a backend DB
+func openJSONS(file string) (backend.Backend, error) {
 	fd, err := os.Open(file)
 	if err != nil {
 		return nil, err
 	}
 	dec := json.NewDecoder(fd)
 
-	db, _ := leveldb.Open(storage.NewMemStorage(), nil)
+	db := backend.OpenMemory()
 
 	for {
 		var row map[string][]byte
@@ -54,7 +58,9 @@ func openJSONS(file string) (*leveldb.DB, error) {
 			return nil, err
 		}
 
-		db.Put(row["k"], row["v"], nil)
+		if err := db.Put(row["k"], row["v"]); err != nil {
+			return nil, err
+		}
 	}
 
 	return db, nil

lib/fs/mtimefs.go

@@ -13,8 +13,8 @@ import (
 // The database is where we store the virtual mtimes
 type database interface {
 	Bytes(key string) (data []byte, ok bool)
-	PutBytes(key string, data []byte)
-	Delete(key string)
+	PutBytes(key string, data []byte) error
+	Delete(key string) error
 }
 
 // The MtimeFS is a filesystem with nanosecond mtime precision, regardless

lib/fs/mtimefs_test.go

@@ -236,8 +236,9 @@ func TestMtimeFSInsensitive(t *testing.T) {
 
 type mapStore map[string][]byte
 
-func (s mapStore) PutBytes(key string, data []byte) {
+func (s mapStore) PutBytes(key string, data []byte) error {
 	s[key] = data
+	return nil
 }
 
 func (s mapStore) Bytes(key string) (data []byte, ok bool) {
@@ -245,8 +246,9 @@ func (s mapStore) Bytes(key string) (data []byte, ok bool) {
 	return
 }
 
-func (s mapStore) Delete(key string) {
+func (s mapStore) Delete(key string) error {
 	delete(s, key)
+	return nil
 }
 
 // failChtimes does nothing, and fails

lib/model/folder_recvonly_test.go

@@ -16,6 +16,7 @@ import (
 
 	"github.com/syncthing/syncthing/lib/config"
 	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/protocol"
 	"github.com/syncthing/syncthing/lib/scanner"
@@ -316,7 +317,7 @@ func setupROFolder() (*model, *sendOnlyFolder) {
 	fcfg.Type = config.FolderTypeReceiveOnly
 	w.SetFolder(fcfg)
  
-	m := newModel(w, myID, "syncthing", "dev", db.OpenMemory(), nil)
+	m := newModel(w, myID, "syncthing", "dev", db.NewLowlevel(backend.OpenMemory()), nil)
 
 	m.ServeBackground()
 

lib/model/folder_sendrecv_test.go

@@ -20,6 +20,7 @@ import (
 
 	"github.com/syncthing/syncthing/lib/config"
 	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/events"
 	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/ignore"
@@ -91,7 +92,7 @@ func createFile(t *testing.T, name string, fs fs.Filesystem) protocol.FileInfo {
 
 func setupSendReceiveFolder(files ...protocol.FileInfo) (*model, *sendReceiveFolder) {
 	w := createTmpWrapper(defaultCfg)
-	model := newModel(w, myID, "syncthing", "dev", db.OpenMemory(), nil)
+	model := newModel(w, myID, "syncthing", "dev", db.NewLowlevel(backend.OpenMemory()), nil)
 	fcfg := testFolderConfigTmp()
 	model.addFolder(fcfg)
 

lib/model/model_test.go

@@ -27,6 +27,7 @@ import (
 
 	"github.com/syncthing/syncthing/lib/config"
 	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/events"
 	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/ignore"
@@ -306,7 +307,7 @@ func TestDeviceRename(t *testing.T) {
 	}
 	cfg := config.Wrap("testdata/tmpconfig.xml", rawCfg, events.NoopLogger)
 
-	db := db.OpenMemory()
+	db := db.NewLowlevel(backend.OpenMemory())
 	m := newModel(cfg, myID, "syncthing", "dev", db, nil)
 
 	if cfg.Devices()[device1].Name != "" {
@@ -402,7 +403,7 @@ func TestClusterConfig(t *testing.T) {
 		},
 	}
 
-	db := db.OpenMemory()
+	db := db.NewLowlevel(backend.OpenMemory())
 
 	wrapper := createTmpWrapper(cfg)
 	m := newModel(wrapper, myID, "syncthing", "dev", db, nil)
@@ -1533,7 +1534,7 @@ func waitForState(t *testing.T, m *model, folder, status string) {
 func TestROScanRecovery(t *testing.T) {
 	testOs := &fatalOs{t}
 
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 	set := db.NewFileSet("default", defaultFs, ldb)
 	set.Update(protocol.LocalDeviceID, []protocol.FileInfo{
 		{Name: "dummyfile", Version: protocol.Vector{Counters: []protocol.Counter{{ID: 42, Value: 1}}}},
@@ -1584,7 +1585,7 @@ func TestROScanRecovery(t *testing.T) {
 func TestRWScanRecovery(t *testing.T) {
 	testOs := &fatalOs{t}
 
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 	set := db.NewFileSet("default", defaultFs, ldb)
 	set.Update(protocol.LocalDeviceID, []protocol.FileInfo{
 		{Name: "dummyfile", Version: protocol.Vector{Counters: []protocol.Counter{{ID: 42, Value: 1}}}},
@@ -1633,7 +1634,7 @@ func TestRWScanRecovery(t *testing.T) {
 }
 
 func TestGlobalDirectoryTree(t *testing.T) {
-	db := db.OpenMemory()
+	db := db.NewLowlevel(backend.OpenMemory())
 	m := newModel(defaultCfgWrapper, myID, "syncthing", "dev", db, nil)
 	m.ServeBackground()
 	m.removeFolder(defaultFolderConfig)
@@ -1886,7 +1887,7 @@ func TestGlobalDirectoryTree(t *testing.T) {
 }
 
 func TestGlobalDirectorySelfFixing(t *testing.T) {
-	db := db.OpenMemory()
+	db := db.NewLowlevel(backend.OpenMemory())
 	m := newModel(defaultCfgWrapper, myID, "syncthing", "dev", db, nil)
 	m.ServeBackground()
 	m.removeFolder(defaultFolderConfig)
@@ -2063,7 +2064,7 @@ func BenchmarkTree_100_10(b *testing.B) {
 }
 
 func benchmarkTree(b *testing.B, n1, n2 int) {
-	db := db.OpenMemory()
+	db := db.NewLowlevel(backend.OpenMemory())
 	m := newModel(defaultCfgWrapper, myID, "syncthing", "dev", db, nil)
 	m.ServeBackground()
 	m.removeFolder(defaultFolderConfig)
@@ -2128,7 +2129,7 @@ func TestIssue3028(t *testing.T) {
 }
 
 func TestIssue4357(t *testing.T) {
-	db := db.OpenMemory()
+	db := db.NewLowlevel(backend.OpenMemory())
 	cfg := defaultCfgWrapper.RawCopy()
 	// Create a separate wrapper not to pollute other tests.
 	wrapper := createTmpWrapper(config.Configuration{})
@@ -2251,7 +2252,7 @@ func TestIssue2782(t *testing.T) {
 }
 
 func TestIndexesForUnknownDevicesDropped(t *testing.T) {
-	dbi := db.OpenMemory()
+	dbi := db.NewLowlevel(backend.OpenMemory())
 
 	files := db.NewFileSet("default", defaultFs, dbi)
 	files.Drop(device1)
@@ -2677,7 +2678,7 @@ func TestInternalScan(t *testing.T) {
 func TestCustomMarkerName(t *testing.T) {
 	testOs := &fatalOs{t}
 
-	ldb := db.OpenMemory()
+	ldb := db.NewLowlevel(backend.OpenMemory())
 	set := db.NewFileSet("default", defaultFs, ldb)
 	set.Update(protocol.LocalDeviceID, []protocol.FileInfo{
 		{Name: "dummyfile"},
@@ -3052,7 +3053,7 @@ func TestPausedFolders(t *testing.T) {
 func TestIssue4094(t *testing.T) {
 	testOs := &fatalOs{t}
 
-	db := db.OpenMemory()
+	db := db.NewLowlevel(backend.OpenMemory())
 	// Create a separate wrapper not to pollute other tests.
 	wrapper := createTmpWrapper(config.Configuration{})
 	m := newModel(wrapper, myID, "syncthing", "dev", db, nil)
@@ -3088,7 +3089,7 @@ func TestIssue4094(t *testing.T) {
 func TestIssue4903(t *testing.T) {
 	testOs := &fatalOs{t}
 
-	db := db.OpenMemory()
+	db := db.NewLowlevel(backend.OpenMemory())
 	// Create a separate wrapper not to pollute other tests.
 	wrapper := createTmpWrapper(config.Configuration{})
 	m := newModel(wrapper, myID, "syncthing", "dev", db, nil)

lib/model/testutils_test.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/syncthing/syncthing/lib/config"
 	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/events"
 	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/protocol"
@@ -102,7 +103,7 @@ func setupModelWithConnectionFromWrapper(w config.Wrapper) (*model, *fakeConnect
 }
 
 func setupModel(w config.Wrapper) *model {
-	db := db.OpenMemory()
+	db := db.NewLowlevel(backend.OpenMemory())
 	m := newModel(w, myID, "syncthing", "dev", db, nil)
 	m.ServeBackground()
 

lib/syncthing/utils.go

@@ -17,6 +17,7 @@ import (
 
 	"github.com/syncthing/syncthing/lib/config"
 	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/events"
 	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/locations"
@@ -124,5 +125,9 @@ func copyFile(src, dst string) error {
 }
 
 func OpenGoleveldb(path string, tuning config.Tuning) (*db.Lowlevel, error) {
-	return db.Open(path, db.Tuning(tuning))
+	ldb, err := backend.Open(path, backend.Tuning(tuning))
+	if err != nil {
+		return nil, err
+	}
+	return db.NewLowlevel(ldb), nil
 }