Add cache

This commits adds rudimentary support for a cache directory, enabled by
default. The cache directory is created if it does not exist. The cache
is used if there's anything in it, newly created snapshot and index
files are written to the cache automatically.

cmd/restic/global.go

@@ -19,6 +19,7 @@ import (
 	"github.com/restic/restic/internal/backend/s3"
 	"github.com/restic/restic/internal/backend/sftp"
 	"github.com/restic/restic/internal/backend/swift"
+	"github.com/restic/restic/internal/cache"
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/options"
 	"github.com/restic/restic/internal/repository"
@@ -38,6 +39,8 @@ type GlobalOptions struct {
 	Quiet        bool
 	NoLock       bool
 	JSON         bool
+	CacheDir     string
+	NoCache      bool
 
 	ctx      context.Context
 	password string
@@ -68,7 +71,8 @@ func init() {
 	f.BoolVarP(&globalOptions.Quiet, "quiet", "q", false, "do not output comprehensive progress report")
 	f.BoolVar(&globalOptions.NoLock, "no-lock", false, "do not lock the repo, this allows some operations on read-only repos")
 	f.BoolVarP(&globalOptions.JSON, "json", "", false, "set output mode to JSON for commands that support it")
-
+	f.StringVar(&globalOptions.CacheDir, "cache-dir", "", "set the cache directory")
+	f.BoolVar(&globalOptions.NoCache, "no-cache", false, "do not use a local cache")
 	f.StringSliceVarP(&globalOptions.Options, "option", "o", []string{}, "set extended option (`key=value`, can be specified multiple times)")
 
 	restoreTerminal()
@@ -322,6 +326,17 @@ func OpenRepository(opts GlobalOptions) (*repository.Repository, error) {
 		return nil, err
 	}
 
+	if opts.NoCache {
+		return s, nil
+	}
+
+	cache, err := cache.New(s.Config().ID, opts.CacheDir)
+	if err != nil {
+		Warnf("unable to open cache: %v\n", err)
+	} else {
+		s.UseCache(cache)
+	}
+
 	return s, nil
 }
 

cmd/restic/integration_helpers_test.go

@@ -199,6 +199,7 @@ func withTestEnvironment(t testing.TB) (env *testEnvironment, cleanup func()) {
 	env.gopts = GlobalOptions{
 		Repo:     env.repo,
 		Quiet:    true,
+		CacheDir: env.cache,
 		ctx:      context.Background(),
 		password: TestPassword,
 		stdout:   os.Stdout,

doc/cache.rst

@@ -0,0 +1,26 @@
+Local Cache
+===========
+
+In order to speed up certain operations, restic manages a local cache of data.
+This document describes the data structures for the local cache with version 1.
+
+Versions
+--------
+
+The cache directory is selected according to the `XDG base dir specification
+<http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html>`__.
+Each repository has its own cache sub-directory, consting of the repository ID
+which is chosen at ``init``. All cache directories for different repos are
+independent of each other.
+
+The cache dir for a repo contains a file named ``version``, which contains a
+single ASCII integer line that stands for the current version of the cache. If
+a lower version number is found the cache is recreated with the current
+version. If a higher version number is found the cache is ignored and left as
+is.
+
+Snapshots and Indexes
+---------------------
+
+Snapshot, Data and Index files are cached in the sub-directories ``snapshots``,
+``data`` and  ``index``, as read from the repository.

doc/manual.rst

@@ -1240,3 +1240,19 @@ instead of the default, set the environment variable like this:
 
     $ export TMPDIR=/var/tmp/restic-tmp
     $ restic -r /tmp/backup backup ~/work
+
+Caching
+-------
+
+Restic keeps a cache with some files from the repository on the local machine.
+This allows faster operations, since meta data does not need to be loaded from
+a remote repository. The cache is automatically created, usually in the
+directory ``.cache/restic`` in the user's home directory. The environment
+variable ``XDG_CACHE_DIR`` or the command line parameter ``--cache-dir`` can
+each be used to specify where the cache is located. The parameter
+``--no-cache`` disables the cache entirely. In this case, all data is loaded
+from the repo.
+
+The cache is ephemeral: When a file cannot be read from the cache, it is loaded
+from the repository.
+

doc/references.rst

@@ -7,3 +7,7 @@ References
 ------------------------
 
 .. include:: rest_backend.rst
+
+------------------------
+
+.. include:: cache.rst

internal/cache/backend.go

@@ -0,0 +1,152 @@
+package cache
+
+import (
+	"context"
+	"io"
+
+	"github.com/restic/restic/internal/debug"
+	"github.com/restic/restic/internal/restic"
+)
+
+// Backend wraps a restic.Backend and adds a cache.
+type Backend struct {
+	restic.Backend
+	restic.Cache
+}
+
+// ensure cachedBackend implements restic.Backend
+var _ restic.Backend = &Backend{}
+
+// Remove deletes a file from the backend and the cache if it has been cached.
+func (b *Backend) Remove(ctx context.Context, h restic.Handle) error {
+	debug.Log("cache Remove(%v)", h)
+	err := b.Backend.Remove(ctx, h)
+	if err != nil {
+		return err
+	}
+
+	return b.Cache.Remove(h)
+}
+
+type teeReader struct {
+	rd  io.Reader
+	wr  io.Writer
+	err error
+}
+
+func (t *teeReader) Read(p []byte) (n int, err error) {
+	n, err = t.rd.Read(p)
+	if t.err == nil && n > 0 {
+		_, t.err = t.wr.Write(p[:n])
+	}
+
+	return n, err
+}
+
+var autoCacheTypes = map[restic.FileType]struct{}{
+	restic.IndexFile:    struct{}{},
+	restic.SnapshotFile: struct{}{},
+}
+
+// Save stores a new file is the backend and the cache.
+func (b *Backend) Save(ctx context.Context, h restic.Handle, rd io.Reader) (err error) {
+	debug.Log("cache Save(%v)", h)
+	if _, ok := autoCacheTypes[h.Type]; !ok {
+		return b.Backend.Save(ctx, h, rd)
+	}
+
+	wr, err := b.Cache.SaveWriter(h)
+	if err != nil {
+		debug.Log("unable to save object to cache: %v", err)
+		return b.Backend.Save(ctx, h, rd)
+	}
+
+	tr := &teeReader{rd: rd, wr: wr}
+	err = b.Backend.Save(ctx, h, tr)
+	if err != nil {
+		wr.Close()
+		b.Cache.Remove(h)
+		return err
+	}
+
+	err = wr.Close()
+	if err != nil {
+		debug.Log("cache writer returned error: %v", err)
+		b.Cache.Remove(h)
+	}
+	return nil
+}
+
+var autoCacheFiles = map[restic.FileType]bool{
+	restic.IndexFile:    true,
+	restic.SnapshotFile: true,
+}
+
+// Load loads a file from the cache or the backend.
+func (b *Backend) Load(ctx context.Context, h restic.Handle, length int, offset int64) (io.ReadCloser, error) {
+	if b.Cache.Has(h) {
+		debug.Log("Load(%v, %v, %v) from cache", h, length, offset)
+		return b.Cache.Load(h, length, offset)
+	}
+
+	debug.Log("Load(%v, %v, %v) delegated to backend", h, length, offset)
+	rd, err := b.Backend.Load(ctx, h, length, offset)
+	if err != nil {
+		if b.Backend.IsNotExist(err) {
+			// try to remove from the cache, ignore errors
+			_ = b.Cache.Remove(h)
+		}
+
+		return nil, err
+	}
+
+	// only cache complete files
+	if offset != 0 || length != 0 {
+		debug.Log("won't store partial file %v", h)
+		return rd, err
+	}
+
+	if _, ok := autoCacheFiles[h.Type]; !ok {
+		debug.Log("wrong type for auto store %v", h)
+		return rd, nil
+	}
+
+	debug.Log("auto-store %v in the cache", h)
+
+	// cache the file, then return cached copy
+	if err = b.Cache.Save(h, rd); err != nil {
+		return nil, err
+	}
+
+	if err = rd.Close(); err != nil {
+		// try to remove from the cache, ignore errors
+		_ = b.Cache.Remove(h)
+		return nil, err
+	}
+
+	// load from the cache and save in the backend
+	return b.Cache.Load(h, 0, 0)
+}
+
+// Stat tests whether the backend has a file. If it does not exist but still
+// exists in the cache, it is removed from the cache.
+func (b *Backend) Stat(ctx context.Context, h restic.Handle) (restic.FileInfo, error) {
+	debug.Log("cache Stat(%v)", h)
+
+	fi, err := b.Backend.Stat(ctx, h)
+	if err != nil {
+		if b.Backend.IsNotExist(err) {
+			// try to remove from the cache, ignore errors
+			_ = b.Cache.Remove(h)
+		}
+
+		return fi, err
+	}
+
+	return fi, err
+}
+
+// IsNotExist returns true if the error is caused by a non-existing file.
+func (b *Backend) IsNotExist(err error) bool {
+	return b.Backend.IsNotExist(err)
+}

internal/cache/backend_test.go

@@ -0,0 +1,114 @@
+package cache
+
+import (
+	"bytes"
+	"context"
+	"math/rand"
+	"testing"
+
+	"github.com/restic/restic/internal/backend"
+	"github.com/restic/restic/internal/backend/mem"
+	"github.com/restic/restic/internal/restic"
+	"github.com/restic/restic/internal/test"
+)
+
+func loadAndCompare(t testing.TB, be restic.Backend, h restic.Handle, data []byte) {
+	buf, err := backend.LoadAll(context.TODO(), be, h)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(buf) != len(data) {
+		t.Fatalf("wrong number of bytes read, want %v, got %v", len(data), len(buf))
+	}
+
+	if !bytes.Equal(buf, data) {
+		t.Fatalf("wrong data returned, want:\n  %02x\ngot:\n  %02x", data[:16], buf[:16])
+	}
+}
+
+func save(t testing.TB, be restic.Backend, h restic.Handle, data []byte) {
+	err := be.Save(context.TODO(), h, bytes.NewReader(data))
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func remove(t testing.TB, be restic.Backend, h restic.Handle) {
+	err := be.Remove(context.TODO(), h)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func randomData(n int) (restic.Handle, []byte) {
+	data := test.Random(rand.Int(), n)
+	id := restic.Hash(data)
+	copy(id[:], data)
+	h := restic.Handle{
+		Type: restic.IndexFile,
+		Name: id.String(),
+	}
+	return h, data
+}
+
+func TestBackend(t *testing.T) {
+	be := mem.New()
+
+	c, cleanup := TestNewCache(t)
+	defer cleanup()
+
+	wbe := c.Wrap(be)
+
+	h, data := randomData(5234142)
+
+	// save directly in backend
+	save(t, be, h, data)
+	if c.Has(h) {
+		t.Errorf("cache has file too early")
+	}
+
+	// load data via cache
+	loadAndCompare(t, wbe, h, data)
+	if !c.Has(h) {
+		t.Errorf("cache dosen't have file after load")
+	}
+
+	// remove via cache
+	remove(t, wbe, h)
+	if c.Has(h) {
+		t.Errorf("cache has file after remove")
+	}
+
+	// save via cache
+	save(t, wbe, h, data)
+	if !c.Has(h) {
+		t.Errorf("cache dosen't have file after load")
+	}
+
+	// load data directly from backend
+	loadAndCompare(t, be, h, data)
+
+	// load data via cache
+	loadAndCompare(t, be, h, data)
+
+	// remove directly
+	remove(t, be, h)
+	if !c.Has(h) {
+		t.Errorf("file not in cache any more")
+	}
+
+	// run stat
+	_, err := wbe.Stat(context.TODO(), h)
+	if err == nil {
+		t.Errorf("expected error for removed file not found, got nil")
+	}
+
+	if !wbe.IsNotExist(err) {
+		t.Errorf("Stat() returned error that does not match IsNotExist(): %v", err)
+	}
+
+	if c.Has(h) {
+		t.Errorf("removed file still in cache after stat")
+	}
+}

internal/cache/cache.go

@@ -0,0 +1,122 @@
+package cache
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strconv"
+
+	"github.com/pkg/errors"
+	"github.com/restic/restic/internal/debug"
+	"github.com/restic/restic/internal/fs"
+	"github.com/restic/restic/internal/restic"
+)
+
+// Cache manages a local cache.
+type Cache struct {
+	Path string
+}
+
+const dirMode = 0700
+const fileMode = 0600
+
+func readVersion(dir string) (v uint, err error) {
+	buf, err := ioutil.ReadFile(filepath.Join(dir, "version"))
+	if os.IsNotExist(err) {
+		return 0, nil
+	}
+
+	if err != nil {
+		return 0, errors.Wrap(err, "ReadFile")
+	}
+
+	ver, err := strconv.ParseUint(string(buf), 10, 32)
+	if err != nil {
+		return 0, errors.Wrap(err, "ParseUint")
+	}
+
+	return uint(ver), nil
+}
+
+const cacheVersion = 1
+
+// ensure Cache implements restic.Cache
+var _ restic.Cache = &Cache{}
+
+var cacheLayoutPaths = map[restic.FileType]string{
+	restic.DataFile:     "data",
+	restic.SnapshotFile: "snapshots",
+	restic.IndexFile:    "index",
+}
+
+// New returns a new cache for the repo ID at dir. If dir is the empty string,
+// the default cache location (according to the XDG standard) is used.
+func New(id string, dir string) (c *Cache, err error) {
+	if dir == "" {
+		dir, err = getXDGCacheDir()
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	cachedir := filepath.Join(dir, id)
+	debug.Log("using cache dir %v", cachedir)
+
+	v, err := readVersion(cachedir)
+	if err != nil {
+		return nil, err
+	}
+
+	if v > cacheVersion {
+		return nil, errors.New("cache version is newer")
+	}
+
+	// create the repo cache dir if it does not exist yet
+	if err = fs.MkdirAll(cachedir, dirMode); err != nil {
+		return nil, err
+	}
+
+	if v < cacheVersion {
+		err = ioutil.WriteFile(filepath.Join(cachedir, "version"), []byte(fmt.Sprintf("%d", cacheVersion)), 0644)
+		if err != nil {
+			return nil, errors.Wrap(err, "WriteFile")
+		}
+	}
+
+	for _, p := range cacheLayoutPaths {
+		if err = fs.MkdirAll(filepath.Join(cachedir, p), dirMode); err != nil {
+			return nil, err
+		}
+	}
+
+	c = &Cache{
+		Path: cachedir,
+	}
+
+	return c, nil
+}
+
+// errNoSuchFile is returned when a file is not cached.
+type errNoSuchFile struct {
+	Type string
+	Name string
+}
+
+func (e errNoSuchFile) Error() string {
+	return fmt.Sprintf("file %v (%v) is not cached", e.Name, e.Type)
+}
+
+// IsNotExist returns true if the error was caused by a non-existing file.
+func (c *Cache) IsNotExist(err error) bool {
+	_, ok := errors.Cause(err).(errNoSuchFile)
+	return ok
+}
+
+// Wrap returns a backend with a cache.
+func (c *Cache) Wrap(be restic.Backend) restic.Backend {
+	return &Backend{
+		Backend: be,
+		Cache:   c,
+	}
+}

internal/cache/dir.go

@@ -0,0 +1,49 @@
+package cache
+
+import (
+	"os"
+	"path/filepath"
+
+	"github.com/pkg/errors"
+	"github.com/restic/restic/internal/debug"
+	"github.com/restic/restic/internal/fs"
+)
+
+// getXDGCacheDir returns the cache directory according to XDG basedir spec, see
+// http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
+func getXDGCacheDir() (string, error) {
+	xdgcache := os.Getenv("XDG_CACHE_HOME")
+	home := os.Getenv("HOME")
+
+	if xdgcache == "" && home == "" {
+		return "", errors.New("unable to locate cache directory (XDG_CACHE_HOME and HOME unset)")
+	}
+
+	cachedir := ""
+	if xdgcache != "" {
+		cachedir = filepath.Join(xdgcache, "restic")
+	} else if home != "" {
+		cachedir = filepath.Join(home, ".cache", "restic")
+	}
+
+	fi, err := fs.Stat(cachedir)
+	if os.IsNotExist(errors.Cause(err)) {
+		err = fs.MkdirAll(cachedir, 0700)
+		if err != nil {
+			return "", errors.Wrap(err, "MkdirAll")
+		}
+
+		fi, err = fs.Stat(cachedir)
+		debug.Log("create cache dir %v", cachedir)
+	}
+
+	if err != nil {
+		return "", errors.Wrap(err, "Stat")
+	}
+
+	if !fi.IsDir() {
+		return "", errors.Errorf("cache dir %v is not a directory", cachedir)
+	}
+
+	return cachedir, nil
+}

internal/cache/file.go

@@ -0,0 +1,194 @@
+package cache
+
+import (
+	"io"
+	"os"
+	"path/filepath"
+
+	"github.com/pkg/errors"
+	"github.com/restic/restic/internal/debug"
+	"github.com/restic/restic/internal/fs"
+	"github.com/restic/restic/internal/restic"
+)
+
+func (c *Cache) filename(h restic.Handle) string {
+	if len(h.Name) < 2 {
+		panic("Name is empty or too short")
+	}
+	subdir := h.Name[:2]
+	return filepath.Join(c.Path, cacheLayoutPaths[h.Type], subdir, h.Name)
+}
+
+func (c *Cache) canBeCached(t restic.FileType) bool {
+	if c == nil {
+		return false
+	}
+
+	if _, ok := cacheLayoutPaths[t]; !ok {
+		return false
+	}
+
+	return true
+}
+
+type readCloser struct {
+	io.Reader
+	io.Closer
+}
+
+// Load returns a reader that yields the contents of the file with the
+// given handle. rd must be closed after use. If an error is returned, the
+// ReadCloser is nil.
+func (c *Cache) Load(h restic.Handle, length int, offset int64) (io.ReadCloser, error) {
+	debug.Log("Load from cache: %v", h)
+	if !c.canBeCached(h.Type) {
+		return nil, errors.New("cannot be cached")
+	}
+
+	f, err := fs.Open(c.filename(h))
+	if err != nil {
+		return nil, errors.Wrap(err, "Open")
+	}
+
+	if offset > 0 {
+		if _, err = f.Seek(offset, io.SeekStart); err != nil {
+			f.Close()
+			return nil, err
+		}
+	}
+
+	rd := readCloser{Reader: f, Closer: f}
+	if length > 0 {
+		rd.Reader = io.LimitReader(f, int64(length))
+	}
+
+	return rd, nil
+}
+
+// SaveWriter returns a writer for the cache object h. It must be closed after writing is finished.
+func (c *Cache) SaveWriter(h restic.Handle) (io.WriteCloser, error) {
+	debug.Log("Save to cache: %v", h)
+	if !c.canBeCached(h.Type) {
+		return nil, errors.New("cannot be cached")
+	}
+
+	p := c.filename(h)
+	err := fs.MkdirAll(filepath.Dir(p), 0700)
+	if err != nil {
+		return nil, errors.Wrap(err, "MkdirAll")
+	}
+
+	f, err := fs.OpenFile(p, os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0400)
+	if err != nil {
+		return nil, errors.Wrap(err, "Create")
+	}
+
+	return f, err
+}
+
+// Save saves a file in the cache.
+func (c *Cache) Save(h restic.Handle, rd io.Reader) error {
+	debug.Log("Save to cache: %v", h)
+	if rd == nil {
+		return errors.New("Save() called with nil reader")
+	}
+
+	f, err := c.SaveWriter(h)
+	if err != nil {
+		return err
+	}
+
+	if _, err = io.Copy(f, rd); err != nil {
+		_ = f.Close()
+		_ = c.Remove(h)
+		return errors.Wrap(err, "Copy")
+	}
+
+	if err = f.Close(); err != nil {
+		return errors.Wrap(err, "Close")
+	}
+
+	return nil
+}
+
+// Remove deletes a file. When the file is not cache, no error is returned.
+func (c *Cache) Remove(h restic.Handle) error {
+	if !c.Has(h) {
+		return nil
+	}
+
+	return fs.Remove(c.filename(h))
+}
+
+// Clear removes all files of type t from the cache that are not contained in
+// the set valid.
+func (c *Cache) Clear(t restic.FileType, valid restic.IDSet) error {
+	debug.Log("Clearing cache for %v: %v valid files", t, len(valid))
+	if !c.canBeCached(t) {
+		return nil
+	}
+
+	list, err := c.list(t)
+	if err != nil {
+		return err
+	}
+
+	for id := range list {
+		if valid.Has(id) {
+			continue
+		}
+
+		if err = fs.Remove(c.filename(restic.Handle{Type: t, Name: id.String()})); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func isFile(fi os.FileInfo) bool {
+	return fi.Mode()&(os.ModeType|os.ModeCharDevice) == 0
+}
+
+// list returns a list of all files of type T in the cache.
+func (c *Cache) list(t restic.FileType) (restic.IDSet, error) {
+	if !c.canBeCached(t) {
+		return nil, errors.New("cannot be cached")
+	}
+
+	list := restic.NewIDSet()
+	dir := filepath.Join(c.Path, cacheLayoutPaths[t])
+	err := filepath.Walk(dir, func(name string, fi os.FileInfo, err error) error {
+		if err != nil {
+			return errors.Wrap(err, "Walk")
+		}
+
+		if !isFile(fi) {
+			return nil
+		}
+
+		id, err := restic.ParseID(filepath.Base(name))
+		if err != nil {
+			return nil
+		}
+
+		list.Insert(id)
+		return nil
+	})
+
+	return list, err
+}
+
+// Has returns true if the file is cached.
+func (c *Cache) Has(h restic.Handle) bool {
+	if !c.canBeCached(h.Type) {
+		return false
+	}
+
+	_, err := fs.Stat(c.filename(h))
+	if err == nil {
+		return true
+	}
+
+	return false
+}

internal/cache/file_test.go

@@ -0,0 +1,259 @@
+package cache
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"math/rand"
+	"testing"
+	"time"
+
+	"github.com/restic/restic/internal/restic"
+	"github.com/restic/restic/internal/test"
+)
+
+func generateRandomFiles(t testing.TB, tpe restic.FileType, c *Cache) restic.IDSet {
+	ids := restic.NewIDSet()
+	for i := 0; i < rand.Intn(15)+10; i++ {
+		buf := test.Random(rand.Int(), 1<<19)
+		id := restic.Hash(buf)
+		h := restic.Handle{Type: tpe, Name: id.String()}
+
+		if c.Has(h) {
+			t.Errorf("index %v present before save", id)
+		}
+
+		err := c.Save(h, bytes.NewReader(buf))
+		if err != nil {
+			t.Fatal(err)
+		}
+		ids.Insert(id)
+	}
+	return ids
+}
+
+// randomID returns a random ID from s.
+func randomID(s restic.IDSet) restic.ID {
+	for id := range s {
+		return id
+	}
+	panic("set is empty")
+}
+
+func load(t testing.TB, c *Cache, h restic.Handle) []byte {
+	rd, err := c.Load(h, 0, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if rd == nil {
+		t.Fatalf("Load() returned nil reader")
+	}
+
+	buf, err := ioutil.ReadAll(rd)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err = rd.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	return buf
+}
+
+func listFiles(t testing.TB, c *Cache, tpe restic.FileType) restic.IDSet {
+	list, err := c.list(tpe)
+	if err != nil {
+		t.Errorf("listing failed: %v", err)
+	}
+
+	return list
+}
+
+func clearFiles(t testing.TB, c *Cache, tpe restic.FileType, valid restic.IDSet) {
+	if err := c.Clear(tpe, valid); err != nil {
+		t.Error(err)
+	}
+}
+
+func TestFiles(t *testing.T) {
+	seed := time.Now().Unix()
+	t.Logf("seed is %v", seed)
+	rand.Seed(seed)
+
+	c, cleanup := TestNewCache(t)
+	defer cleanup()
+
+	var tests = []restic.FileType{
+		restic.SnapshotFile,
+		restic.DataFile,
+		restic.IndexFile,
+	}
+
+	for _, tpe := range tests {
+		t.Run(fmt.Sprintf("%v", tpe), func(t *testing.T) {
+			ids := generateRandomFiles(t, tpe, c)
+			id := randomID(ids)
+
+			h := restic.Handle{Type: tpe, Name: id.String()}
+			id2 := restic.Hash(load(t, c, h))
+
+			if !id.Equal(id2) {
+				t.Errorf("wrong data returned, want %v, got %v", id.Str(), id2.Str())
+			}
+
+			if !c.Has(h) {
+				t.Errorf("cache thinks index %v isn't present", id.Str())
+			}
+
+			list := listFiles(t, c, tpe)
+			if !ids.Equals(list) {
+				t.Errorf("wrong list of index IDs returned, want:\n  %v\ngot:\n  %v", ids, list)
+			}
+
+			clearFiles(t, c, tpe, restic.NewIDSet(id))
+			list2 := listFiles(t, c, tpe)
+			ids.Delete(id)
+			want := restic.NewIDSet(id)
+			if !list2.Equals(want) {
+				t.Errorf("ClearIndexes removed indexes, want:\n  %v\ngot:\n  %v", list2, want)
+			}
+
+			clearFiles(t, c, tpe, restic.NewIDSet())
+			want = restic.NewIDSet()
+			list3 := listFiles(t, c, tpe)
+			if !list3.Equals(want) {
+				t.Errorf("ClearIndexes returned a wrong list, want:\n  %v\ngot:\n  %v", want, list3)
+			}
+		})
+	}
+}
+
+func TestFileSaveWriter(t *testing.T) {
+	seed := time.Now().Unix()
+	t.Logf("seed is %v", seed)
+	rand.Seed(seed)
+
+	c, cleanup := TestNewCache(t)
+	defer cleanup()
+
+	// save about 5 MiB of data in the cache
+	data := test.Random(rand.Int(), 5234142)
+	id := restic.ID{}
+	copy(id[:], data)
+	h := restic.Handle{
+		Type: restic.DataFile,
+		Name: id.String(),
+	}
+
+	wr, err := c.SaveWriter(h)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	n, err := io.Copy(wr, bytes.NewReader(data))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if n != int64(len(data)) {
+		t.Fatalf("wrong number of bytes written, want %v, got %v", len(data), n)
+	}
+
+	if err = wr.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	rd, err := c.Load(h, 0, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	buf, err := ioutil.ReadAll(rd)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(buf) != len(data) {
+		t.Fatalf("wrong number of bytes read, want %v, got %v", len(data), len(buf))
+	}
+
+	if !bytes.Equal(buf, data) {
+		t.Fatalf("wrong data returned, want:\n  %02x\ngot:\n  %02x", data[:16], buf[:16])
+	}
+
+	if err = rd.Close(); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestFileLoad(t *testing.T) {
+	seed := time.Now().Unix()
+	t.Logf("seed is %v", seed)
+	rand.Seed(seed)
+
+	c, cleanup := TestNewCache(t)
+	defer cleanup()
+
+	// save about 5 MiB of data in the cache
+	data := test.Random(rand.Int(), 5234142)
+	id := restic.ID{}
+	copy(id[:], data)
+	h := restic.Handle{
+		Type: restic.DataFile,
+		Name: id.String(),
+	}
+	if err := c.Save(h, bytes.NewReader(data)); err != nil {
+		t.Fatalf("Save() returned error: %v", err)
+	}
+
+	var tests = []struct {
+		offset int64
+		length int
+	}{
+		{0, 0},
+		{5, 0},
+		{32*1024 + 5, 0},
+		{0, 123},
+		{0, 64*1024 + 234},
+		{100, 5234142},
+	}
+
+	for _, test := range tests {
+		t.Run(fmt.Sprintf("%v/%v", test.length, test.offset), func(t *testing.T) {
+			rd, err := c.Load(h, test.length, test.offset)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			buf, err := ioutil.ReadAll(rd)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			if err = rd.Close(); err != nil {
+				t.Fatal(err)
+			}
+
+			o := int(test.offset)
+			l := test.length
+			if test.length == 0 {
+				l = len(data) - o
+			}
+
+			if l > len(data)-o {
+				l = len(data) - o
+			}
+
+			if len(buf) != l {
+				t.Fatalf("wrong number of bytes returned: want %d, got %d", l, len(buf))
+			}
+
+			if !bytes.Equal(buf, data[o:o+l]) {
+				t.Fatalf("wrong data returned, want:\n  %02x\ngot:\n  %02x", data[o:o+16], buf[:16])
+			}
+		})
+	}
+}

internal/cache/testing.go

@@ -0,0 +1,20 @@
+package cache
+
+import (
+	"testing"
+
+	"github.com/restic/restic/internal/restic"
+	"github.com/restic/restic/internal/test"
+)
+
+// TestNewCache returns a cache in a temporary directory which is removed when
+// cleanup is called.
+func TestNewCache(t testing.TB) (*Cache, func()) {
+	dir, cleanup := test.TempDir(t)
+	t.Logf("created new cache at %v", dir)
+	cache, err := New(restic.NewRandomID().String(), dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return cache, cleanup
+}

internal/repository/repository.go

@@ -23,6 +23,7 @@ type Repository struct {
 	key     *crypto.Key
 	keyName string
 	idx     *MasterIndex
+	restic.Cache
 
 	treePM *packerManager
 	dataPM *packerManager
@@ -45,6 +46,16 @@ func (r *Repository) Config() restic.Config {
 	return r.cfg
 }
 
+// UseCache replaces the backend with the wrapped cache.
+func (r *Repository) UseCache(c restic.Cache) {
+	if c == nil {
+		return
+	}
+	debug.Log("using cache")
+	r.Cache = c
+	r.be = c.Wrap(r.be)
+}
+
 // PrefixLength returns the number of bytes required so that all prefixes of
 // all IDs of type t are unique.
 func (r *Repository) PrefixLength(t restic.FileType) (int, error) {
@@ -53,11 +64,11 @@ func (r *Repository) PrefixLength(t restic.FileType) (int, error) {
 
 // LoadAndDecrypt loads and decrypts data identified by t and id from the
 // backend.
-func (r *Repository) LoadAndDecrypt(ctx context.Context, t restic.FileType, id restic.ID) ([]byte, error) {
+func (r *Repository) LoadAndDecrypt(ctx context.Context, t restic.FileType, id restic.ID) (buf []byte, err error) {
 	debug.Log("load %v with id %v", t, id.Str())
 
 	h := restic.Handle{Type: t, Name: id.String()}
-	buf, err := backend.LoadAll(ctx, r.be, h)
+	buf, err = backend.LoadAll(ctx, r.be, h)
 	if err != nil {
 		debug.Log("error loading %v: %v", h, err)
 		return nil, err

internal/restic/cache.go

@@ -0,0 +1,34 @@
+package restic
+
+import "io"
+
+// Cache manages a local cache.
+type Cache interface {
+	// Wrap returns a backend with a cache.
+	Wrap(Backend) Backend
+
+	// IsNotExist returns true if the error was caused by a non-existing file.
+	IsNotExist(err error) bool
+
+	// Load returns a reader that yields the contents of the file with the
+	// given id if it is cached. rd must be closed after use. If an error is
+	// returned, the ReadCloser is nil. The files are still encrypted
+	Load(h Handle, length int, offset int64) (io.ReadCloser, error)
+
+	// SaveIndex saves an index in the cache.
+	Save(Handle, io.Reader) error
+
+	// SaveWriter returns a writer for the to be cached object h. It must be
+	// closed after writing is finished.
+	SaveWriter(Handle) (io.WriteCloser, error)
+
+	// Remove deletes a single file from the cache. If it isn't cached, this
+	// functions must return no error.
+	Remove(Handle) error
+
+	// Clear removes all files of type t from the cache that are not contained in the set.
+	Clear(FileType, IDSet) error
+
+	// Has returns true if the file is cached.
+	Has(Handle) bool
+}