Enable sparseness only conditionally

We can either preallocate storage for a file or sparsify it. This
detects a pack file as sparse if it contains an all zero block or
consists of only one block. As the file sparsification is just an
approximation, hide it behind a `--sparse` parameter.

cmd/restic/cmd_restore.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"runtime"
 	"strings"
 	"time"
 
@@ -42,6 +43,7 @@ type RestoreOptions struct {
 	InsensitiveInclude []string
 	Target             string
 	snapshotFilterOptions
+	Sparse bool
 	Verify bool
 }
 
@@ -58,6 +60,9 @@ func init() {
 	flags.StringVarP(&restoreOptions.Target, "target", "t", "", "directory to extract data to")
 
 	initSingleSnapshotFilterOptions(flags, &restoreOptions.snapshotFilterOptions)
+	if runtime.GOOS != "windows" {
+		flags.BoolVar(&restoreOptions.Sparse, "sparse", false, "restore files as sparse (not supported on windows)")
+	}
 	flags.BoolVar(&restoreOptions.Verify, "verify", false, "verify restored files content")
 }
 
@@ -147,7 +152,7 @@ func runRestore(opts RestoreOptions, gopts GlobalOptions, args []string) error {
 		return err
 	}
 
-	res, err := restorer.NewRestorer(ctx, repo, id)
+	res, err := restorer.NewRestorer(ctx, repo, id, opts.Sparse)
 	if err != nil {
 		Exitf(2, "creating restorer failed: %v\n", err)
 	}

internal/restorer/filerestorer.go

@@ -7,6 +7,7 @@ import (
 
 	"golang.org/x/sync/errgroup"
 
+	"github.com/restic/chunker"
 	"github.com/restic/restic/internal/crypto"
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/errors"
@@ -27,6 +28,7 @@ const (
 type fileInfo struct {
 	lock       sync.Mutex
 	inProgress bool
+	sparse     bool
 	size       int64
 	location   string      // file on local filesystem relative to restorer basedir
 	blobs      interface{} // blobs of the file
@@ -51,6 +53,8 @@ type fileRestorer struct {
 
 	workerCount int
 	filesWriter *filesWriter
+	zeroChunk   restic.ID
+	sparse      bool
 
 	dst   string
 	files []*fileInfo
@@ -61,7 +65,8 @@ func newFileRestorer(dst string,
 	packLoader repository.BackendLoadFn,
 	key *crypto.Key,
 	idx func(restic.BlobHandle) []restic.PackedBlob,
-	connections uint) *fileRestorer {
+	connections uint,
+	sparse bool) *fileRestorer {
 
 	// as packs are streamed the concurrency is limited by IO
 	workerCount := int(connections)
@@ -71,6 +76,8 @@ func newFileRestorer(dst string,
 		idx:         idx,
 		packLoader:  packLoader,
 		filesWriter: newFilesWriter(workerCount),
+		zeroChunk:   restic.Hash(make([]byte, chunker.MinSize)),
+		sparse:      sparse,
 		workerCount: workerCount,
 		dst:         dst,
 		Error:       restorerAbortOnAllErrors,
@@ -133,7 +140,16 @@ func (r *fileRestorer) restoreFiles(ctx context.Context) error {
 				packOrder = append(packOrder, packID)
 			}
 			pack.files[file] = struct{}{}
+			if blob.ID.Equal(r.zeroChunk) {
+				file.sparse = r.sparse
+			}
 		})
+		if len(fileBlobs) == 1 {
+			// no need to preallocate files with a single block, thus we can always consider them to be sparse
+			// in addition, a short chunk will never match r.zeroChunk which would prevent sparseness for short files
+			file.sparse = r.sparse
+		}
+
 		if err != nil {
 			// repository index is messed up, can't do anything
 			return err
@@ -253,7 +269,7 @@ func (r *fileRestorer) downloadPack(ctx context.Context, pack *packInfo) error {
 						file.inProgress = true
 						createSize = file.size
 					}
-					return r.filesWriter.writeToFile(r.targetPath(file.location), blobData, offset, createSize)
+					return r.filesWriter.writeToFile(r.targetPath(file.location), blobData, offset, createSize, file.sparse)
 				}
 				err := sanitizeError(file, writeToFile())
 				if err != nil {

internal/restorer/filerestorer_test.go

@@ -147,10 +147,10 @@ func newTestRepo(content []TestFile) *TestRepo {
 	return repo
 }
 
-func restoreAndVerify(t *testing.T, tempdir string, content []TestFile, files map[string]bool) {
+func restoreAndVerify(t *testing.T, tempdir string, content []TestFile, files map[string]bool, sparse bool) {
 	repo := newTestRepo(content)
 
-	r := newFileRestorer(tempdir, repo.loader, repo.key, repo.Lookup, 2)
+	r := newFileRestorer(tempdir, repo.loader, repo.key, repo.Lookup, 2, sparse)
 
 	if files == nil {
 		r.files = repo.files
@@ -188,6 +188,7 @@ func TestFileRestorerBasic(t *testing.T) {
 	tempdir, cleanup := rtest.TempDir(t)
 	defer cleanup()
 
+	for _, sparse := range []bool{false, true} {
 		restoreAndVerify(t, tempdir, []TestFile{
 			{
 				name: "file1",
@@ -211,7 +212,8 @@ func TestFileRestorerBasic(t *testing.T) {
 					{"data3-1", "pack3-1"},
 				},
 			},
-	}, nil)
+		}, nil, sparse)
+	}
 }
 
 func TestFileRestorerPackSkip(t *testing.T) {
@@ -221,6 +223,7 @@ func TestFileRestorerPackSkip(t *testing.T) {
 	files := make(map[string]bool)
 	files["file2"] = true
 
+	for _, sparse := range []bool{false, true} {
 		restoreAndVerify(t, tempdir, []TestFile{
 			{
 				name: "file1",
@@ -242,7 +245,8 @@ func TestFileRestorerPackSkip(t *testing.T) {
 					{"data1-6", "pack1"},
 				},
 			},
-	}, files)
+		}, files, sparse)
+	}
 }
 
 func TestErrorRestoreFiles(t *testing.T) {
@@ -264,7 +268,7 @@ func TestErrorRestoreFiles(t *testing.T) {
 		return loadError
 	}
 
-	r := newFileRestorer(tempdir, repo.loader, repo.key, repo.Lookup, 2)
+	r := newFileRestorer(tempdir, repo.loader, repo.key, repo.Lookup, 2, false)
 	r.files = repo.files
 
 	err := r.restoreFiles(context.TODO())
@@ -304,7 +308,7 @@ func testPartialDownloadError(t *testing.T, part int) {
 		return loader(ctx, h, length, offset, fn)
 	}
 
-	r := newFileRestorer(tempdir, repo.loader, repo.key, repo.Lookup, 2)
+	r := newFileRestorer(tempdir, repo.loader, repo.key, repo.Lookup, 2, false)
 	r.files = repo.files
 	r.Error = func(s string, e error) error {
 		// ignore errors as in the `restore` command

internal/restorer/fileswriter.go

@@ -26,6 +26,7 @@ type partialFile struct {
 	*os.File
 	size   int64 // File size, tracked for sparse writes (not on Windows).
 	users  int   // Reference count.
+	sparse bool
 }
 
 func newFilesWriter(count int) *filesWriter {
@@ -38,7 +39,7 @@ func newFilesWriter(count int) *filesWriter {
 	}
 }
 
-func (w *filesWriter) writeToFile(path string, blob []byte, offset int64, createSize int64) error {
+func (w *filesWriter) writeToFile(path string, blob []byte, offset int64, createSize int64, sparse bool) error {
 	bucket := &w.buckets[uint(xxhash.Sum64String(path))%uint(len(w.buckets))]
 
 	acquireWriter := func() (*partialFile, error) {
@@ -62,7 +63,7 @@ func (w *filesWriter) writeToFile(path string, blob []byte, offset int64, create
 			return nil, err
 		}
 
-		wr := &partialFile{File: f, users: 1}
+		wr := &partialFile{File: f, users: 1, sparse: sparse}
 		if createSize < 0 {
 			info, err := f.Stat()
 			if err != nil {
@@ -72,7 +73,7 @@ func (w *filesWriter) writeToFile(path string, blob []byte, offset int64, create
 		}
 		bucket.files[path] = wr
 
-		if createSize >= 0 {
+		if createSize >= 0 && !sparse {
 			err := preallocateFile(wr.File, createSize)
 			if err != nil {
 				// Just log the preallocate error but don't let it cause the restore process to fail.

internal/restorer/fileswriter_test.go

@@ -16,16 +16,16 @@ func TestFilesWriterBasic(t *testing.T) {
 	f1 := dir + "/f1"
 	f2 := dir + "/f2"
 
-	rtest.OK(t, w.writeToFile(f1, []byte{1}, 0, 2))
+	rtest.OK(t, w.writeToFile(f1, []byte{1}, 0, 2, false))
 	rtest.Equals(t, 0, len(w.buckets[0].files))
 
-	rtest.OK(t, w.writeToFile(f2, []byte{2}, 0, 2))
+	rtest.OK(t, w.writeToFile(f2, []byte{2}, 0, 2, false))
 	rtest.Equals(t, 0, len(w.buckets[0].files))
 
-	rtest.OK(t, w.writeToFile(f1, []byte{1}, 1, -1))
+	rtest.OK(t, w.writeToFile(f1, []byte{1}, 1, -1, false))
 	rtest.Equals(t, 0, len(w.buckets[0].files))
 
-	rtest.OK(t, w.writeToFile(f2, []byte{2}, 1, -1))
+	rtest.OK(t, w.writeToFile(f2, []byte{2}, 1, -1, false))
 	rtest.Equals(t, 0, len(w.buckets[0].files))
 
 	buf, err := ioutil.ReadFile(f1)

internal/restorer/restorer.go

@@ -18,6 +18,7 @@ import (
 type Restorer struct {
 	repo   restic.Repository
 	sn     *restic.Snapshot
+	sparse bool
 
 	Error        func(location string, err error) error
 	SelectFilter func(item string, dstpath string, node *restic.Node) (selectedForRestore bool, childMayBeSelected bool)
@@ -26,9 +27,10 @@ type Restorer struct {
 var restorerAbortOnAllErrors = func(location string, err error) error { return err }
 
 // NewRestorer creates a restorer preloaded with the content from the snapshot id.
-func NewRestorer(ctx context.Context, repo restic.Repository, id restic.ID) (*Restorer, error) {
+func NewRestorer(ctx context.Context, repo restic.Repository, id restic.ID, sparse bool) (*Restorer, error) {
 	r := &Restorer{
 		repo:         repo,
+		sparse:       sparse,
 		Error:        restorerAbortOnAllErrors,
 		SelectFilter: func(string, string, *restic.Node) (bool, bool) { return true, true },
 	}
@@ -219,7 +221,7 @@ func (res *Restorer) RestoreTo(ctx context.Context, dst string) error {
 	}
 
 	idx := NewHardlinkIndex()
-	filerestorer := newFileRestorer(dst, res.repo.Backend().Load, res.repo.Key(), res.repo.Index().Lookup, res.repo.Connections())
+	filerestorer := newFileRestorer(dst, res.repo.Backend().Load, res.repo.Key(), res.repo.Index().Lookup, res.repo.Connections(), res.sparse)
 	filerestorer.Error = res.Error
 
 	debug.Log("first pass for %q", dst)

internal/restorer/restorer_test.go

@@ -324,7 +324,7 @@ func TestRestorer(t *testing.T) {
 			_, id := saveSnapshot(t, repo, test.Snapshot)
 			t.Logf("snapshot saved as %v", id.Str())
 
-			res, err := NewRestorer(context.TODO(), repo, id)
+			res, err := NewRestorer(context.TODO(), repo, id, false)
 			if err != nil {
 				t.Fatal(err)
 			}
@@ -447,7 +447,7 @@ func TestRestorerRelative(t *testing.T) {
 			_, id := saveSnapshot(t, repo, test.Snapshot)
 			t.Logf("snapshot saved as %v", id.Str())
 
-			res, err := NewRestorer(context.TODO(), repo, id)
+			res, err := NewRestorer(context.TODO(), repo, id, false)
 			if err != nil {
 				t.Fatal(err)
 			}
@@ -682,7 +682,7 @@ func TestRestorerTraverseTree(t *testing.T) {
 			defer cleanup()
 			sn, id := saveSnapshot(t, repo, test.Snapshot)
 
-			res, err := NewRestorer(context.TODO(), repo, id)
+			res, err := NewRestorer(context.TODO(), repo, id, false)
 			if err != nil {
 				t.Fatal(err)
 			}
@@ -764,7 +764,7 @@ func TestRestorerConsistentTimestampsAndPermissions(t *testing.T) {
 		},
 	})
 
-	res, err := NewRestorer(context.TODO(), repo, id)
+	res, err := NewRestorer(context.TODO(), repo, id, false)
 	rtest.OK(t, err)
 
 	res.SelectFilter = func(item string, dstpath string, node *restic.Node) (selectedForRestore bool, childMayBeSelected bool) {
@@ -824,7 +824,7 @@ func TestVerifyCancel(t *testing.T) {
 
 	_, id := saveSnapshot(t, repo, snapshot)
 
-	res, err := NewRestorer(context.TODO(), repo, id)
+	res, err := NewRestorer(context.TODO(), repo, id, false)
 	rtest.OK(t, err)
 
 	tempdir, cleanup := rtest.TempDir(t)

internal/restorer/restorer_unix_test.go

@@ -36,7 +36,7 @@ func TestRestorerRestoreEmptyHardlinkedFileds(t *testing.T) {
 		},
 	})
 
-	res, err := NewRestorer(context.TODO(), repo, id)
+	res, err := NewRestorer(context.TODO(), repo, id, false)
 	rtest.OK(t, err)
 
 	res.SelectFilter = func(item string, dstpath string, node *restic.Node) (selectedForRestore bool, childMayBeSelected bool) {
@@ -85,8 +85,9 @@ func TestRestorerSparseFiles(t *testing.T) {
 	arch := archiver.New(repo, target, archiver.Options{})
 	_, id, err := arch.Snapshot(context.Background(), []string{"/zeros"},
 		archiver.SnapshotOptions{})
+	rtest.OK(t, err)
 
-	res, err := NewRestorer(repo, id)
+	res, err := NewRestorer(context.TODO(), repo, id, true)
 	rtest.OK(t, err)
 
 	tempdir, cleanup := rtest.TempDir(t)
@@ -102,6 +103,7 @@ func TestRestorerSparseFiles(t *testing.T) {
 	content, err := ioutil.ReadFile(filename)
 	rtest.OK(t, err)
 
+	rtest.Equals(t, len(zeros[:]), len(content))
 	rtest.Equals(t, zeros[:], content)
 
 	fi, err := os.Stat(filename)

internal/restorer/sparsewrite.go

@@ -8,6 +8,10 @@ import "bytes"
 // WriteAt writes p to f.File at offset. It tries to do a sparse write
 // and updates f.size.
 func (f *partialFile) WriteAt(p []byte, offset int64) (n int, err error) {
+	if !f.sparse {
+		return f.File.WriteAt(p, offset)
+	}
+
 	n = len(p)
 	end := offset + int64(n)