repository: add SaveBlobAsync method

cmd/restic/cmd_copy.go

@@ -203,7 +203,7 @@ func copyTreeBatched(ctx context.Context, srcRepo restic.Repository, dstRepo res
 		startTime := time.Now()
 
 		// call WithBlobUploader() once and then loop over all selectedSnapshots
-		err := dstRepo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaver) error {
+		err := dstRepo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 			for len(selectedSnapshots) > 0 && (batchSize < targetSize || time.Since(startTime) < minDuration) {
 				sn := selectedSnapshots[0]
 				selectedSnapshots = selectedSnapshots[1:]
@@ -242,7 +242,7 @@ func copyTreeBatched(ctx context.Context, srcRepo restic.Repository, dstRepo res
 }
 
 func copyTree(ctx context.Context, srcRepo restic.Repository, dstRepo restic.Repository,
-	visitedTrees restic.AssociatedBlobSet, rootTreeID restic.ID, printer progress.Printer, uploader restic.BlobSaver) (uint64, error) {
+	visitedTrees restic.AssociatedBlobSet, rootTreeID restic.ID, printer progress.Printer, uploader restic.BlobSaverWithAsync) (uint64, error) {
 
 	wg, wgCtx := errgroup.WithContext(ctx)
 

cmd/restic/cmd_debug.go

@@ -353,7 +353,7 @@ func loadBlobs(ctx context.Context, opts DebugExamineOptions, repo restic.Reposi
 		return err
 	}
 
-	err = repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaver) error {
+	err = repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		for _, blob := range list {
 			printer.S("      loading blob %v at %v (length %v)", blob.ID, blob.Offset, blob.Length)
 			if int(blob.Offset+blob.Length) > len(pack) {

cmd/restic/cmd_recover.go

@@ -153,7 +153,7 @@ func runRecover(ctx context.Context, gopts global.Options, term ui.Terminal) err
 	}
 
 	var treeID restic.ID
-	err = repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaver) error {
+	err = repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		var err error
 		treeID, err = data.SaveTree(ctx, uploader, tree)
 		if err != nil {

cmd/restic/cmd_rewrite.go

@@ -194,7 +194,7 @@ func filterAndReplaceSnapshot(ctx context.Context, repo restic.Repository, sn *d
 
 	var filteredTree restic.ID
 	var summary *data.SnapshotSummary
-	err := repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaver) error {
+	err := repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		var err error
 		filteredTree, summary, err = filter(ctx, sn, uploader)
 		return err

internal/archiver/archiver.go

@@ -874,7 +874,7 @@ func (arch *Archiver) Snapshot(ctx context.Context, targets []string, opts Snaps
 
 	var rootTreeID restic.ID
 
-	err = arch.Repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaver) error {
+	err = arch.Repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		wg, wgCtx := errgroup.WithContext(ctx)
 		start := time.Now()
 

internal/archiver/archiver_test.go

@@ -56,7 +56,7 @@ func saveFile(t testing.TB, repo archiverRepo, filename string, filesystem fs.FS
 		return err
 	}
 
-	err := repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	err := repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		wg, ctx := errgroup.WithContext(ctx)
 		arch.runWorkers(ctx, wg, uploader)
 
@@ -219,7 +219,7 @@ func TestArchiverSave(t *testing.T) {
 			arch.summary = &Summary{}
 
 			var fnr futureNodeResult
-			err := repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaver) error {
+			err := repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 				wg, ctx := errgroup.WithContext(ctx)
 				arch.runWorkers(ctx, wg, uploader)
 
@@ -296,7 +296,7 @@ func TestArchiverSaveReaderFS(t *testing.T) {
 			arch.summary = &Summary{}
 
 			var fnr futureNodeResult
-			err = repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaver) error {
+			err = repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 				wg, ctx := errgroup.WithContext(ctx)
 				arch.runWorkers(ctx, wg, uploader)
 
@@ -415,27 +415,37 @@ type blobCountingRepo struct {
 	saved map[restic.BlobHandle]uint
 }
 
-func (repo *blobCountingRepo) WithBlobUploader(ctx context.Context, fn func(ctx context.Context, uploader restic.BlobSaver) error) error {
+func (repo *blobCountingRepo) WithBlobUploader(ctx context.Context, fn func(ctx context.Context, uploader restic.BlobSaverWithAsync) error) error {
-	return repo.archiverRepo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaver) error {
+	return repo.archiverRepo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		return fn(ctx, &blobCountingSaver{saver: uploader, blobCountingRepo: repo})
 	})
 }
 
 type blobCountingSaver struct {
-	saver            restic.BlobSaver
+	saver            restic.BlobSaverWithAsync
 	blobCountingRepo *blobCountingRepo
 }
 
+func (repo *blobCountingSaver) count(exists bool, h restic.BlobHandle) {
+	if exists {
+		return
+	}
+	repo.blobCountingRepo.m.Lock()
+	repo.blobCountingRepo.saved[h]++
+	repo.blobCountingRepo.m.Unlock()
+}
+
 func (repo *blobCountingSaver) SaveBlob(ctx context.Context, t restic.BlobType, buf []byte, id restic.ID, storeDuplicate bool) (restic.ID, bool, int, error) {
 	id, exists, size, err := repo.saver.SaveBlob(ctx, t, buf, id, storeDuplicate)
-	if exists {
+	repo.count(exists, restic.BlobHandle{ID: id, Type: t})
 	return id, exists, size, err
 }
-	h := restic.BlobHandle{ID: id, Type: t}
+
-	repo.blobCountingRepo.m.Lock()
+func (repo *blobCountingSaver) SaveBlobAsync(ctx context.Context, t restic.BlobType, buf []byte, id restic.ID, storeDuplicate bool, cb func(newID restic.ID, known bool, size int, err error)) {
-	repo.blobCountingRepo.saved[h]++
+	repo.saver.SaveBlobAsync(ctx, t, buf, id, storeDuplicate, func(newID restic.ID, known bool, size int, err error) {
-	repo.blobCountingRepo.m.Unlock()
+		repo.count(known, restic.BlobHandle{ID: newID, Type: t})
-	return id, exists, size, err
+		cb(newID, known, size, err)
+	})
 }
 
 func appendToFile(t testing.TB, filename string, data []byte) {
@@ -840,7 +850,7 @@ func TestArchiverSaveDir(t *testing.T) {
 			defer back()
 
 			var treeID restic.ID
-			err := repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+			err := repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 				wg, ctx := errgroup.WithContext(ctx)
 				arch.runWorkers(ctx, wg, uploader)
 				meta, err := testFS.OpenFile(test.target, fs.O_NOFOLLOW, true)
@@ -906,7 +916,7 @@ func TestArchiverSaveDirIncremental(t *testing.T) {
 		arch.summary = &Summary{}
 
 		var fnr futureNodeResult
-		err := repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+		err := repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 			wg, ctx := errgroup.WithContext(ctx)
 			arch.runWorkers(ctx, wg, uploader)
 			meta, err := testFS.OpenFile(tempdir, fs.O_NOFOLLOW, true)
@@ -1096,7 +1106,7 @@ func TestArchiverSaveTree(t *testing.T) {
 			}
 
 			var treeID restic.ID
-			err := repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+			err := repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 				wg, ctx := errgroup.WithContext(ctx)
 				arch.runWorkers(ctx, wg, uploader)
 
@@ -2095,8 +2105,8 @@ type failSaveRepo struct {
 	err       error
 }
 
-func (f *failSaveRepo) WithBlobUploader(ctx context.Context, fn func(ctx context.Context, uploader restic.BlobSaver) error) error {
+func (f *failSaveRepo) WithBlobUploader(ctx context.Context, fn func(ctx context.Context, uploader restic.BlobSaverWithAsync) error) error {
-	return f.archiverRepo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaver) error {
+	return f.archiverRepo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		return fn(ctx, &failSaveSaver{saver: uploader, failSaveRepo: f})
 	})
 }
@@ -2115,6 +2125,11 @@ func (f *failSaveSaver) SaveBlob(ctx context.Context, t restic.BlobType, buf []b
 	return f.saver.SaveBlob(ctx, t, buf, id, storeDuplicate)
 }
 
+func (f *failSaveSaver) SaveBlobAsync(ctx context.Context, tpe restic.BlobType, buf []byte, id restic.ID, storeDuplicate bool, cb func(newID restic.ID, known bool, sizeInRepo int, err error)) {
+	newID, known, sizeInRepo, err := f.SaveBlob(ctx, tpe, buf, id, storeDuplicate)
+	cb(newID, known, sizeInRepo, err)
+}
+
 func TestArchiverAbortEarlyOnError(t *testing.T) {
 	var testErr = errors.New("test error")
 
@@ -2425,7 +2440,7 @@ func TestRacyFileTypeSwap(t *testing.T) {
 			ctx, cancel := context.WithCancel(context.Background())
 			defer cancel()
 
-			_ = repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaver) error {
+			_ = repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 				wg, ctx := errgroup.WithContext(ctx)
 
 				arch := New(repo, fs.Track{FS: statfs}, Options{})

internal/checker/checker_test.go

@@ -527,7 +527,7 @@ func TestCheckerBlobTypeConfusion(t *testing.T) {
 	}
 
 	var id restic.ID
-	test.OK(t, repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaver) error {
+	test.OK(t, repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		var err error
 		id, err = data.SaveTree(ctx, uploader, damagedTree)
 		return err
@@ -536,7 +536,7 @@ func TestCheckerBlobTypeConfusion(t *testing.T) {
 	buf, err := repo.LoadBlob(ctx, restic.TreeBlob, id, nil)
 	test.OK(t, err)
 
-	test.OK(t, repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaver) error {
+	test.OK(t, repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		var err error
 		_, _, _, err = uploader.SaveBlob(ctx, restic.DataBlob, buf, id, false)
 		return err
@@ -561,7 +561,7 @@ func TestCheckerBlobTypeConfusion(t *testing.T) {
 	}
 
 	var rootID restic.ID
-	test.OK(t, repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaver) error {
+	test.OK(t, repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		var err error
 		rootID, err = data.SaveTree(ctx, uploader, rootTree)
 		return err

internal/data/testing.go

@@ -136,7 +136,7 @@ func TestCreateSnapshot(t testing.TB, repo restic.Repository, at time.Time, dept
 	}
 
 	var treeID restic.ID
-	test.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	test.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		treeID = fs.saveTree(ctx, uploader, seed, depth)
 		return nil
 	}))

internal/data/tree_test.go

@@ -107,7 +107,7 @@ func TestEmptyLoadTree(t *testing.T) {
 
 	tree := data.NewTree(0)
 	var id restic.ID
-	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		var err error
 		// save tree
 		id, err = data.SaveTree(ctx, uploader, tree)

internal/repository/fuzz_test.go

@@ -20,7 +20,7 @@ func FuzzSaveLoadBlob(f *testing.F) {
 		id := restic.Hash(blob)
 		repo, _, _ := TestRepositoryWithVersion(t, 2)
 
-		rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+		rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 			_, _, _, err := uploader.SaveBlob(ctx, restic.DataBlob, blob, id, false)
 			return err
 		}))

internal/repository/prune.go

@@ -563,7 +563,7 @@ func (plan *PrunePlan) Execute(ctx context.Context, printer progress.Printer) er
 	if len(plan.repackPacks) != 0 {
 		printer.P("repacking packs\n")
 		bar := printer.NewCounter("packs repacked")
-		err := repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaver) error {
+		err := repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 			return CopyBlobs(ctx, repo, repo, uploader, plan.repackPacks, plan.keepBlobs, bar, printer.P)
 		})
 		if err != nil {

internal/repository/prune_internal_test.go

@@ -47,7 +47,7 @@ func TestPruneMaxUnusedDuplicate(t *testing.T) {
 		{bufs[1], bufs[3]},
 		{bufs[2], bufs[3]},
 	} {
-		rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+		rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 			for _, blob := range blobs {
 				id, _, _, err := uploader.SaveBlob(ctx, restic.DataBlob, blob, restic.ID{}, true)
 				keep.Insert(restic.BlobHandle{Type: restic.DataBlob, ID: id})

internal/repository/prune_test.go

@@ -25,7 +25,7 @@ func testPrune(t *testing.T, opts repository.PruneOptions, errOnUnused bool) {
 	createRandomBlobs(t, random, repo, 5, 0.5, true)
 	keep, _ := selectBlobs(t, random, repo, 0.5)
 
-	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		// duplicate a few blobs to exercise those code paths
 		for blob := range keep {
 			buf, err := repo.LoadBlob(ctx, blob.Type, blob.ID, nil)
@@ -133,7 +133,7 @@ func TestPruneSmall(t *testing.T) {
 	const numBlobsCreated = 55
 
 	keep := restic.NewBlobSet()
-	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		// we need a minum of 11 packfiles, each packfile will be about 5 Mb long
 		for i := 0; i < numBlobsCreated; i++ {
 			buf := make([]byte, blobSize)

internal/repository/repack.go

@@ -32,7 +32,7 @@ func CopyBlobs(
 	ctx context.Context,
 	repo restic.Repository,
 	dstRepo restic.Repository,
-	dstUploader restic.BlobSaver,
+	dstUploader restic.BlobSaverWithAsync,
 	packs restic.IDSet,
 	keepBlobs repackBlobSet,
 	p *progress.Counter,
@@ -57,7 +57,7 @@ func repack(
 	ctx context.Context,
 	repo restic.Repository,
 	dstRepo restic.Repository,
-	uploader restic.BlobSaver,
+	uploader restic.BlobSaverWithAsync,
 	packs restic.IDSet,
 	keepBlobs repackBlobSet,
 	p *progress.Counter,

internal/repository/repack_test.go

@@ -20,7 +20,7 @@ func randomSize(random *rand.Rand, min, max int) int {
 func createRandomBlobs(t testing.TB, random *rand.Rand, repo restic.Repository, blobs int, pData float32, smallBlobs bool) {
 	// two loops to allow creating multiple pack files
 	for blobs > 0 {
-		rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+		rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 			for blobs > 0 {
 				blobs--
 				var (
@@ -70,7 +70,7 @@ func createRandomWrongBlob(t testing.TB, random *rand.Rand, repo restic.Reposito
 	// invert first data byte
 	buf[0] ^= 0xff
 
-	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		_, _, _, err := uploader.SaveBlob(ctx, restic.DataBlob, buf, id, false)
 		return err
 	}))
@@ -150,7 +150,7 @@ func findPacksForBlobs(t *testing.T, repo restic.Repository, blobs restic.BlobSe
 }
 
 func repack(t *testing.T, repo restic.Repository, be backend.Backend, packs restic.IDSet, blobs restic.BlobSet) {
-	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		return repository.CopyBlobs(ctx, repo, repo, uploader, packs, blobs, nil, nil)
 	}))
 
@@ -265,7 +265,7 @@ func testRepackCopy(t *testing.T, version uint) {
 	_, keepBlobs := selectBlobs(t, random, repo, 0.2)
 	copyPacks := findPacksForBlobs(t, repo, keepBlobs)
 
-	rtest.OK(t, repoWrapped.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	rtest.OK(t, repoWrapped.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		return repository.CopyBlobs(ctx, repoWrapped, dstRepoWrapped, uploader, copyPacks, keepBlobs, nil, nil)
 	}))
 	rebuildAndReloadIndex(t, dstRepo)
@@ -303,7 +303,7 @@ func testRepackWrongBlob(t *testing.T, version uint) {
 	_, keepBlobs := selectBlobs(t, random, repo, 0)
 	rewritePacks := findPacksForBlobs(t, repo, keepBlobs)
 
-	err := repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	err := repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		return repository.CopyBlobs(ctx, repo, repo, uploader, rewritePacks, keepBlobs, nil, nil)
 	})
 	if err == nil {
@@ -336,7 +336,7 @@ func testRepackBlobFallback(t *testing.T, version uint) {
 	modbuf[0] ^= 0xff
 
 	// create pack with broken copy
-	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		_, _, _, err := uploader.SaveBlob(ctx, restic.DataBlob, modbuf, id, false)
 		return err
 	}))
@@ -346,13 +346,13 @@ func testRepackBlobFallback(t *testing.T, version uint) {
 	rewritePacks := findPacksForBlobs(t, repo, keepBlobs)
 
 	// create pack with valid copy
-	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		_, _, _, err := uploader.SaveBlob(ctx, restic.DataBlob, buf, id, true)
 		return err
 	}))
 
 	// repack must fallback to valid copy
-	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		return repository.CopyBlobs(ctx, repo, repo, uploader, rewritePacks, keepBlobs, nil, nil)
 	}))
 

internal/repository/repair_pack.go

@@ -15,7 +15,7 @@ func RepairPacks(ctx context.Context, repo *Repository, ids restic.IDSet, printe
 	bar.SetMax(uint64(len(ids)))
 	defer bar.Done()
 
-	err := repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaver) error {
+	err := repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		// examine all data the indexes have for the pack file
 		for b := range repo.ListPacksFromIndex(ctx, ids) {
 			blobs := b.Blobs

internal/repository/repository.go

@@ -42,6 +42,7 @@ type Repository struct {
 	opts Options
 
 	packerWg    *errgroup.Group
+	blobWg      *errgroup.Group
 	uploader    *packerUploader
 	treePM      *packerManager
 	dataPM      *packerManager
@@ -559,11 +560,14 @@ func (r *Repository) removeUnpacked(ctx context.Context, t restic.FileType, id r
 	return r.be.Remove(ctx, backend.Handle{Type: t, Name: id.String()})
 }
 
-func (r *Repository) WithBlobUploader(ctx context.Context, fn func(ctx context.Context, uploader restic.BlobSaver) error) error {
+func (r *Repository) WithBlobUploader(ctx context.Context, fn func(ctx context.Context, uploader restic.BlobSaverWithAsync) error) error {
 	wg, ctx := errgroup.WithContext(ctx)
 	r.startPackUploader(ctx, wg)
+	saverCtx := r.startBlobSaver(ctx, wg)
 	wg.Go(func() error {
-		if err := fn(ctx, &blobSaverRepo{repo: r}); err != nil {
+		// must use saverCtx to ensure that the ctx used for saveBlob calls is bound to it
+		// otherwise the blob saver could deadlock in case of an error.
+		if err := fn(saverCtx, &blobSaverRepo{repo: r}); err != nil {
 			return err
 		}
 		if err := r.flush(ctx); err != nil {
@@ -574,14 +578,6 @@ func (r *Repository) WithBlobUploader(ctx context.Context, fn func(ctx context.C
 	return wg.Wait()
 }
 
-type blobSaverRepo struct {
-	repo *Repository
-}
-
-func (r *blobSaverRepo) SaveBlob(ctx context.Context, t restic.BlobType, buf []byte, id restic.ID, storeDuplicate bool) (newID restic.ID, known bool, size int, err error) {
-	return r.repo.saveBlob(ctx, t, buf, id, storeDuplicate)
-}
-
 func (r *Repository) startPackUploader(ctx context.Context, wg *errgroup.Group) {
 	if r.packerWg != nil {
 		panic("uploader already started")
@@ -598,8 +594,40 @@ func (r *Repository) startPackUploader(ctx context.Context, wg *errgroup.Group)
 	})
 }
 
+func (r *Repository) startBlobSaver(ctx context.Context, wg *errgroup.Group) context.Context {
+	// blob upload computations are CPU bound
+	blobWg, blobCtx := errgroup.WithContext(ctx)
+	blobWg.SetLimit(runtime.GOMAXPROCS(0))
+	r.blobWg = blobWg
+
+	wg.Go(func() error {
+		// As the goroutines are only spawned on demand, wait until the context is canceled.
+		// This will either happen on an error while saving a blob or when blobWg.Wait() is called
+		// by flushBlobUploader().
+		<-blobCtx.Done()
+		return blobWg.Wait()
+	})
+	return blobCtx
+}
+
+type blobSaverRepo struct {
+	repo *Repository
+}
+
+func (r *blobSaverRepo) SaveBlob(ctx context.Context, t restic.BlobType, buf []byte, id restic.ID, storeDuplicate bool) (newID restic.ID, known bool, size int, err error) {
+	return r.repo.saveBlob(ctx, t, buf, id, storeDuplicate)
+}
+
+func (r *blobSaverRepo) SaveBlobAsync(ctx context.Context, t restic.BlobType, buf []byte, id restic.ID, storeDuplicate bool, cb func(newID restic.ID, known bool, size int, err error)) {
+	r.repo.saveBlobAsync(ctx, t, buf, id, storeDuplicate, cb)
+}
+
 // Flush saves all remaining packs and the index
 func (r *Repository) flush(ctx context.Context) error {
+	if err := r.flushBlobUploader(); err != nil {
+		return err
+	}
+
 	if err := r.flushPacks(ctx); err != nil {
 		return err
 	}
@@ -607,6 +635,15 @@ func (r *Repository) flush(ctx context.Context) error {
 	return r.idx.Flush(ctx, &internalRepository{r})
 }
 
+func (r *Repository) flushBlobUploader() error {
+	if r.blobWg == nil {
+		return nil
+	}
+	err := r.blobWg.Wait()
+	r.blobWg = nil
+	return err
+}
+
 // FlushPacks saves all remaining packs.
 func (r *Repository) flushPacks(ctx context.Context) error {
 	if r.packerWg == nil {
@@ -994,6 +1031,19 @@ func (r *Repository) saveBlob(ctx context.Context, t restic.BlobType, buf []byte
 	return newID, known, size, err
 }
 
+func (r *Repository) saveBlobAsync(ctx context.Context, t restic.BlobType, buf []byte, id restic.ID, storeDuplicate bool, cb func(newID restic.ID, known bool, size int, err error)) {
+	r.blobWg.Go(func() error {
+		if ctx.Err() != nil {
+			// fail fast if the context is cancelled
+			cb(restic.ID{}, false, 0, context.Cause(ctx))
+			return context.Cause(ctx)
+		}
+		newID, known, size, err := r.saveBlob(ctx, t, buf, id, storeDuplicate)
+		cb(newID, known, size, err)
+		return err
+	})
+}
+
 type backendLoadFn func(ctx context.Context, h backend.Handle, length int, offset int64, fn func(rd io.Reader) error) error
 type loadBlobFn func(ctx context.Context, t restic.BlobType, id restic.ID, buf []byte) ([]byte, error)
 

internal/repository/repository_test.go

@@ -51,7 +51,7 @@ func testSave(t *testing.T, version uint, calculateID bool) {
 
 		id := restic.Hash(data)
 
-		rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+		rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 			// save
 			inputID := restic.ID{}
 			if !calculateID {
@@ -97,7 +97,7 @@ func testSavePackMerging(t *testing.T, targetPercentage int, expectedPacks int)
 	})
 
 	var ids restic.IDs
-	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		// add blobs with size targetPercentage / 100 * repo.PackSize to the repository
 		blobSize := repository.MinPackSize / 100
 		for range targetPercentage {
@@ -147,7 +147,7 @@ func benchmarkSaveAndEncrypt(t *testing.B, version uint) {
 	t.ResetTimer()
 	t.SetBytes(int64(size))
 
-	_ = repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	_ = repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		for i := 0; i < t.N; i++ {
 			_, _, _, err = uploader.SaveBlob(ctx, restic.DataBlob, data, id, true)
 			rtest.OK(t, err)
@@ -168,7 +168,7 @@ func testLoadBlob(t *testing.T, version uint) {
 	rtest.OK(t, err)
 
 	var id restic.ID
-	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		var err error
 		id, _, _, err = uploader.SaveBlob(ctx, restic.DataBlob, buf, restic.ID{}, false)
 		return err
@@ -196,7 +196,7 @@ func TestLoadBlobBroken(t *testing.T) {
 	buf := rtest.Random(42, 1000)
 
 	var id restic.ID
-	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		var err error
 		id, _, _, err = uploader.SaveBlob(ctx, restic.TreeBlob, buf, restic.ID{}, false)
 		return err
@@ -225,7 +225,7 @@ func benchmarkLoadBlob(b *testing.B, version uint) {
 	rtest.OK(b, err)
 
 	var id restic.ID
-	rtest.OK(b, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	rtest.OK(b, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		var err error
 		id, _, _, err = uploader.SaveBlob(ctx, restic.DataBlob, buf, restic.ID{}, false)
 		return err
@@ -361,7 +361,7 @@ func TestRepositoryLoadUnpackedRetryBroken(t *testing.T) {
 
 // saveRandomDataBlobs generates random data blobs and saves them to the repository.
 func saveRandomDataBlobs(t testing.TB, repo restic.Repository, num int, sizeMax int) {
-	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		for i := 0; i < num; i++ {
 			size := rand.Int() % sizeMax
 
@@ -432,7 +432,7 @@ func TestListPack(t *testing.T) {
 	buf := rtest.Random(42, 1000)
 
 	var id restic.ID
-	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaver) error {
+	rtest.OK(t, repo.WithBlobUploader(context.TODO(), func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		var err error
 		id, _, _, err = uploader.SaveBlob(ctx, restic.TreeBlob, buf, restic.ID{}, false)
 		return err

internal/restic/repository.go

@@ -42,7 +42,7 @@ type Repository interface {
 	// WithUploader starts the necessary workers to upload new blobs. Once the callback returns,
 	// the workers are stopped and the index is written to the repository. The callback must use
 	// the passed context and must not keep references to any of its parameters after returning.
-	WithBlobUploader(ctx context.Context, fn func(ctx context.Context, uploader BlobSaver) error) error
+	WithBlobUploader(ctx context.Context, fn func(ctx context.Context, uploader BlobSaverWithAsync) error) error
 
 	// List calls the function fn for each file of type t in the repository.
 	// When an error is returned by fn, processing stops and List() returns the
@@ -162,11 +162,23 @@ type BlobLoader interface {
 }
 
 type WithBlobUploader interface {
-	WithBlobUploader(ctx context.Context, fn func(ctx context.Context, uploader BlobSaver) error) error
+	WithBlobUploader(ctx context.Context, fn func(ctx context.Context, uploader BlobSaverWithAsync) error) error
+}
+
+type BlobSaverWithAsync interface {
+	BlobSaver
+	BlobSaverAsync
 }
 
 type BlobSaver interface {
-	SaveBlob(context.Context, BlobType, []byte, ID, bool) (ID, bool, int, error)
+	// SaveBlob saves a blob to the repository. ctx must be derived from the context created by WithBlobUploader.
+	SaveBlob(ctx context.Context, tpe BlobType, buf []byte, id ID, storeDuplicate bool) (newID ID, known bool, sizeInRepo int, err error)
+}
+
+type BlobSaverAsync interface {
+	// SaveBlobAsync saves a blob to the repository. ctx must be derived from the context created by WithBlobUploader.
+	// The callback is called asynchronously from a different goroutine.
+	SaveBlobAsync(ctx context.Context, tpe BlobType, buf []byte, id ID, storeDuplicate bool, cb func(newID ID, known bool, sizeInRepo int, err error))
 }
 
 // Loader loads a blob from a repository.

internal/restorer/restorer_test.go

@@ -171,7 +171,7 @@ func saveSnapshot(t testing.TB, repo restic.Repository, snapshot Snapshot, getGe
 	defer cancel()
 
 	var treeID restic.ID
-	rtest.OK(t, repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaver) error {
+	rtest.OK(t, repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		treeID = saveDir(t, uploader, snapshot.Nodes, 1000, getGenericAttributes)
 		return nil
 	}))