repository: fix race condition for blobSaver shutdown

wg.Go() may not be called after wg.Wait(). This prevents connecting two
errgroups such that the errors are propagated between them if the child
errgroup dynamically starts goroutines. Instead use just a single errgroup,
and sequence the shutdown using a sync.WaitGroup. This is far simpler
and does not require any "clever" tricks.

internal/archiver/archiver_test.go

@@ -2104,8 +2104,10 @@ type failSaveRepo struct {
 }
 
 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.BlobSaverWithAsync) error {
-		return fn(ctx, &failSaveSaver{saver: uploader, failSaveRepo: f, semaphore: make(chan struct{}, 1)})
+	outerCtx, outerCancel := context.WithCancelCause(ctx)
+	defer outerCancel(f.err)
+	return f.archiverRepo.WithBlobUploader(outerCtx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
+		return fn(ctx, &failSaveSaver{saver: uploader, failSaveRepo: f, semaphore: make(chan struct{}, 1), outerCancel: outerCancel})
 	})
 }
 
@@ -2113,6 +2115,7 @@ type failSaveSaver struct {
 	saver        restic.BlobSaverWithAsync
 	failSaveRepo *failSaveRepo
 	semaphore    chan struct{}
+	outerCancel  context.CancelCauseFunc
 }
 
 func (f *failSaveSaver) SaveBlob(ctx context.Context, t restic.BlobType, buf []byte, id restic.ID, storeDuplicate bool) (restic.ID, bool, int, error) {
@@ -2130,10 +2133,9 @@ func (f *failSaveSaver) SaveBlobAsync(ctx context.Context, t restic.BlobType, bu
 
 	val := f.failSaveRepo.cnt.Add(1)
 	if val >= f.failSaveRepo.failAfter {
-		// use a canceled context to make SaveBlobAsync fail
-		var cancel context.CancelCauseFunc
-		ctx, cancel = context.WithCancelCause(ctx)
-		cancel(f.failSaveRepo.err)
+		// kill the outer context to make SaveBlobAsync fail
+		// precisely injecting a specific error into the repository is not possible, so just cancel the context
+		f.outerCancel(f.failSaveRepo.err)
 	}
 
 	f.saver.SaveBlobAsync(ctx, t, buf, id, storeDuplicate, func(newID restic.ID, known bool, size int, err error) {
@@ -2141,7 +2143,6 @@ func (f *failSaveSaver) SaveBlobAsync(ctx context.Context, t restic.BlobType, bu
 			if err == nil {
 				panic("expected error")
 			}
-			err = f.failSaveRepo.err
 		}
 		cb(newID, known, size, err)
 		<-f.semaphore
@@ -2149,13 +2150,10 @@ func (f *failSaveSaver) SaveBlobAsync(ctx context.Context, t restic.BlobType, bu
 }
 
 func TestArchiverAbortEarlyOnError(t *testing.T) {
-	var testErr = errors.New("test error")
-
 	var tests = []struct {
 		src       TestDir
 		wantOpen  map[string]uint
 		failAfter uint // error after so many blobs have been saved to the repo
-		err       error
 	}{
 		{
 			src: TestDir{
@@ -2167,10 +2165,7 @@ func TestArchiverAbortEarlyOnError(t *testing.T) {
 			},
 			wantOpen: map[string]uint{
 				filepath.FromSlash("dir/bar"): 1,
-				filepath.FromSlash("dir/baz"): 1,
-				filepath.FromSlash("dir/foo"): 1,
 			},
-			err: testErr,
 		},
 		{
 			src: TestDir{
@@ -2198,7 +2193,6 @@ func TestArchiverAbortEarlyOnError(t *testing.T) {
 			// fails after four to seven files were opened, as the ReadConcurrency allows for
 			// two queued files and one blob queued for saving.
 			failAfter: 4,
-			err:       testErr,
 		},
 	}
 
@@ -2217,10 +2211,11 @@ func TestArchiverAbortEarlyOnError(t *testing.T) {
 				opened: make(map[string]uint),
 			}
 
+			testErr := context.Canceled
 			testRepo := &failSaveRepo{
 				archiverRepo: repo,
 				failAfter:    int32(test.failAfter),
-				err:          test.err,
+				err:          testErr,
 			}
 
 			// at most two files may be queued
@@ -2233,8 +2228,8 @@ func TestArchiverAbortEarlyOnError(t *testing.T) {
 			}
 
 			_, _, _, err := arch.Snapshot(ctx, []string{"."}, SnapshotOptions{Time: time.Now()})
-			if !errors.Is(err, test.err) {
-				t.Errorf("expected error (%v) not found, got %v", test.err, err)
+			if !errors.Is(err, testErr) {
+				t.Errorf("expected error (%v) not found, got %v", testErr, err)
 			}
 
 			t.Logf("Snapshot return error: %v", err)

internal/repository/repository.go

@@ -42,7 +42,8 @@ type Repository struct {
 	opts Options
 
 	packerWg    *errgroup.Group
-	blobWg      *errgroup.Group
+	mainWg      *errgroup.Group
+	blobSaver   *sync.WaitGroup
 	uploader    *packerUploader
 	treePM      *packerManager
 	dataPM      *packerManager
@@ -562,12 +563,14 @@ func (r *Repository) removeUnpacked(ctx context.Context, t restic.FileType, id r
 
 func (r *Repository) WithBlobUploader(ctx context.Context, fn func(ctx context.Context, uploader restic.BlobSaverWithAsync) error) error {
 	wg, ctx := errgroup.WithContext(ctx)
+	// pack uploader + wg.Go below + blob saver (CPU bound)
+	wg.SetLimit(2 + runtime.GOMAXPROCS(0))
+	r.mainWg = wg
 	r.startPackUploader(ctx, wg)
-	saverCtx := r.startBlobSaver(ctx, wg)
+	// blob saver are spawned on demand, use wait group to keep track of them
+	r.blobSaver = &sync.WaitGroup{}
 	wg.Go(func() error {
-		// 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 {
+		if err := fn(ctx, &blobSaverRepo{repo: r}); err != nil {
 			return err
 		}
 		if err := r.flush(ctx); err != nil {
@@ -594,22 +597,6 @@ 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
 }
@@ -624,28 +611,26 @@ func (r *blobSaverRepo) SaveBlobAsync(ctx context.Context, t restic.BlobType, bu
 
 // Flush saves all remaining packs and the index
 func (r *Repository) flush(ctx context.Context) error {
-	if err := r.flushBlobUploader(); err != nil {
-		return err
-	}
+	r.flushBlobSaver()
+	r.mainWg = nil
 
-	if err := r.flushPacks(ctx); err != nil {
+	if err := r.flushPackUploader(ctx); err != nil {
 		return err
 	}
 
 	return r.idx.Flush(ctx, &internalRepository{r})
 }
 
-func (r *Repository) flushBlobUploader() error {
-	if r.blobWg == nil {
-		return nil
+func (r *Repository) flushBlobSaver() {
+	if r.blobSaver == nil {
+		return
 	}
-	err := r.blobWg.Wait()
-	r.blobWg = nil
-	return err
+	r.blobSaver.Wait()
+	r.blobSaver = nil
 }
 
 // FlushPacks saves all remaining packs.
-func (r *Repository) flushPacks(ctx context.Context) error {
+func (r *Repository) flushPackUploader(ctx context.Context) error {
 	if r.packerWg == nil {
 		return nil
 	}
@@ -1032,11 +1017,11 @@ func (r *Repository) saveBlob(ctx context.Context, t restic.BlobType, buf []byte
 }
 
 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 {
+	r.mainWg.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)
+			cb(restic.ID{}, false, 0, ctx.Err())
+			return ctx.Err()
 		}
 		newID, known, size, err := r.saveBlob(ctx, t, buf, id, storeDuplicate)
 		cb(newID, known, size, err)