archiver: reduce memory usage for large files

FutureBlob now uses a Take() method as a more memory-efficient way to
retrieve the futures result. In addition, futures are now collected
while saving the file. As only a limited number of blobs can be queued
for uploading, for a large file nearly all FutureBlobs already have
their result ready, such that the FutureBlob object just consumes
memory.

internal/archiver/archiver.go

@@ -184,17 +184,17 @@ func (arch *Archiver) saveTree(ctx context.Context, t *restic.TreeJSONBuilder) (
 	b := &Buffer{Data: buf}
 	res := arch.blobSaver.Save(ctx, restic.TreeBlob, b)
 
-	res.Wait(ctx)
-	if !res.Known() {
+	sbr := res.Take(ctx)
+	if !sbr.known {
 		s.TreeBlobs++
-		s.TreeSize += uint64(res.Length())
-		s.TreeSizeInRepo += uint64(res.SizeInRepo())
+		s.TreeSize += uint64(sbr.length)
+		s.TreeSizeInRepo += uint64(sbr.sizeInRepo)
 	}
-	// The context was canceled in the meantime, res.ID() might be invalid
+	// The context was canceled in the meantime, id might be invalid
 	if ctx.Err() != nil {
 		return restic.ID{}, s, ctx.Err()
 	}
-	return res.ID(), s, nil
+	return sbr.id, s, nil
 }
 
 // nodeFromFileInfo returns the restic node from an os.FileInfo.

internal/archiver/blob_saver.go

@@ -44,9 +44,7 @@ func (s *BlobSaver) TriggerShutdown() {
 // Save stores a blob in the repo. It checks the index and the known blobs
 // before saving anything. It takes ownership of the buffer passed in.
 func (s *BlobSaver) Save(ctx context.Context, t restic.BlobType, buf *Buffer) FutureBlob {
-	// buf might be freed once the job was submitted, thus calculate the length now
-	length := len(buf.Data)
-	ch := make(chan saveBlobResponse, 1)
+	ch := make(chan SaveBlobResponse, 1)
 	select {
 	case s.ch <- saveBlobJob{BlobType: t, buf: buf, ch: ch}:
 	case <-ctx.Done():
@@ -55,72 +53,62 @@ func (s *BlobSaver) Save(ctx context.Context, t restic.BlobType, buf *Buffer) Fu
 		return FutureBlob{ch: ch}
 	}
 
-	return FutureBlob{ch: ch, length: length}
+	return FutureBlob{ch: ch}
 }
 
 // FutureBlob is returned by SaveBlob and will return the data once it has been processed.
 type FutureBlob struct {
-	ch     <-chan saveBlobResponse
-	length int
-	res    saveBlobResponse
+	ch <-chan SaveBlobResponse
 }
 
-// Wait blocks until the result is available or the context is cancelled.
-func (s *FutureBlob) Wait(ctx context.Context) {
+func (s *FutureBlob) Poll() *SaveBlobResponse {
 	select {
-	case <-ctx.Done():
-		return
 	case res, ok := <-s.ch:
 		if ok {
-			s.res = res
+			return &res
 		}
+	default:
 	}
+	return nil
 }
 
-// ID returns the ID of the blob after it has been saved.
-func (s *FutureBlob) ID() restic.ID {
-	return s.res.id
+// Take blocks until the result is available or the context is cancelled.
+func (s *FutureBlob) Take(ctx context.Context) SaveBlobResponse {
+	select {
+	case res, ok := <-s.ch:
+		if ok {
+			return res
 		}
-
-// Known returns whether or not the blob was already known.
-func (s *FutureBlob) Known() bool {
-	return s.res.known
+	case <-ctx.Done():
 	}
-
-// Length returns the raw length of the blob.
-func (s *FutureBlob) Length() int {
-	return s.length
-}
-
-// SizeInRepo returns the number of bytes added to the repo (including
-// compression and crypto overhead).
-func (s *FutureBlob) SizeInRepo() int {
-	return s.res.size
+	return SaveBlobResponse{}
 }
 
 type saveBlobJob struct {
 	restic.BlobType
 	buf *Buffer
-	ch  chan<- saveBlobResponse
+	ch  chan<- SaveBlobResponse
 }
 
-type saveBlobResponse struct {
+type SaveBlobResponse struct {
 	id         restic.ID
+	length     int
+	sizeInRepo int
 	known      bool
-	size  int
 }
 
-func (s *BlobSaver) saveBlob(ctx context.Context, t restic.BlobType, buf []byte) (saveBlobResponse, error) {
-	id, known, size, err := s.repo.SaveBlob(ctx, t, buf, restic.ID{}, false)
+func (s *BlobSaver) saveBlob(ctx context.Context, t restic.BlobType, buf []byte) (SaveBlobResponse, error) {
+	id, known, sizeInRepo, err := s.repo.SaveBlob(ctx, t, buf, restic.ID{}, false)
 
 	if err != nil {
-		return saveBlobResponse{}, err
+		return SaveBlobResponse{}, err
 	}
 
-	return saveBlobResponse{
+	return SaveBlobResponse{
 		id:         id,
+		length:     len(buf),
+		sizeInRepo: sizeInRepo,
 		known:      known,
-		size:  size,
 	}, nil
 }
 

internal/archiver/blob_saver_test.go

@@ -54,8 +54,8 @@ func TestBlobSaver(t *testing.T) {
 	}
 
 	for i, blob := range results {
-		blob.Wait(ctx)
-		if blob.Known() {
+		sbr := blob.Take(ctx)
+		if sbr.known {
 			t.Errorf("blob %v is known, that should not be the case", i)
 		}
 	}

internal/archiver/file_saver.go

@@ -129,6 +129,15 @@ func (s *FileSaver) saveFile(ctx context.Context, chnker *chunker.Chunker, snPat
 	chnker.Reset(f, s.pol)
 
 	var results []FutureBlob
+	complete := func(sbr SaveBlobResponse) {
+		if !sbr.known {
+			stats.DataBlobs++
+			stats.DataSize += uint64(sbr.length)
+			stats.DataSizeInRepo += uint64(sbr.sizeInRepo)
+		}
+
+		node.Content = append(node.Content, sbr.id)
+	}
 
 	node.Content = []restic.ID{}
 	var size uint64
@@ -168,6 +177,17 @@ func (s *FileSaver) saveFile(ctx context.Context, chnker *chunker.Chunker, snPat
 		}
 
 		s.CompleteBlob(f.Name(), uint64(len(chunk.Data)))
+
+		// collect already completed blobs
+		for len(results) > 0 {
+			sbr := results[0].Poll()
+			if sbr == nil {
+				break
+			}
+			results[0] = FutureBlob{}
+			results = results[1:]
+			complete(*sbr)
+		}
 	}
 
 	err = f.Close()
@@ -176,15 +196,10 @@ func (s *FileSaver) saveFile(ctx context.Context, chnker *chunker.Chunker, snPat
 		return fnr
 	}
 
-	for _, res := range results {
-		res.Wait(ctx)
-		if !res.Known() {
-			stats.DataBlobs++
-			stats.DataSize += uint64(res.Length())
-			stats.DataSizeInRepo += uint64(res.SizeInRepo())
-		}
-
-		node.Content = append(node.Content, res.ID())
+	for i, res := range results {
+		results[i] = FutureBlob{}
+		sbr := res.Take(ctx)
+		complete(sbr)
 	}
 
 	node.Size = size

internal/archiver/file_saver_test.go

@@ -34,7 +34,7 @@ func startFileSaver(ctx context.Context, t testing.TB) (*FileSaver, context.Cont
 	wg, ctx := errgroup.WithContext(ctx)
 
 	saveBlob := func(ctx context.Context, tpe restic.BlobType, buf *Buffer) FutureBlob {
-		ch := make(chan saveBlobResponse)
+		ch := make(chan SaveBlobResponse)
 		close(ch)
 		return FutureBlob{ch: ch}
 	}