archiver: Asynchronously complete FutureFile

After reading and chunking all data in a file, the FutureFile still has
to wait until the FutureBlobs are completed. This was done synchronously
which results in blocking the file saver and prevents the next file from
being read.

By replacing the FutureBlob with a callback, it becomes possible to
complete the FutureFile asynchronously.

internal/archiver/blob_saver.go

@@ -43,51 +43,18 @@ 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 {
-	ch := make(chan SaveBlobResponse, 1)
+func (s *BlobSaver) Save(ctx context.Context, t restic.BlobType, buf *Buffer, cb func(res SaveBlobResponse)) {
 	select {
-	case s.ch <- saveBlobJob{BlobType: t, buf: buf, ch: ch}:
+	case s.ch <- saveBlobJob{BlobType: t, buf: buf, cb: cb}:
 	case <-ctx.Done():
 		debug.Log("not sending job, context is cancelled")
-		close(ch)
-		return FutureBlob{ch: ch}
 	}
-
-	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
-}
-
-func (s *FutureBlob) Poll() *SaveBlobResponse {
-	select {
-	case res, ok := <-s.ch:
-		if ok {
-			return &res
-		}
-	default:
-	}
-	return nil
-}
-
-// 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
-		}
-	case <-ctx.Done():
-	}
-	return SaveBlobResponse{}
 }
 
 type saveBlobJob struct {
 	restic.BlobType
 	buf *Buffer
-	ch  chan<- SaveBlobResponse
+	cb  func(res SaveBlobResponse)
 }
 
 type SaveBlobResponse struct {
@@ -128,11 +95,9 @@ func (s *BlobSaver) worker(ctx context.Context, jobs <-chan saveBlobJob) error {
 		res, err := s.saveBlob(ctx, job.BlobType, job.buf.Data)
 		if err != nil {
 			debug.Log("saveBlob returned error, exiting: %v", err)
-			close(job.ch)
 			return err
 		}
-		job.ch <- res
-		close(job.ch)
+		job.cb(res)
 		job.buf.Release()
 	}
 }