archiver: Fix race condition resulting in files containing null IDs

In some rare cases files could be created which contain null IDs (all
zero) in their content list. This was caused by a race condition between
growing the `Content` slice and inserting the blob IDs into it. In some
cases the blob ID was written to the old slice, which a short time
afterwards was replaced with a larger copy, that did not yet contain the
blob ID.

internal/archiver/blob_saver_test.go

@@ -48,14 +48,19 @@ func TestBlobSaver(t *testing.T) {
 
 	var wait sync.WaitGroup
 	var results []SaveBlobResponse
+	var lock sync.Mutex
 
 	wait.Add(20)
 	for i := 0; i < 20; i++ {
 		buf := &Buffer{Data: []byte(fmt.Sprintf("foo%d", i))}
 		idx := i
+		lock.Lock()
 		results = append(results, SaveBlobResponse{})
+		lock.Unlock()
 		b.Save(ctx, restic.DataBlob, buf, func(res SaveBlobResponse) {
+			lock.Lock()
 			results[idx] = res
+			lock.Unlock()
 			wait.Done()
 		})
 	}

internal/archiver/file_saver.go

@@ -199,7 +199,10 @@ func (s *FileSaver) saveFile(ctx context.Context, chnker *chunker.Chunker, snPat
 
 		// add a place to store the saveBlob result
 		pos := idx
+
+		lock.Lock()
 		node.Content = append(node.Content, restic.ID{})
+		lock.Unlock()
 
 		s.saveBlob(ctx, restic.DataBlob, buf, func(sbr SaveBlobResponse) {
 			lock.Lock()