archiver: Check that saved file does not have null IDs in content

Null IDs in the file content indicate that something went wrong. Thus
fails before saving the affected file.

internal/archiver/archiver_test.go

@@ -1969,7 +1969,7 @@ type failSaveRepo struct {
 func (f *failSaveRepo) SaveBlob(ctx context.Context, t restic.BlobType, buf []byte, id restic.ID, storeDuplicate bool) (restic.ID, bool, int, error) {
 	val := atomic.AddInt32(&f.cnt, 1)
 	if val >= f.failAfter {
-		return restic.ID{}, false, 0, f.err
+		return restic.Hash(buf), false, 0, f.err
 	}
 
 	return f.Repository.SaveBlob(ctx, t, buf, id, storeDuplicate)

internal/archiver/file_saver.go

@@ -128,6 +128,11 @@ func (s *FileSaver) saveFile(ctx context.Context, chnker *chunker.Chunker, snPat
 			if isCompleted {
 				panic("completed twice")
 			}
+			for _, id := range fnr.node.Content {
+				if id.IsNull() {
+					panic("completed file with null ID")
+				}
+			}
 			isCompleted = true
 			finish(fnr)
 		}

internal/archiver/file_saver_test.go

@@ -35,7 +35,12 @@ 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, cb func(SaveBlobResponse)) {
-		cb(SaveBlobResponse{})
+		cb(SaveBlobResponse{
+			id:         restic.Hash(buf.Data),
+			length:     len(buf.Data),
+			sizeInRepo: len(buf.Data),
+			known:      false,
+		})
 	}
 
 	workers := uint(runtime.NumCPU())