Fix git.Blob.DataAsync(): close pipe since we return a NopCloser (#16899)

* make sure headGitRepo is closed on err too

* refactor

* Fix git.Blob.DataAsync(): exec cancel since we already read all bytes (close pipe since we return a NopCloser)

modules/git/blob_nogogit.go

@@ -47,8 +47,8 @@ func (b *Blob) DataAsync() (io.ReadCloser, error) {
 
 	if size < 4096 {
 		bs, err := ioutil.ReadAll(io.LimitReader(rd, size))
+		defer cancel()
 		if err != nil {
-			cancel()
 			return nil, err
 		}
 		_, err = rd.Discard(1)
@@ -106,12 +106,12 @@ func (b *blobReader) Read(p []byte) (n int, err error) {
 
 // Close implements io.Closer
 func (b *blobReader) Close() error {
+	defer b.cancel()
 	if b.n > 0 {
 		for b.n > math.MaxInt32 {
 			n, err := b.rd.Discard(math.MaxInt32)
 			b.n -= int64(n)
 			if err != nil {
-				b.cancel()
 				return err
 			}
 			b.n -= math.MaxInt32
@@ -119,14 +119,12 @@ func (b *blobReader) Close() error {
 		n, err := b.rd.Discard(int(b.n))
 		b.n -= int64(n)
 		if err != nil {
-			b.cancel()
 			return err
 		}
 	}
 	if b.n == 0 {
 		_, err := b.rd.Discard(1)
 		b.n--
-		b.cancel()
 		return err
 	}
 	return nil