Add missing unpacker.Wait for image import

- For remote snapshotters, the unpack phase serves as an important step for
  preparing the remote snapshot. With the missing unpacker.Wait, the
  snapshotter `Prepare` context is always canceled.
- This patch allows remote snapshotter based archives to be imported via
  the transfer service or `ctr image import`

Signed-off-by: Edgar Lee <edgarhinshunlee@gmail.com>

pkg/transfer/local/import.go

@@ -113,10 +113,20 @@ func (ts *localTransferService) importStream(ctx context.Context, i transfer.Ima
 	}
 
 	if err := images.WalkNotEmpty(ctx, handler, index); err != nil {
+		if unpacker != nil {
+			// wait for unpacker to cleanup
+			unpacker.Wait()
+		}
 		// TODO: Handle Not Empty as a special case on the input
 		return err
 	}
 
+	if unpacker != nil {
+		if _, err = unpacker.Wait(); err != nil {
+			return err
+		}
+	}
+
 	for _, desc := range descriptors {
 		imgs, err := is.Store(ctx, desc, ts.images)
 		if err != nil {