move calling stats handlers into closure to avoid holding two locks

Author: arjan-bal

internal/transport/handler_server.go

@@ -272,19 +272,15 @@ func (ht *serverHandlerTransport) writeStatus(s *ServerStream, st *status.Status
 				}
 			}
 		}
-	})
-
-	if err == nil { // transport has not been closed
 		// Note: The trailer fields are compressed with hpack after this call returns.
 		// No WireLength field is set here.
-		s.hdrMu.Lock()
 		for _, sh := range ht.stats {
 			sh.HandleRPC(s.Context(), &stats.OutTrailer{
 				Trailer: s.trailer.Copy(),
 			})
 		}
-		s.hdrMu.Unlock()
-	}
+	})
+
 	ht.Close(errors.New("finished writing status"))
 	return err
 }