Merge pull request vitessio#3920 from jvaidya/fix_reparent_order

During reparent, first stop queries to vttablet, then set the db to RO

Author: sougou

go/vt/vttablet/tabletmanager/rpc_replication.go

@@ -298,13 +298,7 @@ func (agent *ActionAgent) DemoteMaster(ctx context.Context) (string, error) {
 		}
 	}()
 
-	// Set the server read-only. Note all active connections are not
-	// affected.
-	if err := agent.MysqlDaemon.SetReadOnly(true); err != nil {
-		return "", err
-	}
-
-	// Now disallow queries, to make sure nobody is writing to the
+	// First, disallow queries, to make sure nobody is writing to the
 	// database.
 	tablet := agent.Tablet()
 	// We don't care if the QueryService state actually changed because we'll
@@ -314,6 +308,12 @@ func (agent *ActionAgent) DemoteMaster(ctx context.Context) (string, error) {
 		return "", fmt.Errorf("SetServingType(serving=false) failed: %v", err)
 	}
 
+	// Now, set the server read-only. Note all active connections are not
+	// affected.
+	if err := agent.MysqlDaemon.SetReadOnly(true); err != nil {
+		return "", err
+	}
+
 	// If using semi-sync, we need to disable master-side.
 	if err := agent.fixSemiSync(topodatapb.TabletType_REPLICA); err != nil {
 		return "", err