Support reversetraffic for read-only tablets

Signed-off-by: Matt Lord <mattalord@gmail.com>

go/vt/vtctl/workflow/server.go

@@ -3186,16 +3186,40 @@ func (s *Server) WorkflowSwitchTraffic(ctx context.Context, req *vtctldatapb.Wor
  }
  direction := TrafficSwitchDirection(req.Direction)
  if direction == DirectionBackward {
- ts, startState, err = s.getWorkflowState(ctx, startState.SourceKeyspace, ts.reverseWorkflow)
- if err != nil {
- return nil, err
- }
  if ts.IsMultiTenantMigration() {
  // In a multi-tenant migration, multiple migrations would be writing to the same table, so we can't stop writes like
  // we do with MoveTables, using denied tables, since it would block all other migrations as well as traffic for
  // tenants which have already been migrated.
  return nil, vterrors.Errorf(vtrpcpb.Code_INVALID_ARGUMENT, "cannot reverse traffic for multi-tenant migrations")
  }
+ if !startState.WritesSwitched && !slices.Contains(req.TabletTypes, topodatapb.TabletType_PRIMARY) {
+ // We don't need to do anything but update the routing rules in the
+ // other direction.
+ ts.targetKeyspace = startState.SourceKeyspace
+ ts.sourceKeyspace = startState.TargetKeyspace
+ if err := ts.switchTableReads(ctx, req.Cells, req.TabletTypes, true, direction); err != nil {
+ return nil, vterrors.Wrapf(err, "failed to reverse traffic for %v tablets in workflow %s",
+ req.TabletTypes, req.Workflow)
+ }
+ resp := &vtctldatapb.WorkflowSwitchTrafficResponse{
+ Summary: fmt.Sprintf("ReverseTraffic was successful for workflow %s.%s", req.Keyspace, req.Workflow),
+ StartState: startState.String(),
+ }
+ _, currentState, err := s.getWorkflowState(ctx, req.Keyspace, req.Workflow)
+ if err != nil {
+ resp.CurrentState = fmt.Sprintf("Error reloading workflow state after switching traffic: %v", err)
+ } else {
+ resp.CurrentState = currentState.String()
+ }
+ return resp, nil
+ }
+ // Update the starting state so that we're using the reverse workflow so that we can
+ // move forward with a normal traffic switch operation.
+ ts, startState, err = s.getWorkflowState(ctx, startState.SourceKeyspace, ts.reverseWorkflow)
+ if err != nil {
+ return nil, err
+ }
+
  }
 
  ts.force = req.GetForce()