Use fresh context while canceling a migration

Signed-off-by: Rohit Nayak <rohit@planetscale.com>

go/vt/vtctl/workflow/traffic_switcher.go

@@ -1135,30 +1135,46 @@ func (ts *trafficSwitcher) switchDeniedTables(ctx context.Context) error {
 	return nil
 }
 
+// cancelMigration attempts to revert all changes made during the migration so that we can get back to the
+// state when traffic switching (or reversing) was initiated.
 func (ts *trafficSwitcher) cancelMigration(ctx context.Context, sm *StreamMigrator) {
 	var err error
+
+	if ctx.Err() != nil {
+		// We are creating a new context for cancelMigration, but we still record any error,
+		// for any forensics in case of failures, to help determine whether the migration is being
+		// cancelled due to a client timeout or some other reason.
+		ts.Logger().Infof("In Cancel migration: original context invalid: %s", ctx.Err())
+	}
+
+	// We create a new context while canceling the migration, so that we are independent of the original
+	// context being cancelled prior to or during the cancel operation.
+	cmTimeout := 60 * time.Second
+	cmCtx, cmCancel := context.WithTimeout(context.Background(), cmTimeout)
+	defer cmCancel()
+
 	if ts.MigrationType() == binlogdatapb.MigrationType_TABLES {
-		err = ts.switchDeniedTables(ctx)
+		err = ts.switchDeniedTables(cmCtx)
 	} else {
-		err = ts.changeShardsAccess(ctx, ts.SourceKeyspaceName(), ts.SourceShards(), allowWrites)
+		err = ts.changeShardsAccess(cmCtx, ts.SourceKeyspaceName(), ts.SourceShards(), allowWrites)
 	}
 	if err != nil {
 		ts.Logger().Errorf("Cancel migration failed: %v", err)
 	}
 
-	sm.CancelStreamMigrations(ctx)
+	sm.CancelStreamMigrations(cmCtx)
 
 	err = ts.ForAllTargets(func(target *MigrationTarget) error {
 		query := fmt.Sprintf("update _vt.vreplication set state='Running', message='' where db_name=%s and workflow=%s",
 			encodeString(target.GetPrimary().DbName()), encodeString(ts.WorkflowName()))
-		_, err := ts.TabletManagerClient().VReplicationExec(ctx, target.GetPrimary().Tablet, query)
+		_, err := ts.TabletManagerClient().VReplicationExec(cmCtx, target.GetPrimary().Tablet, query)
 		return err
 	})
 	if err != nil {
 		ts.Logger().Errorf("Cancel migration failed: could not restart vreplication: %v", err)
 	}
 
-	err = ts.deleteReverseVReplication(ctx)
+	err = ts.deleteReverseVReplication(cmCtx)
 	if err != nil {
 		ts.Logger().Errorf("Cancel migration failed: could not delete reverse vreplication streams: %v", err)
 	}