ddl: rollingback add index meets panic leads json unmarshal object error

ddl/db_test.go

@@ -5039,3 +5039,23 @@ func (s *testSerialDBSuite) TestDDLExitWhenCancelMeetPanic(c *C) {
 	c.Assert(job.ErrorCount, Equals, int64(4))
 	c.Assert(job.Error.Error(), Equals, "[ddl:-1]panic in handling DDL logic and error count beyond the limitation 3, cancelled")
 }
+
+// Close issue #23321.
+// See https://github.com/pingcap/tidb/issues/23321
+func (s *testSerialDBSuite) TestJsonUnmarshalErrWhenPanicInCancellingPath(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+
+	tk.MustExec("drop table if exists test_add_index_after_add_col")
+	tk.MustExec("create table test_add_index_after_add_col(a int, b int not null default '0');")
+	tk.MustExec("insert into test_add_index_after_add_col values(1, 2),(2,2);")
+	tk.MustExec("alter table test_add_index_after_add_col add column c int not null default '0';")
+
+	c.Assert(failpoint.Enable("github.com/pingcap/tidb/ddl/mockExceedErrorLimit", `return(true)`), IsNil)
+	defer func() {
+		c.Assert(failpoint.Disable("github.com/pingcap/tidb/ddl/mockExceedErrorLimit"), IsNil)
+	}()
+
+	_, err := tk.Exec("alter table test_add_index_after_add_col add unique index cc(c);")
+	c.Assert(err.Error(), Equals, "[kv:1062]DDL job cancelled by panic in rollingback, error msg: Duplicate entry '0' for key 'cc'")
+}

ddl/ddl_worker.go

@@ -585,6 +585,13 @@ func chooseLeaseTime(t, max time.Duration) time.Duration {
 // countForPanic records the error count for DDL job.
 func (w *worker) countForPanic(job *model.Job) {
 	// If run DDL job panic, just cancel the DDL jobs.
+	if job.State == model.JobStateRollingback {
+		job.State = model.JobStateCancelled
+		msg := fmt.Sprintf("DDL job cancelled by panic in rollingback, error msg: %s", terror.ToSQLError(job.Error).Message)
+		job.Error = terror.GetErrClass(job.Error).Synthesize(terror.ErrCode(job.Error.Code()), msg)
+		logutil.Logger(w.logCtx).Warn(msg)
+		return
+	}
 	job.State = model.JobStateCancelling
 	job.ErrorCount++