alter table modify column to a time type may generate incorrect value

[lcwangchao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

My local time zone is Asia/Shanghai

> create table t(t int);
> insert into t values(NULL);
> set @@time_zone='UTC' --- set a different time zone with local
> alter table t modify column t timestamp not null;
> select now(),t from t;

2. What did you expect to see? (Required)

The value of now() and t should not differs too much.

3. What did you see instead (Required)

These two columns differs two hours

> select t, now() from t
+---------------------+---------------------+
| t                   | now()               |
+---------------------+---------------------+
| 2024-09-12 04:09:36 | 2024-09-12 12:13:47 |
+---------------------+---------------------+

4. What is your TiDB version? (Required)

[lcwangchao]

This bug is introduced by #55873 . In this PR, we use StaticEvalContext to replace mock.Context. However, these two contexts behaviors a little different for GetCurrentTime

For mock.Context, it uses SessionEvalContext as its inner expression context, with uses getStmtTimestamp to return a current time:

tidb/pkg/expression/contextsession/sessionctx.go

Lines 236 to 238 in d6db1e8

	func (ctx *SessionEvalContext) CurrentTime() (time.Time, error) {
	return getStmtTimestamp(ctx.sctx)
	}

And getStmtCurrentTime returns a time with location time.Local:

tidb/pkg/expression/contextsession/sessionctx.go

Line 338 in d6db1e8

return time.Unix(int64(seconds), int64(fractionalSeconds*float64(time.Second))), nil

For StaticEvalContext, it returns a current time with a location same with EvalContext.Location():

tidb/pkg/expression/contextstatic/evalctx.go

Line 65 in d6db1e8

tm = tm.In(loc)

So, if the out side code has a strong assumption that the returned current time's location, it will fail if we replace mock.Context to StaticEvalContext. Unfortunately, there are at least two places have this strong assumption:

In getTimeCurrentTimeStamp, it can cause DDL inserting wrong values in reorging:

tidb/pkg/expression/helper.go

Lines 68 to 82 in d6db1e8

	func getTimeCurrentTimeStamp(ctx EvalContext, tp byte, fsp int) (t types.Time, err error) {
	value := types.NewTime(types.ZeroCoreTime, tp, fsp)
	defaultTime, err := getStmtTimestamp(ctx)
	if err != nil {
	return value, err
	}
	value.SetCoreTime(types.FromGoTime(defaultTime.Truncate(time.Duration(math.Pow10(9-fsp)) * time.Nanosecond)))
	if tp == mysql.TypeTimestamp || tp == mysql.TypeDatetime || tp == mysql.TypeDate {
	err = value.ConvertTimeZone(time.Local, ctx.Location())
	if err != nil {
	return value, err
	}
	}
	return value, nil
	}

In evalNowWithFsp, it can cause lazy cursor fetch return wrong now() value:

tidb/pkg/expression/builtin_time.go

Lines 2504 to 2533 in d6db1e8

	func evalNowWithFsp(ctx EvalContext, fsp int) (types.Time, bool, error) {
	nowTs, err := getStmtTimestamp(ctx)
	if err != nil {
	return types.ZeroTime, true, err
	}
	failpoint.Inject("injectNow", func(val failpoint.Value) {
	nowTs = time.Unix(int64(val.(int)), 0)
	})
	// In MySQL's implementation, now() will truncate the result instead of rounding it.
	// Results below are from MySQL 5.7, which can prove it.
	// mysql> select now(6), now(3), now();
	// +----------------------------+-------------------------+---------------------+
	// | now(6) | now(3) | now() |
	// +----------------------------+-------------------------+---------------------+
	// | 2019-03-25 15:57:56.612966 | 2019-03-25 15:57:56.612 | 2019-03-25 15:57:56 |
	// +----------------------------+-------------------------+---------------------+
	result, err := convertTimeToMysqlTime(nowTs, fsp, types.ModeTruncate)
	if err != nil {
	return types.ZeroTime, true, err
	}
	err = result.ConvertTimeZone(time.Local, location(ctx))
	if err != nil {
	return types.ZeroTime, true, err
	}
	return result, false, nil
	}