wait_ts information in slow log is not always accuracy

[crazycs520]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mock a delay in TiDB code:

--- a/session/session.go
+++ b/session/session.go
@@ -1567,6 +1567,10 @@ func (s *session) Txn(active bool) (kv.Transaction, error) {
        }
        if s.txn.pending() {
                defer func(begin time.Time) {
+                       if !s.sessionVars.InRestrictedSQL {
+                               fmt.Printf("sleep--\n")
+                               time.Sleep(50 * time.Millisecond)
+                       }
                        s.sessionVars.DurationWaitTS = time.Since(begin)
                }(time.Now())
                // Transaction is lazy initialized.

then:

set @@tidb_slow_log_threshold=30;
begin;
select * from t where a=1 and (select sleep(0.03)) is null;

Check the related slow-log of last SQL:

# Time: 2021-01-06T17:52:17.927353+08:00
# Txn_start_ts: 422032634769833985
# Query_time: 0.051914883
# Wait_TS: 0.051791473
...
begin;

# Time: 2021-01-06T17:52:24.252546+08:00
# Txn_start_ts: 422032634769833985
# Query_time: 0.032830658
# Wait_TS: 0.051791473
....
select * from t where a=1 and (select sleep(0.03)) is null;

As you can see, the Query_time is 0.032630017 second, but the Wait_TS is 0.050233804, it's wired, since Wait_TS should be 0 for the query select ..., and the Wait_TS should only emerging in the slow log of the SQL begin.

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

test> select tidb_version();
+-------------------------------------------------------------------+
| tidb_version()                                                    |
+-------------------------------------------------------------------+
| Release Version: v4.0.0-beta.2-1973-g51794e9d3-dirty              |
| Edition: Community                                                |
| Git Commit Hash: 51794e9d304896a315d80ccb4603f59dc75f26d9         |
| Git Branch: master                                                |
| UTC Build Time: 2021-01-06 09:23:45                               |
| GoVersion: go1.15.6                                               |
| Race Enabled: false                                               |
| TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306 |
| Check Table Before Drop: false                                    |
+-------------------------------------------------------------------+