executor: use before statement snapshot for UnionScan

[bobotu]

What problem does this PR solve?

Issue Number: close #19054

What's Changed:

Cherry-pick #19276 to release-3.0

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Release note

• Fix data race in UnionScan

[jackysp]

Missing the test cases?

[bobotu]

MySQL [test]> explain update /*+ TIDB_INLJ(t1) */ t t1, (select a, b from t) t2 set t1.b = t2.b where t1.a = t2.a + 1000;
+----------------------------+----------+------+------------------------------------------------------------------------------+
| id                         | count    | task | operator info                                                                |
+----------------------------+----------+------+------------------------------------------------------------------------------+
| Update_8                   | N/A      | root | N/A                                                                          |
| └─HashLeftJoin_10          | 99998.00 | root | inner join, inner:Projection_16, equal:[eq(test.t1.a, plus(test.t.a, 1000))] |
|   ├─UnionScan_13           | 99998.00 | root |                                                                              |
|   │ └─TableReader_15       | 99998.00 | root | data:TableScan_14                                                            |
|   │   └─TableScan_14       | 99998.00 | cop  | table:t, range:[-inf,+inf], keep order:false                                 |
|   └─Projection_16          | 99998.00 | root | test.t.a, test.t.b, plus(test.t.a, 1000)                                     |
|     └─UnionScan_17         | 99998.00 | root |                                                                              |
|       └─TableReader_19     | 99998.00 | root | data:TableScan_18                                                            |
|         └─TableScan_18     | 99998.00 | cop  | table:t, range:[-inf,+inf], keep order:false                                 |
+----------------------------+----------+------+------------------------------------------------------------------------------+

The execution plan in v3.0 seems cannot reproduce race.

[jackysp]

LGTM

[cfzjywxk]

LGTM

[qw4990]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

@bobotu merge failed.

[qw4990]

/rebuild

[qw4990]

/rebuild