executor, util: fix UnionScan Next() skip reading data when passed chunk capacity is 0 (#36961)

[ti-srebot]

cherry-pick #36961 to release-6.2
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/37335

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/tidb.git pr/37335:release-6.2-81a93a6977e5

---

What problem does this PR solve?

Issue Number: close #36903

Problem Summary:

What is changed and how it works?

In the old UnionScan executor code, if an empty chk is passed to UnionScan.Next(chk) , the UnionScan executor will not read any data.

This bug could be trigger in select pkey from t_vwvgdc where 0 <> 0 union select pkey from t_vwvgdc;
The union first children return empty chunk, and the chunk is reused for the second children (union scan) to read data.

mysql> explain select pkey from t_vwvgdc where 0 <> 0 union select pkey from t_vwvgdc;
+------------------------------+---------+-----------+----------------+-------------------------------------------------------+
| id                           | estRows | task      | access object  | operator info                                         |
+------------------------------+---------+-----------+----------------+-------------------------------------------------------+
| HashAgg_13                   | 24.20   | root      |                | group by:Column#7, funcs:firstrow(Column#7)->Column#7 |
| └─Union_14                   | 29.00   | root      |                |                                                       |
|   ├─TableDual_16             | 0.00    | root      |                | rows:0                                                |
|   └─UnionScan_18             | 29.00   | root      |                |                                                       |
|     └─TableReader_20         | 29.00   | root      |                | data:TableFullScan_19                                 |
|       └─TableFullScan_19     | 29.00   | cop[tikv] | table:t_vwvgdc | keep order:false, stats:pseudo                        |
+------------------------------+---------+-----------+----------------+-------------------------------------------------------+
6 rows in set (0.00 sec)

Now I change req.GrowAndReset(us.maxChunkSize) to ensure capacity > 0 for the req.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Fix a bug that UnionScan's Next() function skips reading data when the passed chunk's capacity is 0

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• xhebox

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[ti-srebot]

/run-all-tests

[ti-srebot]

@tiancaiamao you're already a collaborator in bot's repo.

[tiancaiamao]

6.2 is not LTS version