session: force tidb_multi_statement_mode=OFF when mode=WARN (#25723)

[ti-srebot]

cherry-pick #25723 to release-4.0
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/25749

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

git push git@github.com:ti-srebot/tidb.git pr/25749:release-4.0-1cab530b793a

---

What problem does this PR solve?

Problem Summary:

In TiDB 5.0 the default for tidb_multi_statement_mode=OFF, but for upgrades the previous default from 4.0 could be inherited. This changes the value for upgrades as well.

From the docs, "Only the default value of OFF can be considered safe". This was always known to be the case, WARN was introduced to provide a nagging upgrade path for users. Now its time.

What is changed and how it works?

What's Changed:

For users upgrading from TiDB 4.0, the value of tidb_multi_statement_mode is now OFF. It is recommended to use the multi-statement feature of your client library instead, see the documentation on tidb_multi_statement_mode for additional details.

Related changes

• Need to cherry-pick to the release branch
• The cherry pick to 4.0 will look a bit different as it also needs to change the sysvar default.

Check List

Tests

• Manual test (add detailed scripts or steps below)

Bootstrap tasks are hard to test with automated testing. I launched a 4.0.13 server and then ran the SQL statement in this bootstrap task and confirmed only 1 row was modified, and the value changed to OFF.

Side effects

• Breaking backward compatibility

Release note

• For users upgrading from TiDB 4.0, the value of tidb_multi_statement_mode is now OFF. It is recommended to use the multi-statement feature of your client library instead, see the documentation on tidb_multi_statement_mode for additional details.

[ti-srebot]

/run-all-tests

[morgo]

This is similar to the comment in #25750 : I need to be really careful to not break the upgrade since the bootstrap task numbers are shared between versions.

The way I implemented this was to take the next number along and merge the intended task with the task needed here. I also needed to change the default in the sysvars.go file.

[ti-chi-bot]

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• bb7133
• xhebox

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[morgo]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: c6d570d

[eurekaka]

@morgo shall we bump currentBootstrapVersion to version52?

[morgo]

@morgo shall we bump currentBootstrapVersion to version52?

Yes, I missed it :( It will be handled in #26006