server, sessionctx: improved mysql compatibility with support for init_connect (#23713)

[ti-srebot]

cherry-pick #23713 to release-5.0
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/26072

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

git push git@github.com:ti-srebot/tidb.git pr/26072:release-5.0-9c75cfa4e2bb

---

What problem does this PR solve?

Issue Number: close #18894

Problem Summary:

MySQL supports the ability to execute a set of statements on first connection. This PR adds the same feature to TiDB.

It can be useful for some applications, since SQL modes can be set, user variables, etc. It is also used extensively in the mysql-test testsuite.

What is changed and how it works?

Proposal: xxx

What's Changed:

How it Works:

Related changes

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

Check List

Tests

• Integration test: server integration tests.

Side effects

• Performance regression
  • Regrettably checking if initconnect returned an error is in the query-serving path. This was required to return the client an error.

Release note

• TiDB now supports the mysql system variable init_connect and associated functionality.

[ti-srebot]

/run-all-tests

[ti-srebot]

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

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• AilinKid
• 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.

[AilinKid]

/merge

[ti-chi-bot]

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

Commit hash: df04318

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.