server: use max_allowed_packet to limit the packet size. (#33651)

[ti-srebot]

cherry-pick #33651 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/34053

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

git push [email protected]:ti-srebot/tidb.git pr/34053:release-5.0-4d3a3c259a58

---

What problem does this PR solve?

Issue Number: close #31422

Problem Summary:

The max_allowed_packet exists but did not work on previous versions.

What is changed and how it works?

1. truncate the value of max_allowed_packet if it is not a multiple of 1024
2. report an error if the accumulated length of a packet is longer than max_allowed_packet

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

Fix the issue that system variable `max_allowed_packet` does not work.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has not been approved.

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-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick, it must first be approved by the collaborators.

AFTER it has been approved by collaborators, please ping the release team in a comment to request a cherry pick review.

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 kubernetes/test-infra repository.

[ti-srebot]

/run-all-tests

[ti-srebot]

@CbcWestwolf please accept the invitation then you can push to the cherry-pick pull requests. Comment with /cherry-pick-invite if the invitation is expired.
https://github.com/ti-srebot/tidb/invitations

[CbcWestwolf]

/close

[ti-chi-bot]

@CbcWestwolf: Closed this PR.

In response to this:

/close

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 kubernetes/test-infra repository.