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

[ti-srebot]

cherry-pick #33651 to release-6.0
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/34059

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

git push [email protected]:ti-srebot/tidb.git pr/34059:release-6.0-4d3a3c259a58

---

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]

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[ti-srebot]

/run-all-tests

[CbcWestwolf]

/run-all-tests

[xhebox]

closed for DMR version