server, sessionctx: improved mysql compatibility with support for init_connect

[morgo]

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.

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

I will work on tidb-test tests. Because this feature is dependent on the MySQL client protocol, it could be in server/tidb_test.go, but the existing tests there do not differentiate user privileges (this only applies to non-super users). I thought it would be better to write a tidb-test.

• Manual test (add detailed scripts or steps below)

• Connect as root. Run set global init_connect='select sleep(1);`

• Create a user CREATE USER u1;
• Connect as the new user: mysql -uu1. You should notice a 1 second pause.

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.

[morgo]

Closing for now.