server: set connection to TCP socket when unix and TCP used (#23463)

[ti-srebot]

cherry-pick #23463 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/23513

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

git push git@github.com:ti-srebot/tidb.git pr/23513:release-4.0-fab0b934a47f

---

What problem does this PR solve?

Issue Number: Fixes #23460

Problem Summary:

If a unix socket configuration is present, the server will set connections to type UnixSocket irrespective of if it was used. This is a problem when the server is listening on both, since connections of type unixSocket do not validate the host portion of the user.

Although ideally the function isUnixSocket() could be a clientConnection property, this improves the security, because if both types of socket are used, the function will return FALSE.

What is changed and how it works?

Fixes #23460

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• Unit test

Side effects

• Breaking backward compatibility (fixes security issue. It is possible, but unlikely users depend on it)

Release note

• When TiDB was configured to listen on TCP and UNIX sockets, connections over TCP did not correctly validate that the remote host was permitted to connect.

[ti-srebot]

/run-all-tests

[ti-srebot]

@morgo please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/tidb/invitations

[morgo]

/run-check-dev

[morgo]

/run-check_dev

[djshow832]

/lgtm

[xhebox]

LGTM

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• djshow832
• 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 writing /lgtm in a comment.
Reviewer can cancel approval by writing /lgtm cancel in a comment.

[xhebox]

/merge

[ti-chi-bot]

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

Commit hash: 8f4c61d

[morgo]

/merge

[morgo]

/run-unit-test

[djshow832]

/run-unit-test

[djshow832]

/merge

[zhouqiang-cl]

/merge