session/variable: forbid changing @@global.require_secure_transport to 'on' with SEM enabled (#47677)

[ti-chi-bot]

This is an automated cherry-pick of #47677

What problem does this PR solve?

Issue Number: close #47665

Problem Summary:

The configuration for TiDB Cloud dedicated cluster is:

require_secure_transport: OFF
ssl_ca : /var/lib/tidb-server-tls/ca.crt
ssl_cert : /var/lib/tidb-server-tls/tls.crt
ssl_key: /var/lib/tidb-server-tls/tls.key

We just provide the ca.crt file for the the users to connect the TiDB cluster. When require_secure_transport = OFF, there is no problem.

But if the user set the value of require_secure_transport to 'ON', the users can not connect to the cluster anymore!

This is because the new configuration require the mysql client to pass all ca.crt tls.crt tls.key correctly, while the user only have the ca.crt file.

What is changed and how it works?

To prevent the user from losing access of the cluster, we can forbid changing require_secure_transport system variable to 'on' as a workaround, then the user can not modify it and trigger the issue.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Modify the config.toml.example:

# Security Enhanced Mode (SEM) restricts the "SUPER" privilege and requires fine-grained privileges instead.
enable-sem = true

./bin/tidb-server -config ./pkg/config/config.toml.example
mysql -h 127.0.0.1 -u root -P 4000

mysql> set @@global.require_secure_transport = on;
ERROR 1105 (HY000): require_secure_transport can not be set to ON with SEM(security enhanced mode) enabled

• No need to test

  • I checked and no code files have been changed.

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

Please refer to Release Notes Language Style Guide to write a quality release note.

Forbid changing @@global.require_secure_transport variable to 'on' when SEM(security enhanced mode) is enabled

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/build	9e7920a	link	true	/test build
idc-jenkins-ci-tidb/check_dev_2	9e7920a	link	true	/test check-dev2
idc-jenkins-ci-tidb/check_dev	9e7920a	link	true	/test check-dev
idc-jenkins-ci-tidb/unit-test	9e7920a	link	true	/test unit-test
idc-jenkins-ci-tidb/mysql-test	9e7920a	link	true	/test mysql-test

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[djshow832]

Duplicated with #47689