sessionctx/variable: make lc_time_names read only

[espresso98]

What problem does this PR solve?

Issue Number: close #29949

Problem Summary:
In TiDB lc_time_names variable is a noop, but allowing it to be changed might lead to unexpected results.

What is changed and how it works?

The system variable lc_time_names is now ReadOnly. It will return an error if we try to change it from its default (en_US).

This is a similar handling to optimizer_swtich. In other cases (where user queries may depend on syntax), we should instead use tidb_enable_noop_functions so there is a flag to permit the syntax. But that is unlikely to be required as it is not common functionality.

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

The system variable `lc_time_names` is now read only.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• dveeden
• morgo

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[morgo]

I have added the compatibility-breaker tag because it might not be suitable for cherry pick. But this is not a commonly used variable, so I do not expect any compatibility issues from applications.

[dveeden]

[email protected] [(none)]> SELECT @@lc_time_names;
+-----------------+
| @@lc_time_names |
+-----------------+
| en_US           |
+-----------------+
1 row in set (0.00 sec)

[email protected] [(none)]> SET SESSION lc_time_names='en_IN';
ERROR 1238 (HY000): Variable 'lc_time_names' is a read only variable
[email protected] [(none)]> SET GLOBAL lc_time_names='en_IN';
ERROR 1238 (HY000): Variable 'lc_time_names' is a read only variable
[email protected] [(none)]> SELECT tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v5.4.0-alpha-214-g5da98be90
Edition: Community
Git Commit Hash: 5da98be90ce011a8463fd9473d0856a936bae782
Git Branch: fix-lc_time_names
UTC Build Time: 2021-11-25 11:26:59
GoVersion: go1.17.1
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false
1 row in set (0.00 sec)

[xiongjiwei]

/merge

[ti-chi-bot]

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

Commit hash: 5da98be