*: add support for disabling noop variables

[morgo]

What problem does this PR solve?

Issue Number: close #8895

Problem Summary:

TiDB features a set of "noop sysvars", which are imported from MySQL 5.6 and are used for compatibility with applications that expect certain system variables to be present. These sysvars do nothing: and without reading the source code, it's not possible to differentiate between a regular sysvar and a noop sysvar.

Noops violate the principal of least surprise, since when the system variables are present they are expected to do something. It requires exceptional expertise to have read all the docs or notice the feature does not work as intended. Often the database team won't have the application-level expertise to know if the application is working correctly, because they rely on errors being returned (of which there will be none).

Thus, adding support to disable noop variables makes safe migrations easier.

What is changed and how it works?

A new system variable tidb_enable_noop_variables is introduced. The default is ON, which is compatible with the existing behavior. When set to OFF, TiDB has the following behaviors:

• Setting a noop sysvar returns a warning.
• SHOW [GLOBAL] VARIABLES no longer displays noop sysvars.
• SELECT @@noopvars returns a warning.

The warnings returned by setting and selecting a noop sysvar differ. The statement select * from information_schema.CLIENT_ERRORS_SUMMARY_GLOBAL; can be used to check if a TiDB server has been asked to get or set a noop system variable.

Check List

Tests

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

mysql> SET GLOBAL tidb_enable_noop_variables=0;
Query OK, 0 rows affected (0.00 sec)

mysql> SET query_cache_type=2;
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> select * from information_schema.CLIENT_ERRORS_SUMMARY_GLOBAL;
+--------------+--------------------------------------------------------------------------+-------------+---------------+---------------------+---------------------+
| ERROR_NUMBER | ERROR_MESSAGE                                                            | ERROR_COUNT | WARNING_COUNT | FIRST_SEEN          | LAST_SEEN           |
+--------------+--------------------------------------------------------------------------+-------------+---------------+---------------------+---------------------+
|         8144 | setting %s has no effect in TiDB                                         |           0 |             1 | 2022-06-19 13:23:21 | 2022-06-19 13:23:21 |
|         1193 | Unknown system variable '%-.64s'                                         |           2 |             2 | 2022-06-19 13:23:02 | 2022-06-19 13:23:10 |
|         1229 | Variable '%-.64s' is a GLOBAL variable and should be set with SET GLOBAL |           1 |             1 | 2022-06-19 13:23:07 | 2022-06-19 13:23:07 |
+--------------+--------------------------------------------------------------------------+-------------+---------------+---------------------+---------------------+
3 rows in set (0.00 sec)

• 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

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

A new system variable `tidb_enable_noop_variables` is introduced. The default is `ON`, which is compatible with the existing behavior. When set to `OFF`, TiDB has the following behaviors:

* Setting a noop sysvar returns a warning.
* SHOW [GLOBAL] VARIABLES no longer displays noop sysvars.
* SELECT @@noopvars returns a warning.

The warnings returned by setting and selecting a noop sysvar differ. The statement `select * from information_schema.CLIENT_ERRORS_SUMMARY_GLOBAL;` can be used to check if a TiDB server has been asked to get or set a noop system variable.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• bb7133
• xhebox

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/a0a31d92a65093d2cb30a38ea13bb1a18023540e

[xhebox]

Rest LGTM

[morgo]

/run-unit-test

[bb7133]

LGTM

[bb7133]

/merge

[ti-chi-bot]

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

Commit hash: 6a63dc9

[hawkingrei]

/run-mysql-test
/run-unit-test

[hawkingrei]

/run-mysql-test

[hawkingrei]

/run-mysql-test

[sre-bot]

TiDB MergeCI notify

🔴 Bad News! [1] CI still failing after this pr merged.
These failed integration tests don't seem to be introduced by the current PR.

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci-tidb/integration-common-test	🔴 failed 1, success 10, total 11	23 min	Existing failure
idc-jenkins-ci/integration-cdc-test	🟢 all 34 tests passed	28 min	Existing passed
idc-jenkins-ci-tidb/common-test	🟢 all 12 tests passed	10 min	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	8 min 33 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-1	🟢 all 26 tests passed	8 min 5 sec	Existing passed
idc-jenkins-ci-tidb/integration-ddl-test	🟢 all 6 tests passed	7 min 45 sec	Existing passed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	5 min 41 sec	Existing passed
idc-jenkins-ci-tidb/mybatis-test	🟢 all 1 tests passed	3 min 13 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	2 min 43 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed