MDEV-17746: perfschema.dml_threads failed in buildbot with wrong errno

[gkodinov]

The test was trying to update the performance_schema.threads row for the current connection. And if that row was not found there's nothing to update hence no update not allowed error.
If the maximum number of instrumented threads is hit the assumption that there's always a row in performance_schema.thread for the current thread does not hold. This can happen under heavy concurrent load. mtr sets the maximum number of instrumented threads to 400. This is not much.
Stabilized the test as follows:

• Added a SELECT with the same conditon to ensure there's a row to be updated by the UPDATE following the select.
• Increased the performance_schema_max_thread_instances to 10k for the test.

[gemini-code-assist]

Code Review

This pull request updates the dml_threads performance schema test to query the current connection's thread ID before attempting an invalid update, ensuring the target row exists. It also introduces a master option file setting performance_schema_max_thread_instances to 50,000. The reviewer pointed out that this value is higher than the 10,000 specified in the pull request description, which could cause unnecessary memory consumption, and recommended lowering it to 10,000 and adding a trailing newline.

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[Copilot]

Pull request overview

Stabilizes the perfschema.dml_threads MTR test under heavy concurrency by ensuring the session’s performance_schema.threads row exists before running an UPDATE ... WHERE PROCESSLIST_ID=CONNECTION_ID() that is expected to error, and by increasing the PFS thread-instance sizing for this test run.

Changes:

• Adds a deterministic SELECT (with --replace_column) to confirm a matching performance_schema.threads row exists before the error-expected update.
• Increases performance_schema_max_thread_instances for the test via a new dml_threads-master.opt.
• Updates the expected result output accordingly.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
mysql-test/suite/perfschema/t/dml_threads.test	Adds a pre-check SELECT (with column replacement) before the error-expected update.
mysql-test/suite/perfschema/t/dml_threads-master.opt	Sets a higher performance_schema_max_thread_instances for this test to reduce flakiness under load.
mysql-test/suite/perfschema/r/dml_threads.result	Records the new SELECT output in the expected result.

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[vuvova]

Looks strange. What do you mean "can happen under heavy concurrent load" ? There is no concurrent load in mtr, every test always runs on a dedicated server, "concurrent" mtr means many servers running concurrently, not all tests on a single server.

[gkodinov]

Hitting that limit is the only possible explanation on why this particular query fails to update and the one prior to it is passing. What causes the thread depletion is a subject to a whole different study. But this bug is about stability of the test.

[svoj]

./mtr perfschema.dml_threads --repeat=100 fails reliably with the fix on the second iteration.

[svoj]

Simplified test:

--source include/have_perfschema.inc

select NAME, processlist_id from performance_schema.threads where processlist_id=connection_id();
--error ER_WRONG_PERFSCHEMA_USAGE
update performance_schema.threads
  set thread_id=12 where PROCESSLIST_ID=connection_id();

./mtr --repeat=2

select NAME, processlist_id from performance_schema.threads where processlist_id=connection_id();
NAME	processlist_id
thread/sql/one_connection	4
update performance_schema.threads
set thread_id=12 where PROCESSLIST_ID=connection_id();
ERROR HY000: Invalid performance_schema usage
main.AAA                                 [ pass ]      4
select NAME, processlist_id from performance_schema.threads where processlist_id=connection_id();
NAME	processlist_id
thread/sql/one_connection	8
update performance_schema.threads
set thread_id=12 where PROCESSLIST_ID=connection_id();
main.AAA                                 [ 2 fail ]
        Test ended at 2026-07-16 19:22:06

CURRENT_TEST: main.AAA
mysqltest: At line 5: query 'update performance_schema.threads
set thread_id=12 where PROCESSLIST_ID=connection_id()' succeeded - should have failed with error ER_WRONG_PERFSCHEMA_USAGE (1683)...

It demonstrates a valid row in the table whereas update still succeeds.