MDEV-39706 Assertion `!thd || !coordinator_thd' failed

[Thirunarayanan]

Problem:

• This assert was introduced in commit 0152c61 (MDEV-39261), which sets coordinator_thd in clone_oldest_view() and resets it to nullptr in batch_cleanup() at the end of the batch.
  When innodb_trx_purge_view_update_only_debug is enabled, InnoDB fail to reset coordinator_thd. As a result, InnoDB fails with assert in next batch

Solution:

trx_purge(): Reset coordinator_thd to nullptr when innodb_trx_purge_view_update_only_debug is enabled.

Note:

Avoided adding test case because cannot --source wait_all_purged.inc with
innodb_trx_purge_view_update_only_debug=1 purge never removes history, So the history list
never reaches 0 and the wait would hang forever.

[gemini-code-assist]

Code Review

This pull request introduces a debug-only helper method set_coordinator_thd to the purge_sys_t class and updates trx_purge to reset the coordinator thread to nullptr when the debug flag srv_purge_view_update_only_debug is active. The review feedback suggests improving code readability and style by formatting multi-statement blocks across multiple lines and adding standard spacing around the assignment operator.

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[gemini-code-assist]

Putting multiple statements on a single line inside a block, especially within a macro invocation like ut_d(...), reduces readability and makes debugging (e.g., setting breakpoints or stepping through code) more difficult. Formatting the block across multiple lines is much cleaner and conforms better to standard C++ readability guidelines.

  ut_d(if (srv_purge_view_update_only_debug) {
         purge_sys.set_coordinator_thd(nullptr);
         return 0;
       });

[gemini-code-assist]

For better readability and adherence to standard C++ style conventions (such as the Google C++ Style Guide), binary operators like the assignment operator = should have spaces on both sides.

#ifdef UNIV_DEBUG
  void set_coordinator_thd(THD *thd) noexcept
  {
    coordinator_thd = thd;
  }
#endif /* UNIV_DEBUG */

[dr-m]

Heh, this rather reasonable piece of advice conflicts with our CODING_STANDARDS.md.

[dr-m]

Can you write a test case with the following, or with some other nonzero value?

SET GLOBAL innodb_max_purge_lag_wait=1;

[dr-m]

Can you please add a test case that executes for example the following (which should avoid a hang):

SET GLOBAL innodb_max_purge_lag_wait=1;

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