MDEV-40063 Corruption due to race in SET GLOBAL innodb_log_archive=ON

[dr-m]

There was a race condition between log_t::write_checkpoint() and the execution of SET GLOBAL innodb_log_archive=ON (enabling log archiving). We had wrongly allowed the concurrent execution of log_t::set_archive() and log_t::write_checkpoint(). The result was that log_sys.next_checkpoint_no was corrupted. This could have broken crash recovery.

log_t::write_checkpoint(): When we are releasing log_sys.latch while durably writing the checkpoint header block, assign log_sys.resize_log to log_sys.log to inform other threads that a checkpoint is in progress. Previously, we only did this when innodb_log_archive=ON holds.

log_t::resize_start(): Relax a debug assertion for the logic change.

This addresses this problem found in #4817 and originally introduced in #4405.

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[gemini-code-assist]

Code Review

This pull request refactors the checkpoint writing logic in InnoDB (buf0flu.cc and log0log.cc) to block concurrent set_archive() calls by setting resize_log = log earlier and handling its cleanup. The review feedback suggests refactoring the control flow to avoid using a goto statement to jump into an if block, which would improve code readability and maintainability.

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[gemini-code-assist]

Jumping into an if block using goto is a highly discouraged control flow pattern in C++ that hurts readability and maintainability. We can completely avoid the goto statement and the checkpoint_completed label by restructuring the code. Since resize_log.m_file != log.m_file is guaranteed when archive_header_was_reset is true, we can safely place the resize_log.m_file cleanup block directly after the if-else chain. This simplifies the logic and makes it much cleaner.

  if (!archive)
  {
    archived_lsn= end_lsn;
  }

[gemini-code-assist]

By placing the cleanup block here after the if-else chain, we can completely eliminate the goto checkpoint_completed; statement and the corresponding label inside the if (!archive) block. This makes the control flow linear and much easier to follow.

  if (resize_log.m_file == log.m_file)
  {
    /* We may have assigned resize_log= log to keep set_archived() out. */
#ifdef HAVE_PMEM
    ut_ad(!is_mmap());
#endif
    resize_log.m_file= OS_FILE_CLOSED;
  }