MDEV-33075 [backport/2f5174e556] [10.11] Resolve server shutdown issues on macOS, Solaris, and FreeBSD

[sitano]

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Backport of 2f5174e556cd247133aa14d7b37372ae49fe83c5: MDEV-33075 Resolve server shutdown issues on macOS, Solaris, and FreeBSD.

This commit addresses multiple server shutdown problems observed on macOS, Solaris, and FreeBSD:

1. Corrected a non-portable assumption where socket shutdown was expected to wake up poll() with listening sockets in the main thread.

Use more robust self-pipe to wake up poll() by writing to the pipe's write end.

2. Fixed a random crash on macOS from pthread_kill(signal_handler) when the signal_handler was detached and the thread had already exited.

Use more robust kill(getpid(), SIGTERM) to wake up the signal handler thread.

3. Made sure, that signal handler thread always exits once abort_loop is set, and also calls my_thread_end() and clears signal_thread_in_use when exiting.

This fixes warning "1 thread did not exit" by my_global_thread_end() seen on FreeBSD/macOS when the process is terminated via signal.

Additionally, the shutdown code underwent light refactoring for better readability and maintainability:

• Modified break_connect_loop() to no longer wait for the main thread, aligning behavior with Windows (since 10.4).
• Removed dead code related to the unused USE_ONE_SIGNAL_HAND preprocessor constant.
• Eliminated support for #ifndef HAVE_POLL in handle_connection_sockets This code is also dead, since 10.4

[CLAassistant]

All committers have signed the CLA.

[sitano]

I did not have time to understand if this part needed and missing in 11.x

#ifdef USE_ONE_SIGNAL_HAND
    case THR_SERVER_ALARM:
      process_alarm(sig);			// Trigger alarms.
      break;
#endif

The critical part for me of all those 4 parts is the thread exit on abort in signal_hand - without it the whole 10.x branch just hangs on exit and db init on MacOS.

[sitano]

10.5 seems to be ready green too

[cvicentiu]

@vaintroub can you please confirm if this should be merged or not any more?

[grooverdan]

Fairly sure the 10.5 backport and subsequent merge up handled this. Thanks @sitano for pushing for the backport.