Remove custom signal handling in Triggerer

[potiuk]

There is a bug in CPython (fixed in March 2022 but not yet released) that
makes async.io handle SIGTERM improperly by using async unsafe
functions and hanging the triggerer receive SIGPIPE while handling
SIGTERN/SIGINT and deadlocking itself. Until the bug is handled
we should rather rely on standard handling of the signals rather than
adding our own signal handlers. Seems that even if our signal handler
just run exit(0) - it caused a race condition that led to the hanging.

More details:
* https://bugs.python.org/issue39622
* python/cpython#83803

Fixes: #19260

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[potiuk]

CC: @andrewgodwin @ashb @dstandish -> I was able to reproduce the Ctrl-C problem and it's gone after I removed the custom signal handling in Triggerer, so it looks like the hypothesis of the async.io bug from https://bugs.python.org/issue39622 python/cpython#83803 seems even more plausible.

Pls. take a look and see if my Hypothesis from #23271 (comment) looks sound and maybe we can just fix it permanently also for production.

I believe our custom signal handling of SIGINT and SIGTERM in Triggerer (which then would simply run sys.exit(0) ) is not really needed (default handling of both signals terminates the process eventually). I left SIGQUIT handling though for diagnostics (And QUIT is rarely used anyway for anything else).

[potiuk]

I also found that the standalone problem with hanging was already reported in #19260

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.

[tanelk]

@potiuk It seems that after this we do not change the TriggererJob state to success any more. They stay in running:

There is also register_signals method in the triggerer job, but it is never called.

airflow/airflow/jobs/triggerer_job.py

Lines 67 to 70 in 717a758

	def register_signals(self) -> None:
	"""Register signals that stop child processes"""
	signal.signal(signal.SIGINT, self._exit_gracefully)
	signal.signal(signal.SIGTERM, self._exit_gracefully)

[potiuk]

Thanks @tanelk - reverting it for 2.3.3 then and reopened the original issue.