fix(userspace/falco): make termination and signal handlers more stable

[jasondellaluce]

What type of PR is this?

/kind bug

/kind cleanup

Any specific area of the project related to this PR?

/area engine

What this PR does / why we need it:

Signal handlers are used in Falco to force termination, hot reload, or output reloading. However, the current implementation is not stable for neither thread-safety and async-signal safety (more here: https://docs.oracle.com/cd/E19455-01/806-5257/gen-26/index.html). This PR refactors the way signals are handled to make sure it's safe both on single-threaded (e.g. capturing only syscalls) and multi-threaded use cases (e.g. multiple event sources in parallel).

Accordingly, the event source termination stage has been refactored to be more stable by using atomic values to check 1) that the event source has actually finished processing events, and 2) that the corresponding thread has already been joined. This is safer than simply relying on the thread.joinable() and thread.join() primitives.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[jasondellaluce]

/milestone 0.33.0

[Andreagit97]

what about something like:

Suggested change

	if(! create_handler(SIGINT, ::terminate_signal_handler, ret) ||
	! create_handler(SIGTERM, ::terminate_signal_handler, ret) ||
	! create_handler(SIGUSR1, ::reopen_outputs_signal_handler, ret) ||
	! create_handler(SIGHUP, ::restart_signal_handler, ret))
	return !create_handler(SIGINT, ::terminate_signal_handler, ret) ||
	!create_handler(SIGTERM, ::terminate_signal_handler, ret) ||
	!create_handler(SIGUSR1, ::reopen_outputs_signal_handler, ret) ||
	!create_handler(SIGHUP, ::restart_signal_handler, ret)) ? run_result::fatal("") : run_result::ok();

Or if we don't want to return fatal just run_result::ok() in both cases 🤔 just a nitpick feel free to ignore

[jasondellaluce]

create_handler already sets its own error results in the passed-in ret, so errors are already catched.

[FedeDP]

/approve

[poiana]

LGTM label has been added.

Git tree hash: 9c46687de8ccdb55ef0497400ebf1d701f2787ec

[Andreagit97]

/approve

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Andreagit97, FedeDP, jasondellaluce

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