Webots bypasses python's SIGINT behavior

[MegaIng]

Describe the Bug
Because quit_controller gets registered as the signal handler for SIGINT it becomes impossible to use normal cleanup code for a python process (and presumably similar in other languages).

Steps to Reproduce

1. Write a python script using any controller that tries to do cleanup using try ... finally or atexit
2. Run the script
3. Try to end it with CTRL+C
4. The script exits without executing the finally.

Expected behavior
Webots should not be fully overwriting the signal handler, or at the very least provide an option to not do that. I am honestly not even sure what the point of this code is. The only interesting detail is that a global variable gets set that however AFAICT is not used anywhere.

If this code is for some reason necessary it would be great if this could at least be documented clearly.

System

• Operating System: Linux Ubuntu 24.04
• Graphics Card: Irrelevant

Additional context
Luckily the init code is only executed once and it's possible to overwrite the signal handler back to the python default via signal.signal(signal.SIGINT, signal.default_int_handler), so that's a functional workaround for now. This works perfectly, it doesn't appear that the controller API needs any cleanup done that is disabled by this.

[omichel]

This is strange, as normally the quit_controller function should restore the default signal handler and raise a new signal so that it gets called. See here.

This is a standard "graceful shutdown then die" pattern for signal handlers. It lets the controller clean up or stop waiting before actually terminating, rather than being killed abruptly mid-step.

Is it useful? Yes — without it, a signal like SIGTERM or SIGINT would either be ignored (if caught but no action taken) or kill the process immediately (if using the default handler) without giving the simulation loop a chance to exit cleanly. This approach threads the needle between the two. This is particularly useful with extern controller which are started from the console and can be stopped with Ctrl-C.

[MegaIng]

This is strange, as normally the quit_controller function should restore the default signal handler and raise a new signal so that it gets called. See here.

Yes, but the default signal handler SIG_DFL is not the CPython signal handler.

[omichel]

Yes, but the default signal handler SIG_DFL is not the CPython signal handler.

I see. So, I believe that #6945 probably fixes the problem. Can you test it and confirm?