Bugged interaction with signal handler

[Soham Sarbendu Roy ee20b130]

Same as https://ask.sagemath.org/question/65840/bugged-interaction-with-signal-handler-sagemath-95-linux/

This is a minimal reproducible example of a bug using a typical interrupt handler so that files being written to are not corrupted. Running the script as `python filename.py` works as intended, but the issue occurs on running this script as `sage filename.sage` or as `python filename.py` with any Sage import statement similar to `from sage.all import matrix`.

As commented, the first `time.sleep(3)` can be interrupted correctly, the second `time.sleep(4)` correctly raises the `KeyboardInterrupt()` on exiting the scope of `with DelayedKeyboardInterrupt()`, but the final `time.sleep(5)` call cannot be interrupted, but the hardcoded `KeyboardInterrupt()` at the end still works. Printing `signal.getsignal(signal.SIGINT)` shows that `DelayedKeyboardInterrupt()` correctly restores the handler.

I realise that `cysignals` is preferred with Sage, but am unable to figure out how to use it for the project without major changes.

    import signal
    import time

    class DelayedKeyboardInterrupt:
        def __enter__(self):
            self.sigint_handler = signal.signal(signal.SIGINT, self.handler)
            self.signal_received = False

        def handler(self, sig, frame):
            self.signal_received = True
            print("Delaying exit for file I/O.")

        def __exit__(self, type, value, traceback):
            signal.signal(signal.SIGINT, self.sigint_handler)
            if self.signal_received:
                raise KeyboardInterrupt("delayed")

    print("sleeping unprotected for 3 sec")
    time.sleep(3)  # Ctrl+C here raises KeyboardInterrupt instantly

    with DelayedKeyboardInterrupt():
        print("sleeping protected for 4 sec")
        time.sleep(4)  # Ctrl+C here raises KeyboardInterrupt("delayed") after end of 4 sec

    print("sleeping 'unprotected' for 5 sec")
    time.sleep(5)  # Ctrl+C here does not raise any KeyboardInterrupt

    raise KeyboardInterrupt("hardcoded")  # this raises correctly

Ubuntu 22.04.1 LTS 64-bit, with Sage 9.5 installed as a apt package