Gracefully handle termination signals

[davidfokkema]

On macOS, and presumably similarly on *nix, you can do this:

import pathlib
import signal
import time

from textual.app import App, ComposeResult
from textual.widgets import Placeholder


class MinimalApp(App[None]):
    _signal = None

    def on_mount(self) -> None:
        signal.signal(signalnum=signal.SIGHUP, handler=self.catch_signal)
        signal.signal(signalnum=signal.SIGTERM, handler=self.catch_signal)

    def compose(self) -> ComposeResult:
        yield Placeholder("This is a minimal app.")

    def catch_signal(self, signum, frame) -> None:
        self._signal = signum
        self.exit()


app = MinimalApp()
app.run()

pathlib.Path("signal.txt").write_text(
    f"{time.monotonic()}: Received signal: {app._signal}"
)
print("Done")
print(f"Received signal: {app._signal}")

This will gracefully shutdown the app when the app is killed (with SIGTERM) or when the terminal is closed (sending a SIGHUP to child processes). Currently, in the case of SIGTERM, Textual does shutdown, but not gracefully and stdin/stdout are messed up. Default signal handlers would improve that.

On Windows, it might be a bit more difficult (maybe this would work: https://stackoverflow.com/questions/69323194/how-do-i-handle-windows-terminal-closing-event-in-python) and even on Linux there are reports it might be not as straightforward as hoped. Still, might be worth it?

[github-actions]

We found the following entry in the FAQ which you may find helpful:

• Why doesn't Textual look good on macOS?

Feel free to close this issue if you found an answer in the FAQ. Otherwise, please give us a little time to review.

This is an automated reply, generated by FAQtory

[mzebrak]

Have you tried registering the signal handler on the asyncio loop itself?

What we do is:

    def on_load(self) -> None:
        self._register_quit_signals()

    def _register_quit_signals(self) -> None:
        import signal

        def callback() -> None:
            self.exit()

        loop = asyncio.get_running_loop()  # can't use self._loop since it's not set yet
        for signal_number in [signal.SIGHUP, signal.SIGINT, signal.SIGQUIT, signal.SIGTERM]:
            loop.add_signal_handler(signal_number, callback)

Maybe this will work for you as a temporary workaround?
We haven't observed the mentioned issues with stdin/stdout with this approach.

[davidfokkema]

Hi @mzebrak, I probably wasn't clear enough. We only observe the stdin/stdout issues on 'regular' Textual apps. If we install the signal handlers as shown in my code, the issue disappears. That's why I think Textual should maybe install these event handlers by default. Your code shows another way of installing the signal handlers, which is also great, but requires work on the part of the app author.