Windows signal handler problem

Aug 8, 2023 in GOOEY • PYTHON • SIGNAL • WINDOWS
2 min read

Context

I am using a gradio app inside Gooey to start python cli in a subprocess. I try to handle the signal in the subprocess, but it does not work in the Windows. It works fine in the Mac, but not in the Windows.

Problem

I use the code like this:



class ProcessHandler:
    def __init__(self) -> None:
        self.stdout = []
        self.process = None
        signal.signal(signal.SIGTERM, self.save_stdout)
        signal.signal(signal.SIGINT, self.save_stdout)

    def change_python(self, cmd):
        if "python" in cmd and sys.platform.startswith("win"):
            cmd = cmd.replace("python", os.path.join(f"{INSTALL_ENV}", "python.exe"))
        if "python" in cmd and sys.platform.startswith("darwin"):
            cmd = cmd.replace("python", os.path.join(f"{INSTALL_ENV}", "bin/python"))
        return cmd

    def save_stdout(self, signum, frame):
        with open("log.txt", "w") as f:
            f.write("\n".join(self.stdout))

    def run(self, cmd):
        self.stdout = []
        self.process = subprocess.Popen(
            cmd,
            stdout=subprocess.PIPE,
            stderr=subprocess.STDOUT,
            shell=True,
            text=True,
        )
        while True:
            output = self.process.stdout.readline()
            if self.process.poll() is not None:
                break
            if output:
                if output.strip() == "":
                    continue
                self.stdout.append(output.strip())
                print(output, flush=True, end="")

        return self.process.returncode

    def kill(self):
        if self.process:
            self.process.kill()

I use the signal.signal to handle the signal, but it does not work in the Windows. I tried to use signal.SIGTERM and signal.SIGINT to handle the signal, but it does not work.

Solution

In windows, the signal handler for SIGTERM is useless. Alternative solution is to send signal.CTRL_BREAK_EVENT to the process.

@Goey(
    shutdown_signal=signal.CTRL_BREAK_EVENT,
)
def main():
    pass

class ProcessHandler:
    def __init__(self) -> None:
        self.stdout = []
        self.process = None
        signal.signal(signal.SIGBREAK, self.save_stdout)

Also in the pip, gooey is only 1.0.8, so to use shutdown_signal, we need to install the gooey from the github.

Conclusion

Debug this problem spent me a lot of time, I hope this blog can help you to solve the problem faster.

Learning

chatgpt is a good assistant to help you to debug the problem.
reading the source code is a good way to solve the problem.
set up mvp to debug is much faster than the real app.