python gobject.mainloop gobbles signal events

Question

I have one module which uses python "threading" for concurrency, and "signal" for shutdown hook:

signal.signal(signal.SIGINT, self.shutdownhook)

I have another module which uses dbus and gobject

dbus.mainloop.glib.DBusGMainLoop(set_as_default=True)
....
GObject.threads_init()
mainloop = GObject.MainLoop()
mainloop.run()

When I run them seperately, they both operate as expected and ctrl+c causes termination via "KeyboardInterrupt".

However, when I run them together the mainloop terminates but the shutdown hook is never called - the process does not terminate without kill -9 pid.

Can someone please explain why this occurs, and how best to integrate the two models

Here is a working example which highlights my problem. I cannot exit the program with just a CTRL+C and the shutdown hook is not called in this case either.

import threading
import signal
import sys
from gi.repository import GObject


def runMainloop():
        print('running mainloop')
        mainloop.run()

def shutdown():
        print('shutdown')

def readInput():
        print('readInput')
        print(sys.stdin.readline())

if __name__ == '__main__':
        signal.signal(signal.SIGINT, shutdown)
        signal.signal(signal.SIGTERM, shutdown)
        GObject.threads_init()
        mainloop = GObject.MainLoop()

        mainloopThread = threading.Thread(name='mainloop', target=runMainloop)
        mainloopThread.setDaemon(True)
        mainloopThread.start()
        print('started')

        inputThread = threading.Thread(name='input', target=readInput)
        inputThread.start()
        print('started input')

score 6 · Answer 1 · edited Jun 20 '20 at 09:12

No one is interested, so let me try.

Just to be on the same page:

import signal
from gi.repository import GObject

GObject.threads_init()
mainloop = GObject.MainLoop()

signal.signal(signal.SIGINT, lambda n, f: mainloop.quit())

mainloop.run()

This code works:

import signal
from gi.repository import GObject

signal.signal(signal.SIGINT, lambda n, f: print("kill"))

GObject.threads_init()
mainloop = GObject.MainLoop()
mainloop.run()

I've first registered signal handler, and then initiated the loop. Strange is that it is not called. However result is - as expected...

As a side note - according to their doc... mainloop is deprecated. That is first thing.

Edit

Here is example with reading from stdin inside MainLoop:

import signal
import sys
from gi.repository import GObject, GLib

GObject.threads_init()

def readInput():
    print('readInput\n')
    while True:
        input = sys.stdin.readline()
        print(input)
        if input.strip() == 'exit':
            print('closing main loop')
            mainloop.quit()
            print('terminating thread')
            return

if __name__ == '__main__':
    signal.signal(signal.SIGINT, signal.SIG_DFL)

    mainloop = GObject.MainLoop.new(None, False)
    GObject.timeout_add(1000, readInput)

    # inputThread = threading.Thread(name='input', target=readInput)
    # inputThread.start()
    # print('started input')

    print('running mainloop\n')
    try:
        mainloop.run()
    except KeyboardInterrupt:
        mainloop.quit()

Adding .new(None, False) allows CTRL-C working normally. Took it from here, also here is another thread about integrating pulse audio controller with GLib/GObject loop. There are samples about integration dbus with the loop, but I'm not sure which way you wish to go...

thanks, but that doesn't seem to help the situation. i've updated the question with a better working example. — pstanton, Dec 28 '17 at 05:28
Yes, problem is that `MainLoop` takes all the resources, and since Python goes single threaded since GIL... The only way to integrate with DBus is putting your "stuff" into `MainLoop` event system. There are examples on the net, I'll try to update answer to deal with it. — Michał Zaborowski, Dec 28 '17 at 12:41
ok .. i'm confused because my other threads seem to work just fine, just the signal / shutdown is never triggered. so MainLoop gobbles the signal? maybe i can just add a shutdown hook to MainLoop? — pstanton, Dec 29 '17 at 04:32
According to their docs `MainLoop` takes that signals. I don't get that either - signals can be controlled by main thread... Maybe that is because of doing it outside Python? I mean in C/C++/SharedObject... not sure. — Michał Zaborowski, Dec 29 '17 at 09:49

score 2 · Answer 2 · answered Apr 28 '20 at 13:56

Adding a timer makes the loop receive Unix signals.

import gi
from gi.repository import GLib
import signal

GLib.threads_init()

mainloop = GLib.MainLoop()

signal.signal(signal.SIGTERM, lambda signum, frame: mainloop.quit())

GLib.timeout_add(1000, lambda *args: (print("tick") or True))

try:
    mainloop.run()
except KeyboardInterrupt:
    print()

python gobject.mainloop gobbles signal events

2 Answers2

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