An experiment of signal reentrance in Python

signal_handler_reentrance.py

#!/usr/bin/env python
"""
The result of signal handler reentrance in Python 2.7 and 3.7 might different:

# For Python 2.7

The signal handler will run in order.

```
$ python2 signal_handler_reentrance.py
20190917011037:INFO:__main__:PID = 4219 (ref-command: kill -USR1 4219; sleep 2; kill -USR1 4219)
20190917011037:DEBUG:__main__:in loop 1
20190917011047:DEBUG:__main__:in loop 2
20190917011048:INFO:__main__:in signal handler: t=1568653848, attempt=0
20190917011049:INFO:__main__:in signal handler: t=1568653848, attempt=1
20190917011049:INFO:__main__:in signal handler: t=1568653848, attempt=2
...
20190917011054:INFO:__main__:in signal handler: t=1568653848, attempt=12
20190917011054:INFO:__main__:in signal handler: t=1568653848, attempt=13
20190917011055:INFO:__main__:in signal handler: t=1568653848, attempt=14
20190917011055:INFO:__main__:leaving signal handler: t=1568653848
20190917011055:INFO:__main__:in signal handler: t=1568653855, attempt=0
20190917011056:INFO:__main__:in signal handler: t=1568653855, attempt=1
20190917011056:INFO:__main__:in signal handler: t=1568653855, attempt=2
...
20190917011101:INFO:__main__:in signal handler: t=1568653855, attempt=12
20190917011102:INFO:__main__:in signal handler: t=1568653855, attempt=13
20190917011102:INFO:__main__:in signal handler: t=1568653855, attempt=14
20190917011103:INFO:__main__:leaving signal handler: t=1568653855
20190917011103:DEBUG:__main__:in loop 3
```

# For Python 3.7

The 1st signal hander will suspend on the arrival of 2nd signal.

```
$ /opt/python-3.7/bin/python3 signal_handler_reentrance.py
20190917011309:INFO:__main__:PID = 4350 (ref-command: kill -USR1 4350; sleep 2; kill -USR1 4350)
20190917011309:DEBUG:__main__:in loop 1
20190917011318:INFO:__main__:in signal handler: t=1568653998, attempt=0
20190917011318:INFO:__main__:in signal handler: t=1568653998, attempt=1
20190917011319:INFO:__main__:in signal handler: t=1568653998, attempt=2
20190917011319:INFO:__main__:in signal handler: t=1568653998, attempt=3
20190917011320:INFO:__main__:in signal handler: t=1568653998, attempt=4
20190917011320:INFO:__main__:in signal handler: t=1568654000, attempt=0
20190917011320:INFO:__main__:in signal handler: t=1568654000, attempt=1
20190917011321:INFO:__main__:in signal handler: t=1568654000, attempt=2
...
20190917011326:INFO:__main__:in signal handler: t=1568654000, attempt=12
20190917011326:INFO:__main__:in signal handler: t=1568654000, attempt=13
20190917011327:INFO:__main__:in signal handler: t=1568654000, attempt=14
20190917011327:INFO:__main__:leaving signal handler: t=1568654000
20190917011327:INFO:__main__:in signal handler: t=1568653998, attempt=5
20190917011328:INFO:__main__:in signal handler: t=1568653998, attempt=6
20190917011328:INFO:__main__:in signal handler: t=1568653998, attempt=7
...
20190917011331:INFO:__main__:in signal handler: t=1568653998, attempt=12
20190917011331:INFO:__main__:in signal handler: t=1568653998, attempt=13
20190917011332:INFO:__main__:in signal handler: t=1568653998, attempt=14
20190917011332:INFO:__main__:leaving signal handler: t=1568653998
20190917011332:DEBUG:__main__:in loop 2
```

"""

import logging
import time
import signal
import os

_log = logging.getLogger(__name__)


class SignalHandlerReEntranceExp01:
	def handler(self, *args, **kwds):  # pylint: disable=unused-argument
		t = int(time.time())
		for attempt in range(15):
			_log.info("in signal handler: t=%d, attempt=%d", t, attempt)
			time.sleep(0.5)
		_log.info("leaving signal handler: t=%d", t)

	def loop(self):
		loop_count = 0
		while True:
			loop_count = loop_count + 1
			try:
				_log.debug("in loop %d", loop_count)
				time.sleep(10)
			except Exception:
				_log.exception("caught exception in loop %d", loop_count)


def main():
	logging.basicConfig(level=logging.DEBUG, format="%(asctime)s:%(levelname)s:%(name)s:%(message)s", datefmt="%Y%m%d%H%M%S")
	exp_obj = SignalHandlerReEntranceExp01()
	signal.signal(signal.SIGUSR1, exp_obj.handler)
	pid_value = os.getpid()
	_log.info("PID = %d (ref-command: kill -USR1 %d; sleep 2; kill -USR1 %d)", pid_value, pid_value, pid_value)
	exp_obj.loop()


if __name__ == "__main__":
	main()