file locking using a context manager (with statement) in python

I needed a quick locking mechanism to prevent my daemons from stepping over each other. To have a sane daemon startup (and prevent multiple daemon spawns), we need to ensure that we have an exclusive lock before starting the program.  Googling around didn't lead to show any context managers that actually use the flock syscalls.

So here goes my attempt that seems to work:

	import fcntl
	class FileLock:
	def __init__(self, filename, lock_type):
	self.fh = open(filename, "a")
	self.lock_type = lock_type
	def __enter__(self):
	print "Acquiring lock"
	fcntl.flock(self.fh.fileno(), self.lock_type)
	def __exit__(self, type, value, tb):
	print "Releasing lock"
	fcntl.flock(self.fh.fileno(), fcntl.LOCK_UN)
	self.fh.close()
	def main():
	import os
	import time
	with FileLock("/tmp/some_lock_file", fcntl.LOCK_EX):
	print "Start: %s" % time.ctime()
	print "PID: %s" % os.getpid()
	time.sleep(5)
	print "Finish: %s" % time.ctime()
	if __name__ == '__main__':
	main()

view raw flock.py hosted with ❤ by GitHub

Spinning off some python processes that utilise this context manager shows serialisation taking place:

	Starting process 1
	Starting process 2
	Starting process 3
	Starting process 4
	Starting process 5
	Acquiring lock
	Start: Tue Jul 8 11:51:22 2014
	Acquiring lock
	Acquiring lock
	PID: 826
	Acquiring lock
	Acquiring lock
	Finish: Tue Jul 8 11:51:27 2014
	Releasing lock
	Start: Tue Jul 8 11:51:27 2014
	PID: 825
	Finish: Tue Jul 8 11:51:32 2014
	Releasing lock
	Start: Tue Jul 8 11:51:32 2014
	PID: 824
	Finish: Tue Jul 8 11:51:37 2014
	Releasing lock
	Start: Tue Jul 8 11:51:37 2014
	PID: 823
	Finish: Tue Jul 8 11:51:42 2014
	Releasing lock
	Start: Tue Jul 8 11:51:42 2014
	PID: 822
	Finish: Tue Jul 8 11:51:47 2014
	Releasing lock

view raw process_start.out hosted with ❤ by GitHub

And here's the output of lsof showing locking for the processes spun off above:

	Tue Jul 8 11:51:18 UTC 2014
	Tue Jul 8 11:51:20 UTC 2014
	Tue Jul 8 11:51:22 UTC 2014
	Tue Jul 8 11:51:24 UTC 2014
	COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
	python 822 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 823 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 824 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 825 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 826 vagrant 3wW REG 254,0 0 1973047 /tmp/some_lock_file
	Tue Jul 8 11:51:26 UTC 2014
	COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
	python 822 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 823 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 824 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 825 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 826 vagrant 3wW REG 254,0 0 1973047 /tmp/some_lock_file
	Tue Jul 8 11:51:28 UTC 2014
	COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
	python 822 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 823 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 824 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 825 vagrant 3wW REG 254,0 0 1973047 /tmp/some_lock_file
	Tue Jul 8 11:51:30 UTC 2014
	COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
	python 822 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 823 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 824 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 825 vagrant 3wW REG 254,0 0 1973047 /tmp/some_lock_file
	Tue Jul 8 11:51:32 UTC 2014
	COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
	python 822 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 823 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 824 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 825 vagrant 3wW REG 254,0 0 1973047 /tmp/some_lock_file
	Tue Jul 8 11:51:34 UTC 2014
	COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
	python 822 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 823 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 824 vagrant 3wW REG 254,0 0 1973047 /tmp/some_lock_file
	Tue Jul 8 11:51:36 UTC 2014
	COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
	python 822 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 823 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 824 vagrant 3wW REG 254,0 0 1973047 /tmp/some_lock_file
	Tue Jul 8 11:51:38 UTC 2014
	COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
	python 822 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 823 vagrant 3wW REG 254,0 0 1973047 /tmp/some_lock_file
	Tue Jul 8 11:51:40 UTC 2014
	COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
	python 822 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 823 vagrant 3wW REG 254,0 0 1973047 /tmp/some_lock_file
	Tue Jul 8 11:51:42 UTC 2014
	COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
	python 822 vagrant 3w REG 254,0 0 1973047 /tmp/some_lock_file
	python 823 vagrant 3wW REG 254,0 0 1973047 /tmp/some_lock_file
	Tue Jul 8 11:51:44 UTC 2014
	COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
	python 822 vagrant 3wW REG 254,0 0 1973047 /tmp/some_lock_file
	Tue Jul 8 11:51:46 UTC 2014
	COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
	python 822 vagrant 3wW REG 254,0 0 1973047 /tmp/some_lock_file
	Tue Jul 8 11:51:48 UTC 2014
	Tue Jul 8 11:51:50 UTC 2014

view raw lsof.output hosted with ❤ by GitHub