I'm writing a Python script that may or may not (depending on a bunch of things) run for a long time, and I'd like to make sure that multiple instances (started via cron) don't step on each others toes. The logical way to do this seems to be a PID-based lockfile… But I don't want to re-invent the wheel if there is already code to do this.
So, is there a Python module out there which will manage the details of a PID-based lockfile?
If you can use GPLv2, Mercurial has a module for that:
http://bitbucket.org/mirror/mercurial/src/tip/mercurial/lock.py
Example usage:
from mercurial import error, lock
try:
l = lock.lock("/path/to/lock", timeout=600) # wait at most 10 minutes
# do something
except error.LockHeld:
# couldn't take the lock
else:
l.release()
This might be of help to you: lockfile
I believe you will find the necessary information here. The page in question refers to a package for building daemons in python: this process involves creating a PID lockfile.
i've been pretty unhappy with all of those, so i wrote this:
class Pidfile():
def __init__(self, path, log=sys.stdout.write, warn=sys.stderr.write):
self.pidfile = path
self.log = log
self.warn = warn
def __enter__(self):
try:
self.pidfd = os.open(self.pidfile, os.O_CREAT|os.O_WRONLY|os.O_EXCL)
self.log('locked pidfile %s' % self.pidfile)
except OSError as e:
if e.errno == errno.EEXIST:
pid = self._check()
if pid:
self.pidfd = None
raise ProcessRunningException('process already running in %s as pid %s' % (self.pidfile, pid));
else:
os.remove(self.pidfile)
self.warn('removed staled lockfile %s' % (self.pidfile))
self.pidfd = os.open(self.pidfile, os.O_CREAT|os.O_WRONLY|os.O_EXCL)
else:
raise
os.write(self.pidfd, str(os.getpid()))
os.close(self.pidfd)
return self
def __exit__(self, t, e, tb):
# return false to raise, true to pass
if t is None:
# normal condition, no exception
self._remove()
return True
elif t is PidfileProcessRunningException:
# do not remove the other process lockfile
return False
else:
# other exception
if self.pidfd:
# this was our lockfile, removing
self._remove()
return False
def _remove(self):
self.log('removed pidfile %s' % self.pidfile)
os.remove(self.pidfile)
def _check(self):
"""check if a process is still running
the process id is expected to be in pidfile, which should exist.
if it is still running, returns the pid, if not, return False."""
with open(self.pidfile, 'r') as f:
try:
pidstr = f.read()
pid = int(pidstr)
except ValueError:
# not an integer
self.log("not an integer: %s" % pidstr)
return False
try:
os.kill(pid, 0)
except OSError:
self.log("can't deliver signal to %s" % pid)
return False
else:
return pid
class ProcessRunningException(BaseException):
pass
to be used something like this:
try:
with Pidfile(args.pidfile):
process(args)
except ProcessRunningException:
print "the pid file is in use, oops."
I know this is an old thread, but I also created a simple lock which only relies on python native libraries:
import fcntl
import errno
class FileLock:
def __init__(self, filename=None):
self.filename = os.path.expanduser('~') + '/LOCK_FILE' if filename is None else filename
self.lock_file = open(self.filename, 'w+')
def unlock(self):
fcntl.flock(self.lock_file, fcntl.LOCK_UN)
def lock(self, maximum_wait=300):
waited = 0
while True:
try:
fcntl.flock(self.lock_file, fcntl.LOCK_EX | fcntl.LOCK_NB)
return True
except IOError as e:
if e.errno != errno.EAGAIN:
raise e
else:
time.sleep(1)
waited += 1
if waited >= maximum_wait:
return False
There is a recipe on ActiveState on creating lockfiles.
To generate the filename you can use os.getpid() to get the PID.
