#! /usr/bin/env python
import os, errno, fcntl
class ApplicationLock:
'''
Ensures application is running only once, by using a lock file.
Ensure call to lock works. Then call unlock at program exit.
You cannot read or write to the lock file, but for some reason you can
remove it. Once removed, it is still in a locked state somehow. Another
application attempting to lock against the file will fail, even though
the directory listing does not show the file. Mysterious, but we are glad
the lock integrity is upheld in such a case.
Instance variables:
lockfile -- Full path to lock file
lockfd -- File descriptor of lock file exclusively locked
'''
def __init__ (self, lockfile):
self.lockfile = lockfile
self.lockfd = None
def lock (self):
'''
Creates and holds on to the lock file with exclusive access.
Returns True if lock successful, False if it is not, and raises
an exception upon operating system errors encountered creating the
lock file.
'''
try:
#
# Create or else open and trucate lock file, in read-write mode.
#
# A crashed app might not delete the lock file, so the
# os.O_CREAT | os.O_EXCL combination that guarantees
# atomic create isn't useful here. That is, we don't want to
# fail locking just because the file exists.
#
# Could use os.O_EXLOCK, but that doesn't exist yet in my Python
#
self.lockfd = os.open (self.lockfile,
os.O_TRUNC | os.O_CREAT | os.O_RDWR)
# Acquire exclusive lock on the file, but don't block waiting for it
fcntl.flock (self.lockfd, fcntl.LOCK_EX | fcntl.LOCK_NB)
# Writing to file is pointless, nobody can see it
os.write (self.lockfd, "My Lockfile")
return True
except (OSError, IOError), e:
# Lock cannot be acquired is okay, everything else reraise exception
if e.errno in (errno.EACCES, errno.EAGAIN):
return False
else:
raise
def unlock (self):
try:
# FIRST unlink file, then close it. This way, we avoid file
# existence in an unlocked state
os.unlink (self.lockfile)
# Just in case, let's not leak file descriptors
os.close (self.lockfd)
except (OSError, IOError), e:
# Ignore error destroying lock file. See class doc about how
# lockfile can be erased and everything still works normally.
pass
# Test main routine
if __name__ == '__main__':
import time
applock = ApplicationLock ('Test.lock')
if (applock.lock ()):
# Hint: try running 2nd program instance while this instance sleeps
print ("Obtained lock, sleeping 10 seconds")
time.sleep (10)
print ("Unlocking")
applock.unlock ()
else:
print ("Unable to obtain lock, exiting")
