108 lines
4.2 KiB
Python
108 lines
4.2 KiB
Python
#! /usr/bin/env python2
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# -*- coding: utf-8 -*-
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# The MIT License (MIT)
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# Copyright (c) 2009 Max Polk
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# Permission is hereby granted, free of charge, to any person obtaining a copy of
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# this software and associated documentation files (the "Software"), to deal in
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# the Software without restriction, including without limitation the rights to
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# use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
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# of the Software, and to permit persons to whom the Software is furnished to do
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# so, subject to the following conditions:
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# The above copyright notice and this permission notice shall be included in all
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# copies or substantial portions of the Software.
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# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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# THE SOFTWARE.
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import os
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import errno
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import fcntl
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class FLock:
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'''
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Ensures application is running only once, by using a lock file.
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Ensure call to lock works. Then call unlock at program exit.
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You cannot read or write to the lock file, but for some reason you can
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remove it. Once removed, it is still in a locked state somehow. Another
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application attempting to lock against the file will fail, even though
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the directory listing does not show the file. Mysterious, but we are glad
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the lock integrity is upheld in such a case.
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Instance variables:
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lockfile -- Full path to lock file
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lockfd -- File descriptor of lock file exclusively locked
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'''
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def __init__(self, lockfile):
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self.lockfile = lockfile
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self.lockfd = None
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def lock(self):
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'''
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Creates and holds on to the lock file with exclusive access.
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Returns True if lock successful, False if it is not, and raises
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an exception upon operating system errors encountered creating the
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lock file.
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'''
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try:
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#
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# Create or else open and trucate lock file, in read-write mode.
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#
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# A crashed app might not delete the lock file, so the
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# os.O_CREAT | os.O_EXCL combination that guarantees
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# atomic create isn't useful here. That is, we don't want to
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# fail locking just because the file exists.
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#
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# Could use os.O_EXLOCK, but that doesn't exist yet in my Python
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#
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self.lockfd = os.open(self.lockfile,
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os.O_TRUNC | os.O_CREAT | os.O_RDWR)
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# Acquire exclusive lock on the file,
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# but don't block waiting for it
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fcntl.flock(self.lockfd, fcntl.LOCK_EX | fcntl.LOCK_NB)
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# Writing to file is pointless, nobody can see it
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os.write(self.lockfd, "lockfile")
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return True
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except (OSError, IOError), e:
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# Lock cannot be acquired is okay,
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# everything else reraise exception
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if e.errno in (errno.EACCES, errno.EAGAIN):
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return False
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else:
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raise
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def unlock(self):
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try:
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# FIRST unlink file, then close it. This way, we avoid file
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# existence in an unlocked state
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os.unlink(self.lockfile)
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# Just in case, let's not leak file descriptors
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os.close(self.lockfd)
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except (OSError, IOError):
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# Ignore error destroying lock file. See class doc about how
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# lockfile can be erased and everything still works normally.
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pass
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# Test main routine
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if __name__ == '__main__':
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import time
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applock = FLock('./cmds.lock')
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if (applock.lock()):
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# Hint: try running 2nd program instance while this instance sleeps
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print("Obtained lock, sleeping 10 seconds")
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time.sleep(10)
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print("Unlocking")
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applock.unlock()
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else:
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print("Unable to obtain lock, exiting")
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