File lock not working as expected

Question

I have a Thread-extending class that is supposed to run only one instance at a time (cross-process). In order to achieve that, I'm trying to use a file lock. Here are bits of my code:

class Scanner(Thread):

  def __init__(self, path):
    Thread.__init__(self)
    self.lock_file = open(os.path.join(config.BASEDIR, "scanner.lock"), 'r+')
    fcntl.lockf(self.lock_file, fcntl.LOCK_EX | fcntl.LOCK_NB)

  # Stuff omitted

  def run(self):
    logging.info("Starting scan on %s" % self.path)

    # More stuff omitted

    fcntl.lockf(self.lock_file, fcntl.LOCK_UN)

I was expecting the lockf call to throw an exception if a Scanner thread was already running and not initialize the object at all. However, I can see this in the terminal:

INFO:root:Starting scan on /home/felix/Music
INFO:root:Starting scan on /home/felix/Music
INFO:root:Scan finished
INFO:root:Scan finished

Which suggests that two Scanner threads are running at the same time, no exception thrown. I'm sure I'm missing something really basic here, but I can't seem to figure out what that is. Can anyone help?

Answer 1

Found the solution myself in the end. It was to use fcntl.flock() instead of fcntl.lockf(), with the exact same parameters. Not sure why that made a difference.

Answer 2

You're opening the lock file using r+ which is erasing the previous file and creating a new one. Each thread is locking a different file.

Use w or r+a

Answer 3

Along with using flock, I had to also open the file like so :

fd = os.open(lockfile, os.O_CREAT | os.O_TRUNC | os.O_WRONLY)

It does not work other wise.