DCNmodel/cnmodel/util/filelock.py

# Copyright (c) 2009, Evan Fosmark
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import os
import time
import errno


class FileLockException(Exception):
    pass


class FileLock(object):
    """ A file locking mechanism that has context-manager support so 
        you can use it in a with statement. This should be relatively cross
        compatible as it doesn't rely on msvcrt or fcntl for the locking.
    """

    lock_count = {}  # lock count for each locked file
    lock_handles = {}  # file handle for each locked file

    def __init__(self, file_name, timeout=10, delay=0.05):
        """ Prepare the file locker. Specify the file to lock and optionally
            the maximum timeout and the delay between each attempt to lock.
        """
        self.fd = None
        self.is_locked = False
        self.lockfile = os.path.join(os.getcwd(), "%s.lock" % file_name)
        self.file_name = file_name
        self.timeout = timeout
        self.delay = delay

    def acquire(self):
        """ Acquire the lock, if possible. If the lock is in use, it check again
            every `wait` seconds. It does this until it either gets the lock or
            exceeds `timeout` number of seconds, in which case it throws 
            an exception.
        """
        # Don't try to lock the same file more than once
        if FileLock.lock_count.setdefault(self.lockfile, 0) > 0:
            self.is_locked = True
            self.fd = FileLock.lock_handles[self.lockfile]
            FileLock.lock_count[self.lockfile] += 1
            return

        start_time = time.time()
        while True:
            try:
                # create the cache directory if it does not exist (new installations will not have a cache directory)
                stimdir = os.path.dirname(self.lockfile)
                cachedir = os.path.dirname(stimdir)
                if not os.path.isdir(cachedir):  # make sure the cache exists
                    os.mkdir(cachedir)
                if not os.path.isdir(stimdir):  # and the specific stimulus dir
                    os.mkdir(stimdir)
                self.fd = os.open(self.lockfile, os.O_CREAT | os.O_EXCL | os.O_RDWR)
                open(self.lockfile, "w").write(str(os.getpid()))
                break
            except OSError as e:
                if e.errno != errno.EEXIST:
                    raise
                if (time.time() - start_time) >= self.timeout:
                    try:
                        pid = open(self.lockfile).read()
                    except Exception:
                        pid = "[error reading lockfile: %s]" % sys.exc_info()[0]
                    raise FileLockException(
                        "Timeout occured. (%s is locked by pid %s)"
                        % (self.lockfile, pid)
                    )
                time.sleep(self.delay)

        self.is_locked = True
        FileLock.lock_count[self.lockfile] += 1
        FileLock.lock_handles[self.lockfile] = self.fd

    def release(self):
        """ Get rid of the lock by deleting the lockfile. 
            When working in a `with` statement, this gets automatically 
            called at the end.
        """
        if self.is_locked:
            self.is_locked = False
            FileLock.lock_count[self.lockfile] -= 1
            if FileLock.lock_count[self.lockfile] == 0:
                os.close(self.fd)
                os.unlink(self.lockfile)
                del FileLock.lock_handles[self.lockfile]

    def __enter__(self):
        """ Activated when used in the with statement. 
            Should automatically acquire a lock to be used in the with block.
        """
        if not self.is_locked:
            self.acquire()
        return self

    def __exit__(self, type, value, traceback):
        """ Activated at the end of the with statement.
            It automatically releases the lock if it isn't locked.
        """
        if self.is_locked:
            self.release()

    def __del__(self):
        """ Make sure that the FileLock instance doesn't leave a lockfile
            lying around.
        """
        self.release()