FreeNAS Code

#+
# Copyright 2011 iXsystems, Inc.
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# modification, are permitted providing that the following conditions
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# 2. Redistributions in binary form must reproduce the above copyright
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# THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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#####################################################################

import fcntl
import os

MNTPT = '/mnt'

class MountLock:
    """A mutex for which is used for serializing tasks that need direct
       access to the mountpoints, for whatever reason."""
    def __init__(self, blocking=True, mntpt=MNTPT):

        self._fd = os.open(mntpt, os.O_DIRECTORY)
        # Don't spread lock file descriptors to child processes.
        flags = fcntl.FD_CLOEXEC | fcntl.fcntl(self._fd, fcntl.F_GETFL)
        fcntl.fcntl(self._fd, fcntl.F_SETFL, flags)
        if blocking:
            self.__enter__ = self.lock
        else:
            self.__enter__ = self.lock_try

    def __del__(self):
        if self._fd:
            os.close(self._fd)

    def __exit__(self, exc_type, value, traceback):
        self.unlock()

    def lock_try(self):
        """Try to lock the mountpoint in a non-blocking manner.

        :raises:
            IOError - the lock could not be acquired immediately.
        """
        fcntl.flock(self._fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
            
    def lock(self):
        """Try to lock the mountpoint in a blocking manner.

        XXX: this will not raise an IOError unless EBADF, EINVAL, or
             EOPNOTSUPP occurs when running flock(3) (unless one has stumbled
             across an OS bug). Some portions of FreeNAS incorrectly assume
             that IOError will occur with this API if the lock is unavailable.

        :raises:
             IOError - see the ERRORS section under flock(3) for more details;
                       skip over the EWOULDBLOCK case.
        """
        fcntl.flock(self._fd, fcntl.LOCK_EX)

    def unlock(self):
        """Unlock the mountpoint [in a blocking manner].

        :raises:
             IOError - see the ERRORS section under flock(3) for more details;
                       skip over the EWOULDBLOCK case.
        """
        fcntl.flock(self._fd, fcntl.LOCK_UN)

# Quick and dirty backwards compatibility.
mntlock = MountLock