Synnefo » snf-ganeti

def LockFile(fd):

  """Locks a file using POSIX locks.

  @type fd: int

  @param fd: the file descriptor we need to lock

  """

  try:

    fcntl.flock(fd, fcntl.LOCK_EX | fcntl.LOCK_NB)

  except IOError, err:

    if err.errno == errno.EAGAIN:

      raise errors.LockError("File already locked")

    raise

class FileLock(object):

  """Utility class for file locks.

  """

  def __init__(self, fd, filename):

    """Constructor for FileLock.

    @type fd: file

    @param fd: File object

    @type filename: str

    @param filename: Path of the file opened at I{fd}

    """

    self.fd = fd

    self.filename = filename

  @classmethod

  def Open(cls, filename):

    """Creates and opens a file to be used as a file-based lock.

    @type filename: string

    @param filename: path to the file to be locked

    """

    # Using "os.open" is necessary to allow both opening existing file

    # read/write and creating if not existing. Vanilla "open" will truncate an

    # existing file -or- allow creating if not existing.

    return cls(os.fdopen(os.open(filename, os.O_RDWR | os.O_CREAT), "w+"),

               filename)

  def __del__(self):

    self.Close()

  def Close(self):

    """Close the file and release the lock.

    """

    if hasattr(self, "fd") and self.fd:

      self.fd.close()

      self.fd = None

  def _flock(self, flag, blocking, timeout, errmsg):

    """Wrapper for fcntl.flock.

    @type flag: int

    @param flag: operation flag

    @type blocking: bool

    @param blocking: whether the operation should be done in blocking mode.

    @type timeout: None or float

    @param timeout: for how long the operation should be retried (implies

                    non-blocking mode).

    @type errmsg: string

    @param errmsg: error message in case operation fails.

    """

    assert self.fd, "Lock was closed"

    assert timeout is None or timeout >= 0, \

      "If specified, timeout must be positive"

    assert not (flag & fcntl.LOCK_NB), "LOCK_NB must not be set"

    # When a timeout is used, LOCK_NB must always be set

    if not (timeout is None and blocking):

      flag |= fcntl.LOCK_NB

    if timeout is None:

      self._Lock(self.fd, flag, timeout)

    else:

      try:

        Retry(self._Lock, (0.1, 1.2, 1.0), timeout,

              args=(self.fd, flag, timeout))

      except RetryTimeout:

        raise errors.LockError(errmsg)

  @staticmethod

  def _Lock(fd, flag, timeout):

    try:

      fcntl.flock(fd, flag)

    except IOError, err:

      if timeout is not None and err.errno == errno.EAGAIN:

        raise RetryAgain()

      logging.exception("fcntl.flock failed")

      raise

  def Exclusive(self, blocking=False, timeout=None):

    """Locks the file in exclusive mode.

    @type blocking: boolean

    @param blocking: whether to block and wait until we

        can lock the file or return immediately

    @type timeout: int or None

    @param timeout: if not None, the duration to wait for the lock

        (in blocking mode)

    """

    self._flock(fcntl.LOCK_EX, blocking, timeout,

                "Failed to lock %s in exclusive mode" % self.filename)

  def Shared(self, blocking=False, timeout=None):

    """Locks the file in shared mode.

    @type blocking: boolean

    @param blocking: whether to block and wait until we

        can lock the file or return immediately

    @type timeout: int or None

    @param timeout: if not None, the duration to wait for the lock

        (in blocking mode)

    """

    self._flock(fcntl.LOCK_SH, blocking, timeout,

                "Failed to lock %s in shared mode" % self.filename)

  def Unlock(self, blocking=True, timeout=None):

    """Unlocks the file.

    According to C{flock(2)}, unlocking can also be a nonblocking

    operation::

      To make a non-blocking request, include LOCK_NB with any of the above

      operations.

    @type blocking: boolean

    @param blocking: whether to block and wait until we

        can lock the file or return immediately

    @type timeout: int or None

    @param timeout: if not None, the duration to wait for the lock

        (in blocking mode)

    """

    self._flock(fcntl.LOCK_UN, blocking, timeout,

                "Failed to unlock %s" % self.filename)