lockProtection.py

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# 
# LSST Data Management System
# Copyright 2008, 2009, 2010 LSST Corporation.
# 
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# LSST Project (http://www.lsst.org/).
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"""
classes used to protect method from simultaneous access by different
threads.  The primary class is LockProtected.  
"""
import threading

SharedLock = threading.RLock

class LockProtected(object):
    """
    a class that assists in keeping methods thread-safe.

    This class is intended to be enlisted as a base class to the class being
    protected.  A method that is to be protected from simultaneous access 
    would include (at its beginning) a call to _checkLocked():

    class MyClass(BaseClass, LockProtected):
        def __init__(self, lock=SharedLock):
            LockProtected.__init__(self, lock)
            ...

        def dangerous(self):
            self._checkLocked()
            ...

    Doing so will require that the protected class be "locked" before a 
    protected method can be called or else an UnsafeAccessError will be
    raised.  Locking is done via the with statement:

    mc = MyClass()
    with mc:
        mc.dangerous()

    For the locking to work, the protected class must provide to the
    LockProtected constructor a lock object.  Typically this is a
    lsst.utils.multithreading.SharedLock instance or, if one wants to
    employ thread notification techniques, a
    lsst.utils.multithreading.SharedData instance.  It should at least
    be a reentrant lock--that is, having the behavior of threading.RLock
    (from the standard Python Library).

    This class is primarily intended for protecting methods across multiple
    classes together via a single lock.  That is, it can prevent simultaneous
    access to any protected method across multiple class instances.  To
    accomplish this, each class to be protected should accept a lock as
    a constructor argument.  Then the same lock is passed into all of the
    constructors that need protection together. 
    """

    def __init__(self, lock=None):
        """
        initialize the lock protection
        @param lock   a reentrant lock instance to use.  If None, the
                        protection behavior is disabled.
        """
        # the shared lock
        self._lp_lock = lock

    def __enter__(self):
        if self._lp_lock:
            self._lp_lock.acquire()

    def __exit__(self, exc_type, exc_value, traceback):
        if self._lp_lock:
            self._lp_lock.release()
        return False

    def _checkLocked(self):
        if self._lp_lock and not self._lp_lock._is_owned():
            raise UnsafeAccessError()

class UnsafeAccessError(Exception):
    """
    an exception that is raised when one attempts to access a protected
    method without first acquiring the lock on its class.
    """
    def __init__(self, msg=None):
        if not msg:
            msg = "Programmer Error: failed to obtain lock via with statement"
        Exception.__init__(self, msg)