Class _ModuleLock

Lib/importlib/_bootstrap.py:226–389

A recursive lock implementation which is able to detect deadlocks (e.g. thread 1 trying to take locks A then B, and thread 2 trying to take locks B then A).

class _ModuleLock:
    """A recursive lock implementation which is able to detect deadlocks
    (e.g. thread 1 trying to take locks A then B, and thread 2 trying to
    take locks B then A).
    """

    def __init__(self, name):
        # Create an RLock for protecting the import process for the
        # corresponding module.  Since it is an RLock, a single thread will be
        # able to take it more than once.  This is necessary to support
        # re-entrancy in the import system that arises from (at least) signal
        # handlers and the garbage collector.  Consider the case of:
        #
        #  import foo
        #  -> ...
        #     -> importlib._bootstrap._ModuleLock.acquire
        #        -> ...
        #           -> <garbage collector>
        #              -> __del__
        #                 -> import foo
        #                    -> ...
        #                       -> importlib._bootstrap._ModuleLock.acquire
        #                          -> _BlockingOnManager.__enter__
        #
        # If a different thread than the running one holds the lock then the
        # thread will have to block on taking the lock, which is what we want
        # for thread safety.
        self.lock = _thread.RLock()
        self.wakeup = _thread.allocate_lock()

        # The name of the module for which this is a lock.
        self.name = name

        # Can end up being set to None if this lock is not owned by any thread
        # or the thread identifier for the owning thread.
        self.owner = None

        # Represent the number of times the owning thread has acquired this lock
        # via a list of True.  This supports RLock-like ("re-entrant lock")
        # behavior, necessary in case a single thread is following a circular
        # import dependency and needs to take the lock for a single module
        # more than once.
        #
        # Counts are represented as a list of True because list.append(True)
        # and list.pop() are both atomic and thread-safe in CPython and it's hard
        # to find another primitive with the same properties.
        self.count = []

        # This is a count of the number of threads that are blocking on
        # self.wakeup.acquire() awaiting to get their turn holding this module
        # lock.  When the module lock is released, if this is greater than
        # zero, it is decremented and `self.wakeup` is released one time.  The
        # intent is that this will let one other thread make more progress on
        # acquiring this module lock.  This repeats until all the threads have
        # gotten a turn.
        #
        # This is incremented in self.acquire() when a thread notices it is
        # going to have to wait for another thread to finish.