Method acquire

Lib/importlib/_bootstrap.py:304–370

Acquire the module lock. If a potential deadlock is detected, a _DeadlockError is raised. Otherwise, the lock is always acquired and True is returned.

(self)

        )

    def acquire(self):
        """
        Acquire the module lock.  If a potential deadlock is detected,
        a _DeadlockError is raised.
        Otherwise, the lock is always acquired and True is returned.
        """
        tid = _thread.get_ident()
        with _BlockingOnManager(tid, self):
            while True:
                # Protect interaction with state on self with a per-module
                # lock.  This makes it safe for more than one thread to try to
                # acquire the lock for a single module at the same time.
                with self.lock:
                    if self.count == [] or self.owner == tid:
                        # If the lock for this module is unowned then we can
                        # take the lock immediately and succeed.  If the lock
                        # for this module is owned by the running thread then
                        # we can also allow the acquire to succeed.  This
                        # supports circular imports (thread T imports module A
                        # which imports module B which imports module A).
                        self.owner = tid
                        self.count.append(True)
                        return True

                    # At this point we know the lock is held (because count !=
                    # 0) by another thread (because owner != tid).  We'll have
                    # to get in line to take the module lock.

                    # But first, check to see if this thread would create a
                    # deadlock by acquiring this module lock.  If it would
                    # then just stop with an error.
                    #
                    # It's not clear who is expected to handle this error.
                    # There is one handler in _lock_unlock_module but many
                    # times this method is called when entering the context
                    # manager _ModuleLockManager instead - so _DeadlockError
                    # will just propagate up to application code.
                    #
                    # This seems to be more than just a hypothetical -
                    # https://stackoverflow.com/questions/59509154
                    # https://github.com/encode/django-rest-framework/issues/7078
                    if self.has_deadlock():
                        raise _DeadlockError(f'deadlock detected by {self!r}')

                    # Check to see if we're going to be able to acquire the
                    # lock.  If we are going to have to wait then increment
                    # the waiters so `self.release` will know to unblock us
                    # later on.  We do this part non-blockingly so we don't
                    # get stuck here before we increment waiters.  We have
                    # this extra acquire call (in addition to the one below,
                    # outside the self.lock context manager) to make sure
                    # self.wakeup is held when the next acquire is called (so
                    # we block).  This is probably needlessly complex and we
                    # should just take self.wakeup in the return codepath
                    # above.
                    if self.wakeup.acquire(False):
                        self.waiters.append(None)