Race condition in TagAlongThread

[idanarye]

TagAlongThread's __call__() starts with:

def __call__(self):
    assert self.__alive, '%s is dead' % self
    # We can't use an iteration that's already started - maybe it's already at a too advanced stage?
    if self._iterating.is_set():
        self._not_iterating.wait()

    self._iteration_trigger.set()  # Signal that we want an iteration

    while not self._iterating.wait(1):  # Wait until an iteration starts
        # It is possible that we missed the loop and _iterating was already
        # cleared. If this is the case, _not_iterating will not be set -
        # and we can use it as a signal to stop waiting for iteration.
        if self._not_iterating.is_set():
            break
    else:
        self._not_iterating.wait()  # Wait until it finishes

    # ...

Which should trigger the _loop():

    def _loop(self):
        while self.__alive:
            self._iteration_trigger.wait()
            self._iteration_trigger.clear()

            # Mark that we are now iterating
            self._not_iterating.clear()
            self._iterating.set()

            try:
                self._last_exception, self._last_result = None, self._func()
            except Exception as e:
                self._last_exception = e

            # Mark that we are no longer iterating
            self._iterating.clear()
            self._not_iterating.set()

            time.sleep(self.minimal_sleep)

        # ...

Now, consider this scenario:

• __call__() is called.
• _iterating is not set - so we are not waiting for _not_iterating.
• _iteration_trigger.set()
• We start doing while not self._iterating.wait(1):.
• For some reason, the loop thread does not get scheduled again during the following second - and _iterating.wait(1) returns (with False, of course)
• _not_iterating is still set (the loop thread was not scheduled, so it was not cleared), so we assume the iteration was finished and stop waiting.

The condition requires that a thread will not get scheduled for a whole second - but I can see that happen in a busy process...