Condition.wait() modifies self._waiters without holding the lock (when a wait with timeout times out without the condition being notified).

If this happens to occur in between construction of the _islice and _deque objects in Condition.notify():

    def notify(self, n=1):
        [...]
        all_waiters = self._waiters
        waiters_to_notify = _deque(_islice(all_waiters, n))

then the result is a RuntimeError exception:

File "/usr/lib/python3.4/threading.py", line 358, in notify_all
  self.notify(len(self.\_waiters))
File "/usr/lib/python3.4/threading.py", line 341, in notify
  waiters_to_notify = \_deque(_islice(all_waiters, n))

RuntimeError: deque mutated during iteration

(I have a server which makes extensive use of conditions on which this happens about once a day.)

This patch fixes this bug by moving wait()'s modification of self._waiters to be inside the lock, as suggested by Antoine Pitrou here: http://bugs.python.org/issue17385#msg183875

+1. I agree it's a bug, that the diagnosis is correct, and that the patch will fix it :-)

So, what happens now? What do I need to do to make progress on this?

New changeset 4cce39cfe46c by Antoine Pitrou in branch '3.4':
Issue bpo-22185: Fix an occasional RuntimeError in threading.Condition.wait() caused by mutation of the waiters queue without holding the lock.
http://hg.python.org/cpython/rev/4cce39cfe46c

New changeset 78a38f8bd5d9 by Antoine Pitrou in branch 'default':
Issue bpo-22185: Fix an occasional RuntimeError in threading.Condition.wait() caused by mutation of the waiters queue without holding the lock.
http://hg.python.org/cpython/rev/78a38f8bd5d9

It only needed someone to push your patch, which I just did.
Thank you very much Doug, your contribution is appreciated!