[New-bugs-announce] [issue22185] Occasional RuntimeError from Condition.notify

Doug Zongker report at bugs.python.org
Tue Aug 12 01:47:11 CEST 2014

New submission from Doug Zongker:

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
    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

components: Library (Lib)
files: fix.diff
keywords: patch
messages: 225208
nosy: dougz, pitrou
priority: normal
severity: normal
status: open
title: Occasional RuntimeError from Condition.notify
type: crash
versions: Python 3.4, Python 3.5
Added file: http://bugs.python.org/file36351/fix.diff

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