transient failure in test_multiprocessing.WithProcessesTestCondition

[pitrou]

BPO	11790
Nosy	@pitrou, @vstinner
PRs	[2.7] bpo-11790: Fix sporadic failures in test_multiprocessing.WithProcessesTestCondition #1591

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2011-04-16.19:03:59.103>
created_at = <Date 2011-04-06.23:04:34.959>
labels = ['tests', 'type-bug', 'library']
title = 'transient failure in test_multiprocessing.WithProcessesTestCondition'
updated_at = <Date 2017-05-15.15:32:16.774>
user = 'https://github.com/pitrou'

bugs.python.org fields:

activity = <Date 2017-05-15.15:32:16.774>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2011-04-16.19:03:59.103>
closer = 'pitrou'
components = ['Library (Lib)', 'Tests']
creation = <Date 2011-04-06.23:04:34.959>
creator = 'pitrou'
dependencies = []
files = []
hgrepos = []
issue_num = 11790
keywords = []
message_count = 6.0
messages = ['133182', '133533', '133536', '133901', '133902', '293714']
nosy_count = 4.0
nosy_names = ['pitrou', 'vstinner', 'neologix', 'python-dev']
pr_nums = ['1591']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue11790'
versions = ['Python 3.2', 'Python 3.3']

[pitrou]

http://www.python.org/dev/buildbot/all/builders/x86%20FreeBSD%207.2%202.7/builds/519/steps/test/logs/stdio

======================================================================
FAIL: test_notify_all (test.test_multiprocessing.WithProcessesTestCondition)
----------------------------------------------------------------------

Traceback (most recent call last):
  File "/usr/home/db3l/buildarea/2.7.bolen-freebsd7/build/Lib/test/test_multiprocessing.py", line 757, in test_notify_all
    self.assertReturnsIfImplemented(6, get_value, woken)
  File "/usr/home/db3l/buildarea/2.7.bolen-freebsd7/build/Lib/test/test_multiprocessing.py", line 116, in assertReturnsIfImplemented
    return self.assertEqual(value, res)
AssertionError: 6 != 5

[neologix]

One possible cause for those intermittent failures is the preemtion of a thread while waiting on the condition:

 def wait(self, timeout=None):
   233         assert self._lock._semlock._is_mine(), \
   234                'must acquire() condition before using wait()'
   235 
   236         # indicate that this thread is going to sleep
   237         self._sleeping_count.release()
   238 
   239         # release lock
   240         count = self._lock._semlock._count()
   241         for i in range(count):
   242             self._lock.release()
   243 
<-- here
   244         try:
   245             # wait for notification or timeout
   246             ret = self._wait_semaphore.acquire(True, timeout)

For example, if the last thread/process is preempted after having released the condition's lock (and hence performed a up on the "sleeping" semaphore sooner in the "f" function) but before waiting on the condition's semaphore, since the main thread only waits 0.1s before locking the condition and performing a notify_all on it (it will proceed since all the threads performed an up on "sleeping"), only the threads already waiting on the condition will be woken up, this last thread won't be woken up, triggering a failure in this assertion
764 self.assertReturnsIfImplemented(0, get_value, woken)
with woken.get_value() == 5

It's just a guess, but I'd suggest increasing the sleep before trying to signal the condition a bit:

762 # check no process/thread has woken up
763 time.sleep(10 * DELTA)

[neologix]

Sorry, wrong copy-paste, the failing assertion will of course be this one:
773 self.assertReturnsIfImplemented(6, get_value, woken)

since woken.get_value() == 5

[pitrou]

Indeed, it just seems that the sleep period is sometimes too low. Will commit a patch.

[python-dev]

New changeset 88f1907fe312 by Antoine Pitrou in branch '3.2':
Issue bpo-11790: Fix sporadic failures in test_multiprocessing.WithProcessesTestCondition.
http://hg.python.org/cpython/rev/88f1907fe312

New changeset 0ecfa2ce6561 by Antoine Pitrou in branch 'default':
Issue bpo-11790: Fix sporadic failures in test_multiprocessing.WithProcessesTestCondition.
http://hg.python.org/cpython/rev/0ecfa2ce6561

[vstinner]

New changeset 9d1983b by Victor Stinner in branch '2.7':
bpo-11790: Fix sporadic failures in test_multiprocessing.WithProcessesTestCondition. (bpo-1591)
9d1983b