[issue27930] logging's QueueListener drops log messages

[Roundup Robot]

Roundup Robot added the comment:

New changeset 121b5b41c9e8 by Vinay Sajip in branch 'default':
Fixes #27930: improved QueueListener behaviour.
https://hg.python.org/cpython/rev/121b5b41c9e8

New changeset b2ea0ede3753 by Vinay Sajip in branch '3.5':
Fixes #27930: improved QueueListener behaviour.
https://hg.python.org/cpython/rev/b2ea0ede3753

New changeset 89b185372b2c by Vinay Sajip in branch 'default':
Closes #27930: Merged fix from 3.5.
https://hg.python.org/cpython/rev/89b185372b2c

--
nosy: +python-dev
resolution:  -> fixed
stage:  -> resolved
status: open -> closed

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[issue27930] logging's QueueListener drops log messages

[Vinay Sajip]

Vinay Sajip added the comment:

Removed 3.3 and 3.4 as they are not in scope for non-security issues.

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versions:  -Python 3.3, Python 3.4

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[issue27930] logging's QueueListener drops log messages

[Petr Viktorin]

New submission from Petr Viktorin:

There are two "barrier" like abstractions on Lib/logging/handlers.py in the 
_monitor method.

First _monitor has two loops, what is already kind of a hint something is not 
right.

Second, it has two ways to exit the loop, that also exit the thread:
1) The _stop threading.Event is "set"
2) The _sentinel object is added to the queue

The problem is, the documentation says that the correct way to not loose 
records, the stop method must be called, but, the stop method just sets the 
_stop object and then adds the _sentinel object to the queue.

The loop stops when noticing that _stop is set, and then enters a second 
version of the loop, trying again to see the _sentinel object, but this time 
with non blocking read.

The test case shows the problem, but it also hints about the race conditions by 
the fact that running the test case under "taskset 1" works, so, to reproduce 
the issue, run the test under a multiprocessor environment.

The proper solution would be to have a proper locking mechanism, otherwise, the 
_stop object should not be used, and rely only in seeing the _sentinel field; 
this is what the class DeterministicQueueListener does in the test case.


(Reported by Paulo Andrade at 
https://bugzilla.redhat.com/show_bug.cgi?id=1370484 )

--
components: Library (Lib)
files: test.py
messages: 274139
nosy: encukou, vinay.sajip
priority: normal
severity: normal
status: open
title: logging's QueueListener drops log messages
versions: Python 3.3, Python 3.4, Python 3.5, Python 3.6
Added file: http://bugs.python.org/file44323/test.py

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