[issue4106] multiprocessing occasionally spits out exception during shutdown

[Yaniv Aknin]

Yaniv Aknin  added the comment:

Ugh. Not 100% sure it's related, but I've been getting a similar traceback when 
running pip's test suite (python setup.py test) on OSX 10.6.8 with Python 2.7.2.

Traceback (most recent call last):
  File 
"/usr/local/Cellar/python/2.7.2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/atexit.py",
 line 24, in _run_exitfuncs
func(*targs, **kargs)
  File 
"/usr/local/Cellar/python/2.7.2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/multiprocessing/util.py",
 line 284, in _exit_function
info('process shutting down')
TypeError: 'NoneType' object is not callable

Obviously it's not the exact same bug as fixed here, but Googling the traceback 
led me here and I do think it's the same genre of bug, i.e., multiprocessing's 
use of forking leads to issues when atexit is called (wasn't sure whether to 
open it here or #9207). Also, see 
https://groups.google.com/forum/#!topic/nose-users/fnJ-kAUbYHQ, it seems other 
users of the nose testsuite ran into this.

I'm afraid I won't have time to look much further into this (the reason I'm 
running pip's testsuite is that I'm already trying to make a contribution to 
pip...), but I thought it's best to at least mention it somewhere.

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Antoine Pitrou]

Antoine Pitrou  added the comment:

This should hopefully be fixed now. Feel free to reopen if it isn't.

--
resolution:  -> fixed
stage: patch review -> committed/rejected
status: open -> closed

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Roundup Robot]

Roundup Robot  added the comment:

New changeset d316315a8781 by Antoine Pitrou in branch '2.7':
Issue #4106: Fix occasional exceptions printed out by multiprocessing on 
interpreter shutdown.
http://hg.python.org/cpython/rev/d316315a8781

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Vinay Sajip]

Vinay Sajip  added the comment:

In Antoine's patch, ISTM that the line

created_by_this_process = ...

could also be deleted, as the patch no longer uses that value and it's not used 
anywhere later in the method.

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Antoine Pitrou]

Antoine Pitrou  added the comment:

Ok, I think the reason this doesn't appear in 3.2/3.3 is the fix for issue1856. 
In 2.x (and 3.1) daemon threads can continue executing after the interpreter's 
internal structures have started being destroyed.

The least intrusive solution is to always join the helper thread before 
shutting down the interpreter. Patch attached.

--
keywords: +patch
stage:  -> patch review
Added file: http://bugs.python.org/file23031/mpqshutdown.patch

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Antoine Pitrou]

Antoine Pitrou  added the comment:

Indeed, 2.7 seems still affected.

--
resolution: out of date -> 
versions:  -Python 3.2, Python 3.3

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Jesse Noller]

Jesse Noller  added the comment:

On Wed, Aug 24, 2011 at 3:01 PM, Antoine Pitrou  wrote:
>
> Antoine Pitrou  added the comment:
>
> I can't seem to reproduce this under 3.3. Should it be closed?

I don't think so; it's still applicable to 2.x, and a fix should go
into 2.7 ideally. http://bugs.python.org/issue9207 is the source of
the issue AFAIR

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Antoine Pitrou]

Antoine Pitrou  added the comment:

I can't seem to reproduce this under 3.3. Should it be closed?

--
nosy: +pitrou
resolution:  -> out of date
versions: +Python 3.2, Python 3.3 -Python 2.6

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Miquel Torres]

Miquel Torres  added the comment:

I can confirm this but with Python 2.7.1 on Ubuntu 11.04 64bit

My code was working with a queue that was being fed a two-string tuple.
When i changed it to contain my custom Objects, it still worked correctly, but 
the main program doesn't end until it raises the exception in QueueFeederThread

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versions: +Python 2.7

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Dirk Pranke]

Changes by Dirk Pranke :


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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Jesse Noller]

Jesse Noller  added the comment:

On Tue, Jan 18, 2011 at 6:23 PM, Brian Thorne  wrote:
>
> Brian Thorne  added the comment:
>
> With the example script attached I see the exception every time. On Ubuntu 
> 10.10 with Python 2.6
>
> Since the offending line in multiprocesing/queues.py (233) is a debug 
> statement, just commenting it out seems to stop this exception.
>
> Looking at the util file shows the logging functions to be all of the form:
>
>    if _logger:
>        _logger.log(...
>
> Could it be possible that after the check the _logger global (or the debug 
> function) is destroyed by the exit handler? Can we convince them to stick 
> around until such a time that they cannot be called?
>
> Adding a small delay before joining also seems to work, but is ugly. Why 
> should another Process *have* to have a minimum amount of work to not throw 
> an exception?

See http://bugs.python.org/issue9207 - but yes, the problem is that
the VM is nuking our imported modules before all the processes are
shutdown.

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Brian Thorne]

Brian Thorne  added the comment:

With the example script attached I see the exception every time. On Ubuntu 
10.10 with Python 2.6

Since the offending line in multiprocesing/queues.py (233) is a debug 
statement, just commenting it out seems to stop this exception.

Looking at the util file shows the logging functions to be all of the form:

if _logger:
_logger.log(...

Could it be possible that after the check the _logger global (or the debug 
function) is destroyed by the exit handler? Can we convince them to stick 
around until such a time that they cannot be called?

Adding a small delay before joining also seems to work, but is ugly. Why should 
another Process *have* to have a minimum amount of work to not throw an 
exception?

--
nosy: +Thorney
versions: +Python 2.6 -Python 2.7
Added file: http://bugs.python.org/file20445/test_mult.py

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Matthew Woodcraft]

Changes by Matthew Woodcraft :


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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Greg Brockman]

Greg Brockman  added the comment:

Sure thing.  See http://bugs.python.org/issue9207.

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Jesse Noller]

Jesse Noller  added the comment:

Greg - this is actually a different exception then the original bug report; 
could you please file a new issue with the information you've provided? I'm 
going to need to find a 64bit ubuntu box as I don't have one right now.

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Greg Brockman]

Greg Brockman  added the comment:

I'm on Ubuntu 10.04, 64 bit.

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Jesse Noller]

Jesse Noller  added the comment:

Greg - what platform?

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Greg Brockman]

Greg Brockman  added the comment:

For what it's worth, I think I have a simpler reproducer of this issue.  Using 
freshly-compiled python-from-trunk (as well as multiprocessing-from-trunk), I 
get tracebacks from the following about 30% of the time:

"""
import multiprocessing, time   
def foo(x):
 time.sleep(3) 
multiprocessing.Pool(1).apply(foo, [1])
"""

My tracebacks are of the form:
"""
Exception in thread Thread-1 (most likely raised during interpreter shutdown):
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/threading.py", line 530, in __bootstrap_inner
  File "/usr/local/lib/python2.7/threading.py", line 483, in run
  File "/usr/local/lib/python2.7/multiprocessing/pool.py", line 272, in 
_handle_workers
: 'NoneType' object is not callable
"""

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Skip Montanaro]

Changes by Skip Montanaro :


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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Bobby Impollonia]

Changes by Bobby Impollonia :


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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Jesse Noller]

Jesse Noller  added the comment:

Ah ha. I see it if I run it with the loop set to 3000 - it is pretty rare.

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Jesse Noller]

Jesse Noller  added the comment:

Skip, using this:

 while ((x++ < 500)) ; do echo '!'$i ; ./python.exe test_proc.py; done | 
egrep '!'

I don't see the exception in python-trunk, freshly compiled. It could be 
an OS thing (I'm on OS/X) - I just want to confirm that you're still 
seeing this on trunk

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Jesse Noller]

Changes by Jesse Noller :


--
assignee:  -> jnoller
nosy: +jnoller

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Skip Montanaro]

Skip Montanaro <[EMAIL PROTECTED]> added the comment:

Final comment before I see some feedback from the experts.

I have this code in the worker function's loop:

# quick pause to allow other stuff to happen a bit randomly
t = 0.1 * random.random()
time.sleep(t)

If I eliminate the sleep altogether pretty much all hell breaks loose.
As I reduce the sleep time it gets noisier and noisier.  I switched to
a fixed sleep time and reduced it as far as

time.sleep(0.00015625)

At that point it was complaining about killing worker processes on
many of the runs, maybe 1 out of every 5 or 10 runs.

I suppose the moral of the story is to not use multiprocessing except
when you have long-running tasks.

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Skip Montanaro]

Skip Montanaro <[EMAIL PROTECTED]> added the comment:

Got another one just now, but with just the note about the exception
in the queue feeder thread.  The traceback was swallowed.

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Skip Montanaro]

Skip Montanaro <[EMAIL PROTECTED]> added the comment:

Oh, the range command used in the shell for loop is analogous to Python's 
range() builtin function.

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[issue4106] multiprocessing occasionally spits out exception during shutdown

[Skip Montanaro]

New submission from Skip Montanaro <[EMAIL PROTECTED]>:

I worked up a simple example of using the external processing module 
(0.52) for a friend at work today.  I noticed some cases where it raised 
exceptions during exit.  Not all the time, but not infrequently either.  
This evening I tweaked it for the 2.6 multiprocessing module's API and 
tried it out.  I ran it in a large loop:

   for i in $(range 500) ; do
   echo '!'$i
   python test_proc.py
   end | egrep '!'

Most of the time all I see are the '!' lines from the echo command.  
Every once in awhile I see a traceback though.  For example:

Exception in thread QueueFeederThread (most likely raised during 
interpreter shutdown):
Traceback (most recent call last):
  File "/Users/skip/local/lib/python2.7/threading.py", line 522, in 
__bootstrap_inner
  File "/Users/skip/local/lib/python2.7/threading.py", line 477, in run
  File "/Users/skip/local/lib/python2.7/multiprocessing/queues.py", line 
233, in _feed
: 'NoneType' object is not callable

This occurred once in approximately 1500 runs of the script (three times 
through the above shell loop).  The script used to trigger this 
exception is attached.

--
components: Library (Lib)
files: test_proc.py
messages: 74656
nosy: skip.montanaro
priority: normal
severity: normal
status: open
title: multiprocessing occasionally spits out exception during shutdown
type: behavior
versions: Python 2.7
Added file: http://bugs.python.org/file11771/test_proc.py

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