[issue24573] Using multiprocessing with tkinter frames in Python 3.4.3 crashes in OS X

[Evan Jones]

Evan Jones added the comment:

This is another instance of the following bug: http://bugs.python.org/issue27126

libdispatch (grand central dispatch) is not fork safe. The forkserver approach 
is a good workaround, thanks!

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[issue24573] Using multiprocessing with tkinter frames in Python 3.4.3 crashes in OS X

[R. David Murray]

R. David Murray added the comment:

Your test code works for me on linux and python3.3 and 3.4.1.  That is, I can 
click four buttons and get back the prompt, with no segfault.  It is quite 
possible this is a bug in the Mac version of TK, assuming this is even supposed 
to work.

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[issue24573] Using multiprocessing with tkinter frames in Python 3.4.3 crashes in OS X

[Ronald Oussoren]

Ronald Oussoren added the comment:

This is likely a known issue with using os.fork on OSX: that is unsafe, and 
likely causes crashes, once one of Apple's frameworks has initialized. 

I think it might be better in the long run to make multiprocessing on OSX 
behave the same as on windows, but haven't thought about the consequences of 
this enough to be sure (because there are downsides as well)

BTW. Besides that, the code pattern doesn't match how GUI applications are 
supposed to work on OSX. Applications are basically supposed to be single 
process, even when working with multiple documents. 

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On the road, hence brief. 

Op 6 jul. 2015 om 15:57 heeft R. David Murray rep...@bugs.python.org het 
volgende geschreven:

 
 R. David Murray added the comment:
 
 Your test code works for me on linux and python3.3 and 3.4.1.  That is, I can 
 click four buttons and get back the prompt, with no segfault.  It is quite 
 possible this is a bug in the Mac version of TK, assuming this is even 
 supposed to work.
 
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[issue24573] Using multiprocessing with tkinter frames in Python 3.4.3 crashes in OS X

[Ned Deily]

Ned Deily added the comment:

A web search will find multiple hits for problems with using Tk (and Tkinter) 
with multiprocess on OS X and elsewhere.  On OS X, there is a well-known and 
documented restriction that impacts Tk-based apps:

When launching separate processes using the fork function, you must always 
follow a call to fork with a call to exec or a similar function. Applications 
that depend on the Core Foundation, Cocoa, or Core Data frameworks (either 
explicitly or implicitly) must make a subsequent call to an exec function or 
those frameworks may behave improperly.

https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/Multithreading/AboutThreads/AboutThreads.html

For a specific example of problems using tkinter and multiprocessing see:
http://stackoverflow.com/questions/23599087/multiprocessing-python-core-foundation-error

Some suggest that you *might* be able to get things to work as long as you 
defer importing tkinter (and, thus, initializing Tcl and Tk) until after all of 
the processes are created.  But, in general, it's not a good idea to separate 
tkinter calls across multiple processes.  See Bryan Oakley's advice in the 
above SO link:

You'll need to keep all GUI code in the master process, and have your other 
process communicate with it via a queue.

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resolution:  - third party
stage:  - resolved
status: open - closed

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[issue24573] Using multiprocessing with tkinter frames in Python 3.4.3 crashes in OS X

[Topher Kessler]

Topher Kessler added the comment:

Yeah it is a bug in OS X, fixed by setting the python multiprocessing start 
method to 'forkserver' instead of the default fork.

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[issue24573] Using multiprocessing with tkinter frames in Python 3.4.3 crashes in OS X

[Topher Kessler]

New submission from Topher Kessler:

There may be a bug in how tkinter frames are handled when called in multiple 
processes in OS X.

I am trying to run a simple script that defines a new Frame subclass and then 
attempts to call it multiple times in separate processes using the 
multiprocessing module. When the frame's mainloops are called the process 
crashes. I've included the script where this problem is occurring.

The crash report specifies python, and among a bunch of boilerplate information 
includes the following lines:

Application Specific Information:
*** multi-threaded process forked ***
crashed on child side of fork pre-exec

This is happening in Python 3.4.3, running in OS X 10.10.4. In testing this on 
alternative platforms (Windows and Linux) it appears to work, suggesting it may 
be a bug in OS X's implementation.

--
components: Macintosh, Tkinter
files: tkmultiproc.py
messages: 246344
nosy: ned.deily, ronaldoussoren, tkessler
priority: normal
severity: normal
status: open
title: Using multiprocessing with tkinter frames in Python 3.4.3 crashes in OS X
type: crash
versions: Python 3.4
Added file: http://bugs.python.org/file39874/tkmultiproc.py

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