[ python-Bugs-754449 ] Exceptions when a thread exits

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Mon Mar 8 17:36:27 EST 2004


Bugs item #754449, was opened at 2003-06-14 03:32
Message generated for change (Comment added) made by bcannon
You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=105470&aid=754449&group_id=5470

Category: Threads
Group: Python 2.3
Status: Closed
Resolution: Fixed
Priority: 3
Submitted By: Matthias Klose (doko)
Assigned to: Brett Cannon (bcannon)
Summary: Exceptions when a thread exits

Initial Comment:
[forwarded from http://bugs.debian.org/195812]

The application mentioned is offlineimap, available
from ftp://ftp.debian.org/dists/unstable/main/source/.

This behavior is new to Python 2.3.

When my application exits, I get a lot of these messages:

Traceback (most recent call last):
  File "/usr/lib/python2.3/threading.py", line 426, in
__bootstrap
    self.__stop()
  File "/usr/lib/python2.3/threading.py", line 435, in
__stop
    self.__block.notifyAll()
  File "/usr/lib/python2.3/threading.py", line 239, in
notifyAll
    self.notify(len(self.__waiters))
  File "/usr/lib/python2.3/threading.py", line 221, in
notify
    currentThread() # for side-effect
TypeError: 'NoneType' object is not callable
jgoerzen at christoph:~/tree/offlineimap-3.99.18$
./offlineimap.py  -l log -d maildir -a Personal,Excel
Unhandled exception in thread started by <bound method
ExitNotifyThread.__bootstrap of <ExitNotifyThread(Keep
alive LocalExcel, stopped daemon)>>
Traceback (most recent call last):
  File "/usr/lib/python2.3/threading.py", line 426, in
__bootstrap
    self.__stop()
  File "/usr/lib/python2.3/threading.py", line 435, in
__stop
    self.__block.notifyAll()
  File "/usr/lib/python2.3/threading.py", line 239, in
notifyAll
    self.notify(len(self.__waiters))
  File "/usr/lib/python2.3/threading.py", line 221, in
notify
    currentThread() # for side-effect
TypeError: &#039;NoneType&#039; object is not callable
Unhandled exception in thread started by <bound method
ExitNotifyThread.__bootstrap of <ExitNotifyThread(Keep
alive RemoteExcel, stopped daemon)>>
Traceback (most recent call last):
  File "/usr/lib/python2.3/threading.py", line 426, in
__bootstrap
    self.__stop()
  File "/usr/lib/python2.3/threading.py", line 435, in
__stop
    self.__block.notifyAll()
  File "/usr/lib/python2.3/threading.py", line 239, in
notifyAll
    self.notify(len(self.__waiters))
  File "/usr/lib/python2.3/threading.py", line 221, in
notify
    currentThread() # for side-effect
TypeError: &#039;NoneType&#039; object is not callable


----------------------------------------------------------------------

>Comment By: Brett Cannon (bcannon)
Date: 2004-03-08 14:36

Message:
Logged In: YES 
user_id=357491

OK, this has been fixed in Python 2.4 (rev. 1.41 of Lib/threading.py; just 
removed the two calls to currentThread in the _Condition class that were 
commented to be for their side-effect).

Just because I don't feel rock-solid on my fix to the point of it not 
changing some obscure semantics that I would be willing to put a gun to 
my head over it I am not going to backport this until I hear from 
someone who had this bug and reports that they have not had any issues 
for a long time.  Otherwise I will just leave the fix in 2.4 only.

----------------------------------------------------------------------

Comment By: Brett Cannon (bcannon)
Date: 2004-03-08 14:32

Message:
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OK, this has been fixed in Python 2.4 (rev. 1.41 of Lib/threading.py; just 
removed the two calls to currentThread in the _Condition class that were 
commented to be for their side-effect).

Just because I don't feel rock-solid on my fix to the point of it not 
changing some obscure semantics that I would be willing to put a gun to 
my head over it I am not going to backport this until I hear from 
someone who had this bug and reports that they have not had any issues 
for a long time.  Otherwise I will just leave the fix in 2.4 only.

----------------------------------------------------------------------

Comment By: Brett Cannon (bcannon)
Date: 2004-03-02 20:29

Message:
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To force this error I inserted code to set all attributes of the threading 
module to None and then throw and exception.  After commenting out 
the two calls to currentThread in _Condition the thrown exception to 
trigger the problem propogated to the top correctly.

I did have the clean-up function give out since I set *everything* to 
None, but it doesn't look like a normal issue as shown in the exception 
traceback in the OP.  I don't think it is an issue.

So it seems commenting the two calls to currentThread in _Condition 
solves the problem.  But since this is threading code and the comment 
explicitly states it is for the side-effect I am going to try to get a second 
opinion.

----------------------------------------------------------------------

Comment By: Brett Cannon (bcannon)
Date: 2004-02-16 20:59

Message:
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Does anyone know how to cause this error in isolation?  I have tried a 
bunch of different ways but cannot cause an exception to be raised at the 
correct point in Threading.__bootstrap() to lead to self.__stop() to be 
called while the interpreter is tearing itself down.

----------------------------------------------------------------------

Comment By: Brett Cannon (bcannon)
Date: 2004-02-09 17:18

Message:
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After staring at the code, I am not even sure if the calls for its side-
effect are needed.  If you call currentThread(), it either returns a value 
from _active which is a dict of running Thread instances indexed by 
thread idents, or a new _DummyThread instance that inserts itself into 
_active.  Now the two calls in the _Condition class are purely for this 
possible side-effect.

And yet at no point is anything from _active, through currentThread or 
direct access, even touched by a _Condition method.  The only chance 
this might be an issue is if a _Condition instance uses an RLock instance 
for its locking, which is the default behavior.

But this still makes the side-effect need useless.  RLocks will call 
currentThread on their own and since this is all in the same thread the 
result won't change.  And in this specific case of this bug, the _Condition 
instance is created with a Lock instance since that is what the Thread 
instance uses for constructing the _Condition instance it use, which is just 
thread.allocate_lock() which is obviously not an RLock.

In other words I can't find the point to the side-effect in _Condition.  I 
will try to come up with some testing code that reproduces the error and 
then see if just removing the calls in _Condition remove the error and 
still pass the regression tests.

----------------------------------------------------------------------

Comment By: John Goerzen (jgoerzen)
Date: 2003-06-16 06:26

Message:
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I can confirm that this behavior is not present in Python 2.2 in 
the same version that I am using to test against Python 2.3. 
 
I will be on vacation for most of this and next week.  I'll try to 
get to the logging script before I leave, but I might not get to it 
until I return. 
 
FYI, you can also obtain OfflineIMAP at 
http://quux.org/devel/offlineimap. 
 
 

----------------------------------------------------------------------

Comment By: Brett Cannon (bcannon)
Date: 2003-06-15 23:58

Message:
Logged In: YES 
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OK, following Tim's advice I checked and it seems that Thread 
calls a method while shutting itself down that calls 
Condition.notifyAll which calls currentThread which is a global.  It 
would appear that offlineimap is leaving its threads running, the 
program gets shut down, the threads raise an error while 
shutting down (probably because things are being torn down), 
this triggers the stopping method in Thread, and this raises its 
own exception because of the teardown which is what we are 
seeing as the TypeError.

So the question is whether Condition should store a local 
reference to currentThread or not.  It is not the most pure 
solution since this shutdown issue is not Condition's, but the only 
other solution I can think of is to have Thread keep a reference 
to currentThread, inject it into the current frame's global 
namespace, call Condition.notifyAll, and then remove the 
reference from the frame again.  I vote for the cleaner, less pure 
solution.  =)

Am I insane on this or does it at least sound like this is the 
problem and proper solution?

----------------------------------------------------------------------

Comment By: Brett Cannon (bcannon)
Date: 2003-06-15 23:19

Message:
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Nuts.  For some reason I thought the OP had said when threads 
were exiting.

I will ask on python-dev for a good explanation of what happens 
when Python is shutting down.

----------------------------------------------------------------------

Comment By: Tim Peters (tim_one)
Date: 2003-06-15 18:05

Message:
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Note that the OP said "when my application exits".  When 
Python is tearing down the universe, it systematically sets 
module-global bindings to None.  currentThread() is a module 
global.  I can't make more time for this now, so for more info 
talk about it on Python-Dev.

----------------------------------------------------------------------

Comment By: Brett Cannon (bcannon)
Date: 2003-06-15 16:44

Message:
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Well, I'm stumped.  I checked the diff from when 2.2 was initially 
released until now and the only change that seems to be related 
to any of this is that what is returned by currentThread is not 
saved in a variable.  But since the error is the calling of 
currentThread itself and not saving the return value I don't see 
how that would affect anything.

I also went through offlineimap but I didn't see anything there 
that seemed to be setting currentThread to None.  Although 
since several files do ``import *`` so there still is a possibility of 
overwriting currentThread locally.

So, for my owning learning and to help solve this, I have written 
a tracing function that writes to stderr using the logging package 
when it detects that either currentThread or 
threading.currentThread has been set to None, locally or globally 
(I assume the code is not injecting into builtins so I didn't bother 
checking there).  The file is named tracer.py and I have attached 
it to this bug report.  If you can execute 
``sys.settrace(tracer.trace_currentThread)`` before offlinemap 
starts executing and immediately within each thread (it has to be 
called in *every* thread since tracing functions are no inherited 
from the main thread) it should print out a message when 
currentThread becomes None.  If you *really* want to make this 
robust you can also have it check sys.modules['threading'] every 
time as well, but I figure there is not going to be much renaming 
and masking of currentThread.

----------------------------------------------------------------------

Comment By: Matthias Klose (doko)
Date: 2003-06-15 01:44

Message:
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Please see
http://packages.debian.org/unstable/mail/offlineimap.html

or for the tarball:
http://ftp.debian.org/debian/pool/main/o/offlineimap/offlineimap_3.99.18.tar.gz

----------------------------------------------------------------------

Comment By: Brett Cannon (bcannon)
Date: 2003-06-15 01:19

Message:
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I went to the FTP site and all I found was some huge, 
compressed files (after changing the path to ftp://ftp.debian.org/
debian/dists/sid/main/source); no specific program called 
offlinemap.  If it is in one of those files can you just add the file 
to the bug report?

As for the reported bug, it looks like currentThread is being 
redefined, although how that is happening is beyond me.  I 
checked the 'threading' module and no where is currentThread 
redefined which could lead to None.  Has the app being run been 
changed at all since Python 2.2 was released?

----------------------------------------------------------------------

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