[ python-Bugs-1055820 ] weakref callback vs gc vs threads

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Bugs item #1055820, was opened at 2004-10-27 21:58
Message generated for change (Comment added) made by tim_one
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Category: Python Interpreter Core
Group: None
Status: Open
Resolution: None
Priority: 9
Submitted By: Tim Peters (tim_one)
Assigned to: Tim Peters (tim_one)
Summary: weakref callback vs gc vs threads

Initial Comment:
Oh oh.  It's that time of year again.  I'm sure the 
attached (temp2a.py) can be simplified, perhaps down 
to two objects and one thread.  As is, I *think* it 
demonstrates that invoking a weakref callback can do 
fatal damage, not necessarily because of what the 
callback does, but because simply calling it while gc is 
running can allow other threads to run during gc too, 
and resurrect a piece of cyclic trash T that's already 
been tp_clear()'ed, via invoking a still-living weakref to 
T.

If so, this isn't new in 2.4.  It's a real problem since 
temp2a.py is what's left of ZODB 3.4a1 <wink>.

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>Comment By: Tim Peters (tim_one)
Date: 2004-10-28 04:06

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temp2c.py may be as bad as it gets.  It shows that the 
problem can occur on a gc collection that doesn't see *any* 
object that has a weakref with a callback.

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Comment By: Tim Peters (tim_one)
Date: 2004-10-27 22:37

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Noting that temp2b.py fails in current CVS, 2.3.4, and 2.2.3.  
That's all the Pythons I have handy right now.

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Comment By: Tim Peters (tim_one)
Date: 2004-10-27 22:28

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Yup, temp2b.py probably shows the same problem, with 2 
objects and one thread.  This one is definitely more strained, 
though, since the weakref callback does the damage 
directly.  In temp2a.py, nothing is trying to fool anything, and 
the only damage done by the wr callbacks there is simply in 
releasing the GIL (thus allowing other threads to do perfectly 
ordinary things with weakrefs).

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