[Python-bugs-list] [ python-Bugs-751998 ] Object destruction is broken for slots

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Bugs item #751998, was opened at 2003-06-10 11:55
Message generated for change (Comment added) made by nnorwitz
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Category: Python Interpreter Core
Group: Python 2.2.3
>Status: Closed
Resolution: Fixed
Priority: 6
Submitted By: Kevin Jacobs (jacobs99)
Assigned to: Neal Norwitz (nnorwitz)
Summary: Object destruction is broken for slots

Initial Comment:
The following code worked without errors in
Python 2.3b1, but seems to be broken in
the current Python 2.3 CVS:

class Foo(object):
  __slots__ = ('bar','__dict__')

  def __init__(self):
    self.bar = 1
    self.baz = 2

  def __del__(self):
    print 'In __del__.'
    print '  baz =',self.baz
    print '  bar =',self.bar

foo=Foo()

Python 2.3b1: No error, output:
  In __del__.
    baz = 2
    bar = 1

However, the current CVS outputs:
  In __del__.
    baz = 2
    Exception exceptions.AttributeError: 'bar' in 
    <bound method Foo.__del__ of 
    <__main__.Foo object at 0x403ace6c>> ignored

Somehow, descriptor lookup seems to be failing
in objects that are in the process of being deleted,
since commenting out the __slots__ declaration 
makes the problem go away.

I wish I had time to look into this, but I'm currently 
swamped.  Hopefully this is the result of something
recent and this report will trigger some light-bulbs.
If not, I'll see if I can make time next week.  As it
stands, I see this as a serious bug, since it prevents
finalization from completing on these objects.


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>Comment By: Neal Norwitz (nnorwitz)
Date: 2003-06-16 19:38

Message:
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Checked in a test too:
 * Lib/test/test_descr.py 1.194, 1.113.4.34
 * Objects/typeobject.c 2.126.4.39

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Comment By: Guido van Rossum (gvanrossum)
Date: 2003-06-13 16:57

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Sorry, the checkin barfed and I didn't notice. Try again now.


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Comment By: Neal Norwitz (nnorwitz)
Date: 2003-06-13 16:41

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Hmm, did you check anything in?  The report says it's broken
in CVS, but works in 2.3b1.  It is still broken for me
without the patch.  Did you want me to test the patch and
then backport?

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Comment By: Guido van Rossum (gvanrossum)
Date: 2003-06-13 15:42

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Thanks! Fixed in CVS for 2.3. This should be backported to
2.2 too; assiged to Neal Norwitz for that purpose.


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Comment By: Kevin Jacobs (jacobs99)
Date: 2003-06-10 14:24

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er, see Bug 742911 instead.

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Comment By: Kevin Jacobs (jacobs99)
Date: 2003-06-10 14:21

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Oh, and with the patch, 'make test' completes without
any new errors, my attached test case works, as does
the minimal test case associated with Bug 751998.


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Comment By: Kevin Jacobs (jacobs99)
Date: 2003-06-10 14:19

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Okay, I had a few minutes free.  The problem is that
slots are being deallocated too early -- before tp_dealloc.
I'm attaching a patch that corrects this at the expense of
having to travel up the path of base class tp_dealloc
entries twice.  Here is the new sequence of actions:

1) Find the nearest base with a different tp_dealloc
2) Clear weakrefs
3) Call finalizer
4) Check to see if object was resurrected, if so stop
5) Clear all slots up to nearest base with a different 
    tp_dealloc
6) DECREF dict pointer, if necessary
7) proceed as usual... (call basedealloc, DECREF type, etc.)

Without my patch, step number 5  is done as part of step
1, and bad things happen.


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Comment By: Kevin Jacobs (jacobs99)
Date: 2003-06-10 13:07

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The fix checked in to solve Bug 751998 is the cause of this
potentially more serious one bug.  I have yet to figure 
out why, though I have isolated the problem to revision
2.234 of typeobject.c.  I've not looked to see if this also 
affects Python 2.2.3, though I wouldn't be too suprised if it 
does.

More soon, hopefully.  Maybe...


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