Debugging a difficult refcount issue.

From	buck <workitharder@gmail.com>
Newsgroups	comp.lang.python
Subject	Debugging a difficult refcount issue.
Date	2011-12-17 20:17 -0800
Organization	http://groups.google.com
Message-ID	<18365650.792.1324181855929.JavaMail.geo-discussion-forums@prht13> (permalink)

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I'm getting a fatal python error "Fatal Python error: GC object already tracked"[1].

Using gdb, I've pinpointed the place where the error is detected. It is an empty dictionary which is marked as in-use. This is somewhat helpful since I can reliably find the memory address of the dict, but it does not help me pinpoint the issue. I was able to find the piece of code that allocates the problematic dict via a malloc/LD_PRELOAD interposer, but that code was pure python. I don't think it was the cause.

I believe that the dict was deallocated, cached, and re-allocated via PyDict_New to a C routine with bad refcount logic, then the above error manifests when the dict is again deallocated, cached, and re-allocated.

I tried to pinpoint this intermediate allocation with a similar PyDict_New/LD_PRELOAD interposer, but that isn't working for me[2].

How should I go about debugging this further? I've been completely stuck on this for two days now :(

[1] http://hg.python.org/cpython/file/99af4b44e7e4/Include/objimpl.h#l267
[2] http://stackoverflow.com/questions/8549671/cant-intercept-pydict-new-with-ld-preload

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Debugging a difficult refcount issue. buck <workitharder@gmail.com> - 2011-12-17 20:17 -0800
  Re: Debugging a difficult refcount issue. Paul Rubin <no.email@nospam.invalid> - 2011-12-17 23:55 -0800
    Re: Debugging a difficult refcount issue. buck <workitharder@gmail.com> - 2011-12-18 08:06 -0800
  Re: Debugging a difficult refcount issue. Jack Diederich <jackdied@gmail.com> - 2011-12-18 12:52 -0500
    Re: Debugging a difficult refcount issue. buck <workitharder@gmail.com> - 2011-12-18 18:01 -0800
      Re: Debugging a difficult refcount issue. buck <workitharder@gmail.com> - 2011-12-19 00:09 -0800
      Re: Debugging a difficult refcount issue. buck <workitharder@gmail.com> - 2011-12-19 00:09 -0800
    Re: Debugging a difficult refcount issue. buck <workitharder@gmail.com> - 2011-12-18 18:01 -0800

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