Debugging a difficult refcount issue.

[buck]

Thanks Jack. I think printf is what it will come down to. I plan to put a little code into PyDict_New to print the id and the line at which it was allocated. Hopefully this will show me all the possible suspects and I can figure it out from there.

I hope figuring out the file and line-number from within that code isn't too hard.


On Sunday, December 18, 2011 9:52:46 AM UTC-8, Jack Diederich wrote:
> I don't have any great advice, that kind of issue is hard to pin down.
>  That said, do try using a python compile with --with-debug enabled,
> with that you can turn your unit tests on and off to pinpoint where
> the refcounts are getting messed up.  It also causes python to use
> plain malloc()s so valgrind becomes useful.  Worst case add assertions
> and printf()s in the places you think are most janky.
> 
> -Jack
> 
> On Sat, Dec 17, 2011 at 11:17 PM, buck <workit... at gmail.com> wrote:
> > 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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