[Python-Dev] Nested scopes core dump

[Michael Hudson]

Jeremy Hylton <jeremy@alum.mit.edu> writes:

> >>>>> "MH" == Michael Hudson <mwh21@cam.ac.uk> writes:
> 
>   MH> Jeremy Hylton <jeremy@alum.mit.edu> writes:
>   >> >>>>> "MWH" == Michael Hudson <mwh21@cam.ac.uk> writes:
>   >>
>   MWH> [*] I thought that if you used the same keys when you were
>   MWH> iterating over a dict you were safe.  It seems not, at least as
>   MWH> far as I could tell with mounds of debugging printf's.
>   >>
>   >> I did, too.  Anyone know what the problems is?
> 
>   MH> The dict's resizing, it turns out.
> 
> So a hack to make the iteration safe would be to assign and element
> and then delete it?

Yes.  This would be gross beyond belief though.  Particularly as the
normal case is for freevars to be empty.

>   MH> I note that in PyDict_SetItem, the check to see if the dict
>   MH> needs resizing occurs *before* it is known whether the key is
>   MH> already in the dict.  But if this is the problem, how come we
>   MH> haven't been bitten by this before?
> 
> It's probably unusual for a dictionary to be in this state when the
> compiler decides to update the values.

What I meant was that there are bits and pieces of code in the Python
core that blithely pass keys gotten from PyDict_Next into
PyDict_SetItem.  From what I've just learnt, I'd expect this to
occasionally cause glitches of extreme confusing-ness.  Though on
investigation, I don't think any of these bits of code are sensitive
to getting keys out multiple times (which is what happens in this case
- though you must be able to miss keys too).  Might cause the odd leak
here and there.

Cheers,
M.

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