Guido van Rossum <guido@digicool.com> writes:
> > > 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.
>
> Where?
import.c:PyImport_Cleanup
moduleobject.c:_PyModule_Clear
Hrm, I was sure there were more than that, but there don't seem to be.
Sorry for the alarmism.
> > 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.
>
> I'd fix the dict implementation, except that that's tricky.
I'd got that far...
> Checking for a dup key in PyDict_SetItem() before calling dictresize()
> slows things down. Checking in insertdict() is wrong because
> dictresize() uses that!
Maybe you could do the check for resize *after* the call to
insertdict? I think that would work, but I wouldn't like to go
messing with such a performance critical bit of code without some
careful thinking.
Cheers,
M.
--
You sound surprised. We're talking about a government department
here - they have procedures, not intelligence.
-- Ben Hutchings, cam.misc
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