Re: ‘set-cdr!’ and weak-cdr pairs

[Andy Wingo <wingo@pobox.com>]

Hi :)

On Sun 27 Mar 2011 15:29, ludo@gnu.org (Ludovic Courtès) writes:

> Andy Wingo <wingo@pobox.com> writes:
>
>> On Sun 13 Mar 2011 16:25, ludo@gnu.org (Ludovic Courtès) writes:
>>
>>> The problem is that ‘hash-create-handle!’ above created a weak-cdr
>>> pair—i.e., a pair whose cdr is /not/ scanned for pointers—but ‘set-cdr!’
>>> did not register a disappearing link from O to K+V.  Consequently, O
>>> eventually gets collected, but K+V remains; the storage of O then gets
>>> reused, and the cdr of K+V ends up containing either an unrelated or >an
>>> invalid Scheme object.
>>
>> Given that we don't expose weak-pair constructors or accessors to
>> Scheme, we should not expose weak pairs to Scheme.  What do you think
>> about making it an error to hash-create-handle! on a weak table?  That
>> way you never expose a weak pair to Scheme.  It does appear possible to
>> discriminate in C between calls to create-handle! that occur due to ref
>> / set! and those that are called explicitly.
>
> Yes, sounds good.  Would you like to work on it?  :-)

I have done this now.  There were a few places within Guile that were
using the get-handle / create-handle! API with weak hash tables, which I
fixed.  It's quite possible that user code also does this, but between
random failures to `(car x)' and preventative errors, I chose the
latter.  We can back it down to a warning if that's the right thing to
do, though.

Andy
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