From: ludo@gnu.org (Ludovic Courtès)
To: Andy Wingo <wingo@pobox.com>
Cc: 26503@debbugs.gnu.org
Subject: bug#26503: Local variables reclaimed early vs. finalizers
Date: Wed, 19 Apr 2017 11:50:22 +0200 [thread overview]
Message-ID: <87tw5krajl.fsf@gnu.org> (raw)
In-Reply-To: <87shl4esj8.fsf@igalia.com> (Andy Wingo's message of "Wed, 19 Apr 2017 10:00:11 +0200")
Andy Wingo <wingo@pobox.com> skribis:
> ludo@gnu.org (Ludovic Courtès) writes:
>
>> Consider this code:
>>
>> (use-modules (system foreign))
>>
>> (define %abort
>> (dynamic-func "abort" (dynamic-link)))
>>
>> (let ((ptr (make-pointer 123 %abort)))
>> (display "hello\n")
>> (gc))
>>
>> Guile is free to collect ‘ptr’ when ‘gc’ is called since it has become
>> unreachable at that point; that’s what 2.2.0 does, as explained in
>> ‘NEWS’.
>>
>> However, there’s a finalizer here so collecting ‘ptr’ has an observable
>> side effect. This side effect makes the semantic change visible: the
>> “expected” semantics would be that ‘ptr’ is not subject to GC while it’s
>> in scope.
>
> This would indicate that the user has erroneous expectations ;-)
>
> Note that here since (gc) is in tail position, ptr is in fact not
> protected in any way, even given this mental model, though with a single
> thread it may be that the collection actually happens later in 2.0 given
> that finalizers are run by asyncs. Also ptr is not protected during the
> "display" either, in 2.0; in 2.0 this "let" reduces to "begin" under
> peval since the ptr is not used.
Indeed (in practice ‘ptr’ would happen to be finalized later, but that’s
“out of luck”.)
>> (In 2.0 the finalizer is not called until ‘ptr’ is no longer in scope.)
>>
>> I’m not sure this counts as a bug, but it’s certainly a pitfall when
>> working with finalizers and the FFI.
>>
>> Thoughts?
>
> For me, I don't think this is a bug. Rather the contrary, as it's more
> in spirit with safe-for-space principle that a continuation should only
> keep alive those values that it uses; any other data should be available
> for the GC to reclaim.
>
> In any case, I think this manual section treats the problem adequately,
> for me at least:
>
> https://www.gnu.org/software/guile/manual/html_node/Foreign-Object-Memory-Management.html
>
> Would you like to add something there?
Hmm, I don’t think so (great section, BTW).
I need to chew a bit more on this, but the conclusion is probably that
my expectations were incorrect, indeed. :-)
Thanks,
Ludo’.
next prev parent reply other threads:[~2017-04-19 9:50 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-14 21:56 bug#26503: Local variables reclaimed early vs. finalizers Ludovic Courtès
2017-04-19 8:00 ` Andy Wingo
2017-04-19 9:50 ` Ludovic Courtès [this message]
2017-04-19 12:23 ` Andy Wingo
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