Subject: Re: bug#26503: Local variables reclaimed early vs. finalizers

From: Andy Wingo <[email-masked]>
Date: Wed, 19 Apr 2017 10:00:11 +0200

[email-masked] (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.

> (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?

Andy