GNU bug report logs - #22737
25.1; Finalizer should be optional in dynamic modules

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From: Daniel Colascione <dancol <at> dancol.org>
To: Jess Balint <jbalint <at> gmail.com>, Eli Zaretskii <eliz <at> gnu.org>
Cc: John Wiegley <johnw <at> gnu.org>, 22737 <at> debbugs.gnu.org
Subject: bug#22737: 25.1; Finalizer should be optional in dynamic modules
Date: Fri, 26 Feb 2016 13:55:00 -0800

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On 02/26/2016 01:51 PM, Jess Balint wrote:
> On Fri, Feb 26, 2016 at 3:33 PM, Eli Zaretskii <eliz <at> gnu.org
> <mailto:eliz <at> gnu.org>> wrote:
> 
>     > Date: Fri, 26 Feb 2016 12:53:20 -0600
>     > From: Jess Balint <jbalint <at> gmail.com <mailto:jbalint <at> gmail.com>>
>     > Cc: 22737 <at> debbugs.gnu.org <mailto:22737 <at> debbugs.gnu.org>
>     >
>     >  What will happen if such objects are exposed to Lisp, copied or
>     >  assigned to other Lisp variables, etc.? Won't this cause all kinds of
>     >  trouble, like modifying one such object will magically modify several
>     >  others, which share its storage?
>     >
>     > This is how C code works. If you return a pointer from a function, you may have to free that pointer yourself or
>     > you may not. You may get the same pointer back from multiple calls to the same function. If you use the
>     > pointer after it's been freed, it's your problem. You need to agree with the owner of the pointer how the
>     > memory is to be managed. With pointers, modifications to the underlying data are visible by all who have a
>     > pointer to the data. I wouldn't call this "magically modifying others".
> 
>     In C, yes.  But we are talking about Lisp objects here.
> 
>     Am I the only one who is uneasy with supporting such Lisp objects?  If
>     so, I will shut up and install the changes.  Daniel, John, what's your
>     opinion on this?
> 
>     Thanks.
> 
> 
> All I'm asking for is to allow the code to accept a NULL finalizer. This
> means no finalizer will be called. It's a clear and simple semantic.
> Upside is that I (and others who do not want Emacs to free their
> pointers) will not have to create a no-op function unnecessarily to
> supply a finalizer to Emacs.

A no-op function is trivial though; creating it forces you to think
about whether you actually need to free the resulting memory. I think
it's more important to discourage memory leaks and simplify the
semantics of the finalizer parameter than to make this rare (I think)
use case slightly easier for module implementors.

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