Re: Native module callback when Lisp thread exits

[Eli Zaretskii <eliz@gnu.org>]

> From: Spencer Baugh <sbaugh@janestreet.com>
> Date: Fri, 17 May 2024 13:09:35 -0400
> Cc: mshinwell@janestreet.com
> 
> I have a native module written in OCaml which provides some useful
> functions.  Sometimes, I would like to call these native module
> functions from Lisp threads other than the main Lisp thread.
> 
> For a native module written in C, this works fine.  However, the OCaml
> runtime requires that if it's called from other threads, the thread must
> first call caml_c_thread_register; and when the thread exits, it should
> call caml_c_thread_unregister. See
> https://ocaml.org/manual/5.1/intfc.html#ss:c-thread-register
> 
> My native module can call caml_c_thread_register when the native module
> is first called in a non-main thread; then calling my native module
> functions works fine in Lisp threads.
> 
> However, there's no way for my native module to call
> caml_c_thread_unregister when the thread exits.  That causes memory
> leaks and means that OCaml GC performance degrades over time, since the
> GC scans all known threads.
> 
> Could we add a way for a native module to call some functions when a
> Lisp thread exits?
> 
> Currently, native modules expose a emacs_module_init function which is
> called when the module is loaded.  Maybe a native module could
> optionally also expose an emacs_module_thread_init function which is
> called when a Lisp thread is created (or maybe when the module is first
> used on a Lisp thread).  And likewise, an emacs_module_thread_uninit
> function which is called when a Lisp thread exits.

I don't understand why you need something module-specific.  Shouldn't
a thread-exit-hook be enough?  We could easily add that if that helps.