Native module callback when Lisp thread exits

[Spencer Baugh <sbaugh@janestreet.com>]

Hi,

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.