bug#42482: 27.0.91; emacs modules memory leak

["Milan Stanojević" <mstanojevic@janestreet.com>]

[-- Attachment #1: Type: text/plain, Size: 707 bytes --]

env-make_global_ref adds a reference to the underlying Lisp_Object
and allocates emacs_value from the global storage. env->free_global_ref
on the other hand will only remove a reference to the underlying
Lisp_Object and not free the emacs_value.

Here is a simple recipe to reproduce the problem (I only tested this
on linux). I'm attaching the necessary files.

$ gcc -shared -fpic -std=c99 -I <dir-with-emacs-module.h>
create_global_refs.c -o create_global_refs.so
$ emacs --no-splash -q -l create_global_refs.so -l create_global_refs_test.el

If you look at the memory usage of emacs (for example in htop) you'll
see that with emacs-26 it is constant but with emacs-27 the resident
memory quickly grows.

[-- Attachment #2: create_global_refs.c --]
[-- Type: text/x-csrc, Size: 1053 bytes --]

#include <string.h>
#include <emacs-module.h>

int plugin_is_GPL_compatible;

static emacs_value create_loop (emacs_env *env, ptrdiff_t _nargs, emacs_value _args[], void* _data)
{
  for (int i = 0; i < 1000000; i++)
    env->free_global_ref
      (env,
       env->make_global_ref
       (env,
        env->make_integer(env, 0)));

  char msg[] = "1M global references created and freed";
  emacs_value message = { env->make_string(env, msg, strlen(msg)) };
  env->funcall(env, env->intern(env, "message"), 1, &message);

  return env->intern(env, "nil");
}

int emacs_module_init (struct emacs_runtime *ert) {
  emacs_env* env = ert->get_environment(ert);

  emacs_value loopf = env->make_function(env, 0, 0, create_loop, "create and free 1M refs", NULL);

  emacs_value args[] = { env->intern(env, "create-refs-loop"), loopf };
  env->funcall(env, env->intern(env, "fset"), 2, args);

  emacs_value simple_loop_feature = env->intern(env, "create-global-refs");
  env->funcall(env, env->intern(env, "provide"), 1, &simple_loop_feature);

  return 0;
}

[-- Attachment #3: create_global_refs_test.el --]
[-- Type: text/x-emacs-lisp, Size: 211 bytes --]

(require 'create-global-refs)

(defun run-one-iteration-and-gc ()
  (create-refs-loop)
  (garbage-collect))

(switch-to-buffer (get-buffer-create "*Messages*"))

(run-with-timer 0.1 1 'run-one-iteration-and-gc)