Subject: bug#27587: 25.1; in the dynamic modules api, env->free_global_ref doesn't free anything

From: Valentin Gatien-Baron <[email-masked]>
Date: Wed, 5 Jul 2017 08:43:26 -0400

The following dynamic module takes unbounded memory:

/* gcc -I . -g -ggdb -fPIC foo.c -shared -o foo.so && echo running && emacs
-Q -L . -batch -l foo */
#include <emacs-module.h>

int plugin_is_GPL_compatible;

int emacs_module_init(struct emacs_runtime *ert) {
  emacs_env *env = ert->get_environment(ert);
  while (1) {
    int i;
    for (i = 0; i < 10000; i++) {
      emacs_value v = env->make_string(env, "asdads", 3);
      env->free_global_ref(env, env->make_global_ref(env, v));
    }
    env->funcall(env, env->intern(env, "garbage-collect"), 0, NULL);
  }
}

This is because env->make_global_ref/env->free_global_ref leak memory.
env->free_global_ref fails to remove values from the hash table of
refcounts. The following patch makes the program above run in constant
space.

--- src/emacs-module.c 2017-06-30 16:00:36.776301646 -0400
+++ src/emacs-module.c 2017-06-30 16:05:01.660120043 -0400
@@ -308,7 +308,7 @@
           set_hash_value_slot (h, i, value);
         }
       else
- hash_remove_from_table (h, value);
+ hash_remove_from_table (h, obj);
     }
 }