Subject: bug#32479: Fix tracking of freed emacs_values with enabled --module-assertions

From: Sergey Vinokurov <[email-masked]>
Date: Mon, 20 Aug 2018 09:27:46 +0100

Hi,

I was developing a dynamic module when I noticed that when
'--module-assertions' are enabled I got strange complaints regarding
value to be freed. Namely, Emacs was producing an error message like the
following one:

    Emacs module assertion: Global value was not found in list of 10 globals

I tracked it down to a bug in module_free_global_ref. It's traversal of
the linked list of all allocated global values was flawed and only
considered the head of the list. Please find attached my attempt at
fixing it and a test in Emacs test suite that reproduces the issue.

Regards,
Sergey
From a432465c57f14157f444540af3cea73261580dd6 Mon Sep 17 00:00:00 2001
From: Sergey Vinokurov <[email-masked]>
Date: Sun, 19 Aug 2018 21:31:01 +0100
Subject: [PATCH] Fix detection of freed emacs_values

Compare a value to be freed with all entries of the list
* src/emacs-module.c: Fix traversal of the list that contains all
  emacs_values allocated by a dynamic module
---
 src/emacs-module.c                |  8 ++++----
 test/data/emacs-module/mod-test.c | 19 +++++++++++++++++++
 test/src/emacs-module-tests.el    |  3 +++
 3 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/src/emacs-module.c b/src/emacs-module.c
index e7ba17426b..f2844c40d0 100644
--- a/src/emacs-module.c
+++ b/src/emacs-module.c
@@ -344,20 +344,20 @@ module_free_global_ref (emacs_env *env, emacs_value ref)
       Lisp_Object globals = global_env_private.values;
       Lisp_Object prev = Qnil;
       ptrdiff_t count = 0;
-      for (Lisp_Object tail = global_env_private.values; CONSP (tail);
+      for (Lisp_Object tail = globals; CONSP (tail);
            tail = XCDR (tail))
         {
-          emacs_value global = xmint_pointer (XCAR (globals));
+          emacs_value global = xmint_pointer (XCAR (tail));
           if (global == ref)
             {
               if (NILP (prev))
                 global_env_private.values = XCDR (globals);
               else
-                XSETCDR (prev, XCDR (globals));
+                XSETCDR (prev, XCDR (tail));
               return;
             }
           ++count;
-          prev = globals;
+          prev = tail;
         }
       module_abort ("Global value was not found in list of %"pD"d globals",
                     count);
diff --git a/test/data/emacs-module/mod-test.c b/test/data/emacs-module/mod-test.c
index db05e90bc4..a9b459b4cc 100644
--- a/test/data/emacs-module/mod-test.c
+++ b/test/data/emacs-module/mod-test.c
@@ -156,6 +156,24 @@ Fmod_test_globref_make (emacs_env *env, ptrdiff_t nargs, emacs_value args[],
   return env->make_global_ref (env, lisp_str);
 }
 
+/* Create a few global references from arguments and free them.  */
+static emacs_value
+Fmod_test_globref_free (emacs_env *env, ptrdiff_t nargs, emacs_value args[],
+			void *data)
+{
+  emacs_value refs[10];
+  for (int i = 0; i < 10; i++)
+    {
+      refs[i] = env->make_global_ref (env, args[i % nargs]);
+    }
+  for (int i = 0; i < 10; i++)
+    {
+      env->free_global_ref (env, refs[i]);
+    }
+  return env->intern (env, "ok");
+}
+
+
 
 /* Return a copy of the argument string where every 'a' is replaced
    with 'b'.  */
@@ -339,6 +357,7 @@ emacs_module_init (struct emacs_runtime *ert)
   DEFUN ("mod-test-non-local-exit-funcall", Fmod_test_non_local_exit_funcall,
 	 1, 1, NULL, NULL);
   DEFUN ("mod-test-globref-make", Fmod_test_globref_make, 0, 0, NULL, NULL);
+  DEFUN ("mod-test-globref-free", Fmod_test_globref_free, 4, 4, NULL, NULL);
   DEFUN ("mod-test-string-a-to-b", Fmod_test_string_a_to_b, 1, 1, NULL, NULL);
   DEFUN ("mod-test-userptr-make", Fmod_test_userptr_make, 1, 1, NULL, NULL);
   DEFUN ("mod-test-userptr-get", Fmod_test_userptr_get, 1, 1, NULL, NULL);
diff --git a/test/src/emacs-module-tests.el b/test/src/emacs-module-tests.el
index 90cd37a98a..c67190be5c 100644
--- a/test/src/emacs-module-tests.el
+++ b/test/src/emacs-module-tests.el
@@ -148,6 +148,9 @@ multiply-string
     (garbage-collect) ;; XXX: not enough to really test but it's something..
     (should (string= ref-str mod-str))))
 
+(ert-deftest mod-test-globref-free-test ()
+  (should (eq (mod-test-globref-free 1 'a "test" 'b) 'ok)))
+
 (ert-deftest mod-test-string-a-to-b-test ()
   (should (string= (mod-test-string-a-to-b "aaa") "bbb")))
 
-- 
2.18.0