Fix incorrect handling of module runtime and environment pointers.

We used to store module runtime and environment pointers in the static
lists Vmodule_runtimes and Vmodule_environments.  However, this is
incorrect because these objects have to be kept per-thread.  With this
naive approach, interleaving module function calls in separate threads
leads to environments being removed in the wrong order, which in turn
can cause local module values to be incorrectly garbage-collected.
The fix isn't completely trivial: specbinding the lists wouldn't work
either, because then the garbage collector wouldn't find the
environments in other threads than the current ones, again leading to
objects being garbage-collected incorrectly.  While introducing custom
pseudovector types would fix this, it's simpler to put the runtime and
environment pointers into the specbinding list as new specbinding
kinds.  This works since we need to unwind them anyway, and we only
ever treat the lists as a stack.  The thread switching machinery
ensures that the specbinding lists are thread-local, and that all
elements of the specbinding lists in all threads are marked during
garbage collection.

Module assertions now have to walk the specbinding list for the
current thread, which is more correct since they now only find
environments for the current thread.  As a result, we can now remove
the faulty Vmodule_runtimes and Vmodule_environments variables
entirely.

Also add a unit test that exemplifies the problem.  It interleaves two
module calls in two threads so that the first call ends while the
second one is still active.  Without this change, this test triggers
an assertion failure.

* src/lisp.h (enum specbind_tag): Add new tags for module runtimes and
environments.

* src/eval.c (record_unwind_protect_module): New function to record a
module object in the specpdl list.
(do_one_unbind): Unwind module objects.
(backtrace_eval_unrewind, default_toplevel_binding, lexbound_p)
(Fbacktrace__locals): Deal with new specbinding types.
(mark_specpdl): Mark module environments as needed.

* src/alloc.c (garbage_collect): Remove call to 'mark-modules'.
Garbage collection of module values is now handled as part of marking
the specpdl of each thread.

* src/emacs-module.c (Fmodule_load, funcall_module): Use specpdl to
record module runtimes and environments.
(module_assert_runtime, module_assert_env, value_to_lisp): Walk
through specpdl list instead of list variables.
(mark_module_environment): Rename from 'mark_modules'.  Don't attempt
to walk though current thread's environments only, since that would
miss other threads.
(initialize_environment, finalize_environment): Don't change
Vmodule_environments variable; environments are now in the specpdl
list.
(finalize_environment_unwind, finalize_runtime_unwind): Make 'extern'
since do_one_unbind now calls them.
(finalize_runtime_unwind): Don't change Vmodule_runtimes variable;
runtimes are now in the specpdl list.
(syms_of_module): Remove Vmodule_runtimes and Vmodule_environments.

* test/data/emacs-module/mod-test.c (Fmod_test_funcall): New test
function.
(emacs_module_init): Bind it.

* test/src/emacs-module-tests.el (emacs-module-tests--variable): New
helper type to guard access to state in a thread-safe way.
(emacs-module-tests--wait-for-variable)
(emacs-module-tests--change-variable): New helper functions.
(emacs-module-tests/interleaved-threads): New unit test.

6 files changed, 151 insertions, 67 deletions

diff --git a/src/alloc.c b/src/alloc.c
index 2b3643e35bd..504ef179ecc 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -6061,10 +6061,6 @@ garbage_collect (void)
   mark_fringe_data ();
 #endif
 
-#ifdef HAVE_MODULES
-  mark_modules ();
-#endif
-
   /* Everything is now marked, except for the data in font caches,
      undo lists, and finalizers.  The first two are compacted by
      removing an items which aren't reachable otherwise.  */
diff --git a/src/emacs-module.c b/src/emacs-module.c
index 23b8e8620c9..5f97815ec15 100644
--- a/src/emacs-module.c
+++ b/src/emacs-module.c
@@ -200,8 +200,6 @@ static AVOID module_abort (const char *, ...) ATTRIBUTE_FORMAT_PRINTF (1, 2);
 static emacs_env *initialize_environment (emacs_env *,
 					  struct emacs_env_private *);
 static void finalize_environment (emacs_env *);
-static void finalize_environment_unwind (void *);
-static void finalize_runtime_unwind (void *);
 static void module_handle_nonlocal_exit (emacs_env *, enum nonlocal_exit,
                                          Lisp_Object);
 static void module_non_local_exit_signal_1 (emacs_env *,
@@ -1089,10 +1087,6 @@ module_signal_or_throw (struct emacs_env_private *env)
     }
 }
 
-/* Live runtime and environment objects, for assertions.  */
-static Lisp_Object Vmodule_runtimes;
-static Lisp_Object Vmodule_environments;
-
 DEFUN ("module-load", Fmodule_load, Smodule_load, 1, 1, 0,
        doc: /* Load module FILE.  */)
   (Lisp_Object file)
@@ -1137,9 +1131,9 @@ DEFUN ("module-load", Fmodule_load, Smodule_load, 1, 1, 0,
   rt->private_members = &rt_priv;
   rt->get_environment = module_get_environment;
 
-  Vmodule_runtimes = Fcons (make_mint_ptr (rt), Vmodule_runtimes);
   ptrdiff_t count = SPECPDL_INDEX ();
-  record_unwind_protect_ptr (finalize_runtime_unwind, rt);
+  record_unwind_protect_module (SPECPDL_MODULE_RUNTIME, rt);
+  record_unwind_protect_module (SPECPDL_MODULE_ENVIRONMENT, rt_priv.env);
 
   int r = module_init (rt);
 
@@ -1167,7 +1161,7 @@ funcall_module (Lisp_Object function, ptrdiff_t nargs, Lisp_Object *arglist)
   struct emacs_env_private priv;
   emacs_env *env = initialize_environment (&pub, &priv);
   ptrdiff_t count = SPECPDL_INDEX ();
-  record_unwind_protect_ptr (finalize_environment_unwind, env);
+  record_unwind_protect_module (SPECPDL_MODULE_ENVIRONMENT, env);
 
   USE_SAFE_ALLOCA;
   emacs_value *args = nargs > 0 ? SAFE_ALLOCA (nargs * sizeof *args) : NULL;
@@ -1243,12 +1237,13 @@ module_assert_runtime (struct emacs_runtime *runtime)
   if (! module_assertions)
     return;
   ptrdiff_t count = 0;
-  for (Lisp_Object tail = Vmodule_runtimes; CONSP (tail); tail = XCDR (tail))
-    {
-      if (xmint_pointer (XCAR (tail)) == runtime)
-        return;
-      ++count;
-    }
+  for (const union specbinding *pdl = specpdl; pdl != specpdl_ptr; ++pdl)
+    if (pdl->kind == SPECPDL_MODULE_RUNTIME)
+      {
+        if (pdl->unwind_ptr.arg == runtime)
+          return;
+        ++count;
+      }
   module_abort ("Runtime pointer not found in list of %"pD"d runtimes",
 		count);
 }
@@ -1259,13 +1254,13 @@ module_assert_env (emacs_env *env)
   if (! module_assertions)
     return;
   ptrdiff_t count = 0;
-  for (Lisp_Object tail = Vmodule_environments; CONSP (tail);
-       tail = XCDR (tail))
-    {
-      if (xmint_pointer (XCAR (tail)) == env)
-        return;
-      ++count;
-    }
+  for (const union specbinding *pdl = specpdl; pdl != specpdl_ptr; ++pdl)
+    if (pdl->kind == SPECPDL_MODULE_ENVIRONMENT)
+      {
+        if (pdl->unwind_ptr.arg == env)
+          return;
+        ++count;
+      }
   module_abort ("Environment pointer not found in list of %"pD"d environments",
                 count);
 }
@@ -1323,22 +1318,22 @@ value_to_lisp (emacs_value v)
          environments.  */
       ptrdiff_t num_environments = 0;
       ptrdiff_t num_values = 0;
-      for (Lisp_Object environments = Vmodule_environments;
-           CONSP (environments); environments = XCDR (environments))
-        {
-          emacs_env *env = xmint_pointer (XCAR (environments));
-          struct emacs_env_private *priv = env->private_members;
-          /* The value might be one of the nonlocal exit values.  Note
-             that we don't check whether a nonlocal exit is currently
-             pending, because the module might have cleared the flag
-             in the meantime.  */
-          if (&priv->non_local_exit_symbol == v
-              || &priv->non_local_exit_data == v)
-            goto ok;
-          if (value_storage_contains_p (&priv->storage, v, &num_values))
-            goto ok;
-          ++num_environments;
-        }
+      for (const union specbinding *pdl = specpdl; pdl != specpdl_ptr; ++pdl)
+        if (pdl->kind == SPECPDL_MODULE_ENVIRONMENT)
+          {
+            const emacs_env *env = pdl->unwind_ptr.arg;
+            struct emacs_env_private *priv = env->private_members;
+            /* The value might be one of the nonlocal exit values.  Note
+               that we don't check whether a nonlocal exit is currently
+               pending, because the module might have cleared the flag
+               in the meantime.  */
+            if (&priv->non_local_exit_symbol == v
+                || &priv->non_local_exit_data == v)
+              goto ok;
+            if (value_storage_contains_p (&priv->storage, v, &num_values))
+              goto ok;
+            ++num_environments;
+          }
       /* Also check global values.  */
       if (module_global_reference_p (v, &num_values))
         goto ok;
@@ -1421,18 +1416,14 @@ allocate_emacs_value (emacs_env *env, Lisp_Object obj)
 /* Mark all objects allocated from local environments so that they
    don't get garbage-collected.  */
 void
-mark_modules (void)
+mark_module_environment (void *ptr)
 {
-  for (Lisp_Object tem = Vmodule_environments; CONSP (tem); tem = XCDR (tem))
-    {
-      emacs_env *env = xmint_pointer (XCAR (tem));
-      struct emacs_env_private *priv = env->private_members;
-      for (struct emacs_value_frame *frame = &priv->storage.initial;
-	   frame != NULL;
-	   frame = frame->next)
-        for (int i = 0; i < frame->offset; ++i)
-          mark_object (frame->objects[i].v);
-    }
+  emacs_env *env = ptr;
+  struct emacs_env_private *priv = env->private_members;
+  for (struct emacs_value_frame *frame = &priv->storage.initial; frame != NULL;
+       frame = frame->next)
+    for (int i = 0; i < frame->offset; ++i)
+      mark_object (frame->objects[i].v);
 }
 
 
@@ -1495,7 +1486,6 @@ initialize_environment (emacs_env *env, struct emacs_env_private *priv)
   env->set_function_finalizer = module_set_function_finalizer;
   env->open_channel = module_open_channel;
   env->make_interactive = module_make_interactive;
-  Vmodule_environments = Fcons (make_mint_ptr (env), Vmodule_environments);
   return env;
 }
 
@@ -1505,22 +1495,18 @@ static void
 finalize_environment (emacs_env *env)
 {
   finalize_storage (&env->private_members->storage);
-  eassert (xmint_pointer (XCAR (Vmodule_environments)) == env);
-  Vmodule_environments = XCDR (Vmodule_environments);
 }
 
-static void
+void
 finalize_environment_unwind (void *env)
 {
   finalize_environment (env);
 }
 
-static void
+void
 finalize_runtime_unwind (void *raw_ert)
 {
   struct emacs_runtime *ert = raw_ert;
-  eassert (xmint_pointer (XCAR (Vmodule_runtimes)) == ert);
-  Vmodule_runtimes = XCDR (Vmodule_runtimes);
   finalize_environment (ert->private_members->env);
 }
 
@@ -1610,12 +1596,6 @@ syms_of_module (void)
 		       DEFAULT_REHASH_SIZE, DEFAULT_REHASH_THRESHOLD,
 		       Qnil, false);
 
-  staticpro (&Vmodule_runtimes);
-  Vmodule_runtimes = Qnil;
-
-  staticpro (&Vmodule_environments);
-  Vmodule_environments = Qnil;
-
   DEFSYM (Qmodule_load_failed, "module-load-failed");
   Fput (Qmodule_load_failed, Qerror_conditions,
 	pure_list (Qmodule_load_failed, Qerror));
diff --git a/src/eval.c b/src/eval.c
index 3f9dcf6be6d..d9a424b57a9 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -681,6 +681,10 @@ default_toplevel_binding (Lisp_Object symbol)
 	case SPECPDL_UNWIND_EXCURSION:
 	case SPECPDL_UNWIND_VOID:
 	case SPECPDL_BACKTRACE:
+#ifdef HAVE_MODULES
+        case SPECPDL_MODULE_RUNTIME:
+        case SPECPDL_MODULE_ENVIRONMENT:
+#endif
 	case SPECPDL_LET_LOCAL:
 	  break;
 
@@ -720,6 +724,10 @@ lexbound_p (Lisp_Object symbol)
 	case SPECPDL_UNWIND_EXCURSION:
 	case SPECPDL_UNWIND_VOID:
 	case SPECPDL_BACKTRACE:
+#ifdef HAVE_MODULES
+        case SPECPDL_MODULE_RUNTIME:
+        case SPECPDL_MODULE_ENVIRONMENT:
+#endif
 	case SPECPDL_LET_LOCAL:
 	  break;
 
@@ -3480,6 +3488,15 @@ record_unwind_protect_void (void (*function) (void))
 }
 
 void
+record_unwind_protect_module (enum specbind_tag kind, void *ptr)
+{
+  specpdl_ptr->kind = kind;
+  specpdl_ptr->unwind_ptr.func = NULL;
+  specpdl_ptr->unwind_ptr.arg = ptr;
+  grow_specpdl ();
+}
+
+void
 rebind_for_thread_switch (void)
 {
   union specbinding *bind;
@@ -3529,6 +3546,14 @@ do_one_unbind (union specbinding *this_binding, bool unwinding,
       break;
     case SPECPDL_BACKTRACE:
       break;
+#ifdef HAVE_MODULES
+    case SPECPDL_MODULE_RUNTIME:
+      finalize_runtime_unwind (this_binding->unwind_ptr.arg);
+      break;
+    case SPECPDL_MODULE_ENVIRONMENT:
+      finalize_environment_unwind (this_binding->unwind_ptr.arg);
+      break;
+#endif
     case SPECPDL_LET:
       { /* If variable has a trivial value (no forwarding), and isn't
 	   trapped, we can just set it.  */
@@ -3859,6 +3884,10 @@ backtrace_eval_unrewind (int distance)
 	case SPECPDL_UNWIND_INTMAX:
 	case SPECPDL_UNWIND_VOID:
 	case SPECPDL_BACKTRACE:
+#ifdef HAVE_MODULES
+        case SPECPDL_MODULE_RUNTIME:
+        case SPECPDL_MODULE_ENVIRONMENT:
+#endif
 	  break;
 	case SPECPDL_LET:
 	  { /* If variable has a trivial value (no forwarding), we can
@@ -3994,6 +4023,10 @@ NFRAMES and BASE specify the activation frame to use, as in `backtrace-frame'.
 	  case SPECPDL_UNWIND_EXCURSION:
 	  case SPECPDL_UNWIND_VOID:
 	  case SPECPDL_BACKTRACE:
+#ifdef HAVE_MODULES
+          case SPECPDL_MODULE_RUNTIME:
+          case SPECPDL_MODULE_ENVIRONMENT:
+#endif
 	    break;
 
 	  default:
@@ -4040,6 +4073,14 @@ mark_specpdl (union specbinding *first, union specbinding *ptr)
 	  }
 	  break;
 
+#ifdef HAVE_MODULES
+        case SPECPDL_MODULE_RUNTIME:
+          break;
+        case SPECPDL_MODULE_ENVIRONMENT:
+          mark_module_environment (pdl->unwind_ptr.arg);
+          break;
+#endif
+
 	case SPECPDL_LET_DEFAULT:
 	case SPECPDL_LET_LOCAL:
 	  mark_object (specpdl_where (pdl));
diff --git a/src/lisp.h b/src/lisp.h
index 31614b9125b..a95913c4efe 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -3127,6 +3127,10 @@ enum specbind_tag {
   SPECPDL_UNWIND_EXCURSION,	/* Likewise, on an excursion.  */
   SPECPDL_UNWIND_VOID,		/* Likewise, with no arg.  */
   SPECPDL_BACKTRACE,		/* An element of the backtrace.  */
+#ifdef HAVE_MODULES
+  SPECPDL_MODULE_RUNTIME,       /* A live module runtime.  */
+  SPECPDL_MODULE_ENVIRONMENT,   /* A live module environment.  */
+#endif
   SPECPDL_LET,			/* A plain and simple dynamic let-binding.  */
   /* Tags greater than SPECPDL_LET must be "subkinds" of LET.  */
   SPECPDL_LET_LOCAL,		/* A buffer-local let-binding.  */
@@ -4146,6 +4150,7 @@ extern void record_unwind_protect_intmax (void (*) (intmax_t), intmax_t);
 extern void record_unwind_protect_void (void (*) (void));
 extern void record_unwind_protect_excursion (void);
 extern void record_unwind_protect_nothing (void);
+extern void record_unwind_protect_module (enum specbind_tag, void *);
 extern void clear_unwind_protect (ptrdiff_t);
 extern void set_unwind_protect (ptrdiff_t, void (*) (Lisp_Object), Lisp_Object);
 extern void set_unwind_protect_ptr (ptrdiff_t, void (*) (void *), void *);
@@ -4216,7 +4221,9 @@ extern module_funcptr module_function_address
   (struct Lisp_Module_Function const *);
 extern void *module_function_data (const struct Lisp_Module_Function *);
 extern void module_finalize_function (const struct Lisp_Module_Function *);
-extern void mark_modules (void);
+extern void mark_module_environment (void *);
+extern void finalize_runtime_unwind (void *);
+extern void finalize_environment_unwind (void *);
 extern void init_module_assertions (bool);
 extern void syms_of_module (void);
 #endif
diff --git a/test/src/emacs-module-resources/mod-test.c b/test/src/emacs-module-resources/mod-test.c
index 258a679b207..419621256ae 100644
--- a/test/src/emacs-module-resources/mod-test.c
+++ b/test/src/emacs-module-resources/mod-test.c
@@ -691,6 +691,14 @@ Fmod_test_identity (emacs_env *env, ptrdiff_t nargs, emacs_value *args,
   return args[0];
 }
 
+static emacs_value
+Fmod_test_funcall (emacs_env *env, ptrdiff_t nargs, emacs_value *args,
+                   void *data)
+{
+  assert (0 < nargs);
+  return env->funcall (env, args[0], nargs - 1, args + 1);
+}
+
 /* Lisp utilities for easier readability (simple wrappers).  */
 
 /* Provide FEATURE to Emacs.  */
@@ -780,6 +788,8 @@ emacs_module_init (struct emacs_runtime *ert)
   DEFUN ("mod-test-function-finalizer-calls",
          Fmod_test_function_finalizer_calls, 0, 0, NULL, NULL);
   DEFUN ("mod-test-async-pipe", Fmod_test_async_pipe, 1, 1, NULL, NULL);
+  DEFUN ("mod-test-funcall", Fmod_test_funcall, 1, emacs_variadic_function,
+         NULL, NULL);
 
 #undef DEFUN
 
diff --git a/test/src/emacs-module-tests.el b/test/src/emacs-module-tests.el
index fb4ed4a6842..99d4cafb4af 100644
--- a/test/src/emacs-module-tests.el
+++ b/test/src/emacs-module-tests.el
@@ -506,4 +506,54 @@ See Bug#36226."
     (should (not (multibyte-string-p (mod-test-return-unibyte))))
     (should (equal result "foo\x00zot"))))
 
+(cl-defstruct (emacs-module-tests--variable
+               (:constructor nil)
+               (:constructor emacs-module-tests--make-variable
+                             (name
+                              &aux
+                              (mutex (make-mutex name))
+                              (condvar (make-condition-variable mutex name))))
+               (:copier nil))
+  "A variable that's protected by a mutex."
+  value
+  (mutex nil :read-only t :type mutex)
+  (condvar nil :read-only t :type condition-variable))
+
+(defun emacs-module-tests--wait-for-variable (variable desired)
+  (with-mutex (emacs-module-tests--variable-mutex variable)
+    (while (not (eq (emacs-module-tests--variable-value variable) desired))
+      (condition-wait (emacs-module-tests--variable-condvar variable)))))
+
+(defun emacs-module-tests--change-variable (variable new)
+  (with-mutex (emacs-module-tests--variable-mutex variable)
+    (setf (emacs-module-tests--variable-value variable) new)
+    (condition-notify (emacs-module-tests--variable-condvar variable) :all)))
+
+(ert-deftest emacs-module-tests/interleaved-threads ()
+  (let* ((state-1 (emacs-module-tests--make-variable "1"))
+         (state-2 (emacs-module-tests--make-variable "2"))
+         (thread-1
+          (make-thread
+           (lambda ()
+             (emacs-module-tests--change-variable state-1 'before-module)
+             (mod-test-funcall
+              (lambda ()
+                (emacs-module-tests--change-variable state-1 'in-module)
+                (emacs-module-tests--wait-for-variable state-2 'in-module)))
+             (emacs-module-tests--change-variable state-1 'after-module))
+           "thread 1"))
+         (thread-2
+          (make-thread
+           (lambda ()
+             (emacs-module-tests--change-variable state-2 'before-module)
+             (emacs-module-tests--wait-for-variable state-1 'in-module)
+             (mod-test-funcall
+              (lambda ()
+                (emacs-module-tests--change-variable state-2 'in-module)
+                (emacs-module-tests--wait-for-variable state-1 'after-module)))
+             (emacs-module-tests--change-variable state-2 'after-module))
+           "thread 2")))
+    (thread-join thread-1)
+    (thread-join thread-2)))
+
 ;;; emacs-module-tests.el ends here