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.

1 files changed, 41 insertions, 0 deletions

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));