wplua: store closures only with a weak reference

This allows closures to be properly unrefed when they are no longer
used instead of staying alive until wireplumber exits

Because GClosure has no weak references, we are now sharing the
GPtrArray that holds these references among all the active closures
and each closure is responsible for removing itself from the array
when it is finalized. The lua engine holds a reference to a "store"
object that also has a pointer to the array and when this "store"
is finalized, all closures are invalidated and removed. Even if they
stay alive afterwards, they are only holding a ref to an empty array

lib/wplua/closure.c

@@ -10,11 +10,57 @@
 #include "private.h"
 #include <wp/wp.h>
 
+/* This structure is added to a lua global and it's only referenced from there;
+   When the lua_State closes, it is unrefed and its finalize function below
+   invalidates all the closures so that nothing attempts to call into a lua
+   function after the state is closed */
+typedef struct _WpLuaClosureStore WpLuaClosureStore;
+struct _WpLuaClosureStore
+{
+  GPtrArray *closures;
+};
+
+static WpLuaClosureStore *
+_wplua_closure_store_new (void)
+{
+  WpLuaClosureStore *self = g_rc_box_new (WpLuaClosureStore);
+  self->closures = g_ptr_array_new ();
+  return self;
+}
+
+static void
+_wplua_closure_store_finalize (WpLuaClosureStore * self)
+{
+  for (guint i = self->closures->len; i > 0; i--) {
+    GClosure *c = g_ptr_array_index (self->closures, i-1);
+    g_closure_invalidate (c);
+    g_ptr_array_remove_index_fast (self->closures, i-1);
+  }
+  g_ptr_array_unref (self->closures);
+}
+
+static WpLuaClosureStore *
+_wplua_closure_store_ref (WpLuaClosureStore * self)
+{
+  return g_rc_box_acquire (self);
+}
+
+static void
+_wplua_closure_store_unref (WpLuaClosureStore * self)
+{
+  g_rc_box_release_full (self, (GDestroyNotify) _wplua_closure_store_finalize);
+}
+
+G_DEFINE_BOXED_TYPE(WpLuaClosureStore, _wplua_closure_store,
+    _wplua_closure_store_ref, _wplua_closure_store_unref)
+
+
 typedef struct _WpLuaClosure WpLuaClosure;
 struct _WpLuaClosure
 {
   GClosure closure;
   int func_ref;
+  GPtrArray *closures;
 };
 
 static void
@@ -61,6 +107,13 @@ _wplua_closure_invalidate (lua_State *L, WpLuaClosure *c)
   c->func_ref = LUA_NOREF;
 }
 
+static void
+_wplua_closure_finalize (lua_State *L, WpLuaClosure *c)
+{
+  g_ptr_array_remove_fast (c->closures, c);
+  g_ptr_array_unref (c->closures);
+}
+
 /**
  * wplua_function_to_closure:
  *
@@ -75,12 +128,7 @@ wplua_function_to_closure (lua_State *L, int idx)
 
   GClosure *c = g_closure_new_simple (sizeof (WpLuaClosure), L);
   WpLuaClosure *wlc = (WpLuaClosure *) c;
-  GPtrArray *closures;
-
-  lua_pushliteral (L, "wplua_closures");
-  lua_gettable (L, LUA_REGISTRYINDEX);
-  closures = wplua_toboxed (L, -1);
-  lua_pop (L, 1);
+  WpLuaClosureStore *store;
 
   lua_pushvalue (L, idx);
   wlc->func_ref = luaL_ref (L, LUA_REGISTRYINDEX);
@@ -90,27 +138,30 @@ wplua_function_to_closure (lua_State *L, int idx)
   g_closure_set_marshal (c, _wplua_closure_marshal);
   g_closure_add_invalidate_notifier (c, L,
       (GClosureNotify) _wplua_closure_invalidate);
+  g_closure_add_finalize_notifier (c, L,
+      (GClosureNotify) _wplua_closure_finalize);
 
-  /* keep a ref in lua, so that we can invalidate
-     the closure when lua_State closes */
-  g_ptr_array_add (closures, g_closure_ref (c));
+  /* keep a weak ref of the closure in the store's array,
+     so that we can invalidate the closure when lua_State closes;
+     keep a strong ref of the array in the closure so that
+     _wplua_closure_finalize() works even after the state is closed */
+  lua_pushliteral (L, "wplua_closures");
+  lua_gettable (L, LUA_REGISTRYINDEX);
+  store = wplua_toboxed (L, -1);
+  lua_pop (L, 1);
+
+  g_ptr_array_add (store->closures, c);
+  wlc->closures = g_ptr_array_ref (store->closures);
 
   return c;
 }
 
-static void
-_wplua_closure_destroy (GClosure * c)
-{
-  g_closure_invalidate (c);
-  g_closure_unref (c);
-}
-
 void
 _wplua_init_closure (lua_State *L)
 {
-  GPtrArray *a = g_ptr_array_new_with_free_func (
-      (GDestroyNotify) _wplua_closure_destroy);
   lua_pushliteral (L, "wplua_closures");
-  wplua_pushboxed (L, G_TYPE_PTR_ARRAY, a);
+  wplua_pushboxed (L,
+      _wplua_closure_store_get_type (),
+      _wplua_closure_store_new ());
   lua_settable (L, LUA_REGISTRYINDEX);
 }