[libpeas] Fix the global Lua plugin loader with multiple threads

[Garrett Regier <gregier src gnome org>]

commit 826f902ef2dad8ea45a7d4c5ee9960e9faa6109c
Author: Garrett Regier <garrett regier riftio com>
Date:   Tue Dec 30 08:58:27 2014 -0800

    Fix the global Lua plugin loader with multiple threads
    
    LGI will yield to other threads when calling into C API.
    This caused the return values for another thread's function
    call to be on the Lua stack which caused an assertion.
    
    This also adds multiple assertions that the thread's
    Lua stack does not have any unpopped values.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=741859

 loaders/lua5.1/peas-plugin-loader-lua.c |   91 ++++++++++++++++++++++++++-----
 1 files changed, 77 insertions(+), 14 deletions(-)
---
diff --git a/loaders/lua5.1/peas-plugin-loader-lua.c b/loaders/lua5.1/peas-plugin-loader-lua.c
index eaf7cbb..b98db05 100644
--- a/loaders/lua5.1/peas-plugin-loader-lua.c
+++ b/loaders/lua5.1/peas-plugin-loader-lua.c
@@ -231,20 +231,66 @@ _lua_find_extension_type (lua_State      *L,
   return found_type;
 }
 
+static lua_State *
+thread_enter (PeasPluginLoaderLua *lua_loader,
+              PeasPluginInfo      *info)
+{
+  PeasPluginLoaderLuaPrivate *priv = GET_PRIV (lua_loader);
+  lua_State *L = priv->L;
+  lua_State *NL = info->loader_data;
+
+  priv->lgi_enter_func (priv->lgi_lock);
+
+  if (NL != NULL)
+    {
+      /* We should never have multiple values on the stack */
+      g_assert_cmpint (lua_gettop (NL), ==, 0);
+    }
+  else
+    {
+      luaL_checkstack (L, 2, "");
+
+      lua_pushlightuserdata (L, info);
+      NL = lua_newthread (L);
+      lua_rawset (L, LUA_REGISTRYINDEX);
+
+      info->loader_data = NL;
+    }
+
+  return NL;
+}
+
+static void
+thread_leave (PeasPluginLoaderLua  *lua_loader,
+              PeasPluginInfo       *info,
+              lua_State           **L_ptr)
+{
+  PeasPluginLoaderLuaPrivate *priv = GET_PRIV (lua_loader);
+  lua_State *L = info->loader_data;
+
+  g_assert (*L_ptr == L);
+  *L_ptr = NULL;
+
+  /* The stack should always be empty */
+  g_assert_cmpint (lua_gettop (L), ==, 0);
+
+  priv->lgi_leave_func (priv->lgi_lock);
+}
+
 static gboolean
 peas_plugin_loader_lua_provides_extension (PeasPluginLoader *loader,
                                            PeasPluginInfo   *info,
                                            GType             exten_type)
 {
   PeasPluginLoaderLua *lua_loader = PEAS_PLUGIN_LOADER_LUA (loader);
-  PeasPluginLoaderLuaPrivate *priv = GET_PRIV (lua_loader);
-  lua_State *L = priv->L;
+  lua_State *L;
   GType extension_type;
 
-  priv->lgi_enter_func (priv->lgi_lock);
+  L = thread_enter (lua_loader, info);
+
   extension_type = _lua_find_extension_type (L, info, exten_type);
-  priv->lgi_leave_func (priv->lgi_lock);
 
+  thread_leave (lua_loader, info, &L);
   return extension_type != G_TYPE_INVALID;
 }
 
@@ -256,12 +302,11 @@ peas_plugin_loader_lua_create_extension (PeasPluginLoader *loader,
                                          GParameter       *parameters)
 {
   PeasPluginLoaderLua *lua_loader = PEAS_PLUGIN_LOADER_LUA (loader);
-  PeasPluginLoaderLuaPrivate *priv = GET_PRIV (lua_loader);
-  lua_State *L = priv->L;
+  lua_State *L;
   GType the_type;
   GObject *object = NULL;
 
-  priv->lgi_enter_func (priv->lgi_lock);
+  L = thread_enter (lua_loader, info);
 
   the_type = _lua_find_extension_type (L, info, exten_type);
   if (the_type == G_TYPE_INVALID)
@@ -309,7 +354,7 @@ peas_plugin_loader_lua_create_extension (PeasPluginLoader *loader,
 
 out:
 
-  priv->lgi_leave_func (priv->lgi_lock);
+  thread_leave (lua_loader, info, &L);
   return object;
 }
 
@@ -318,11 +363,10 @@ peas_plugin_loader_lua_load (PeasPluginLoader *loader,
                              PeasPluginInfo   *info)
 {
   PeasPluginLoaderLua *lua_loader = PEAS_PLUGIN_LOADER_LUA (loader);
-  PeasPluginLoaderLuaPrivate *priv = GET_PRIV (lua_loader);
-  lua_State *L = priv->L;
+  lua_State *L;
   gboolean success;
 
-  priv->lgi_enter_func (priv->lgi_lock);
+  L = thread_enter (lua_loader, info);
 
   luaL_checkstack (L, 2, "");
 
@@ -367,7 +411,7 @@ peas_plugin_loader_lua_load (PeasPluginLoader *loader,
   /* Pop the module's table */
   lua_pop (L, 1);
 
-  priv->lgi_leave_func (priv->lgi_lock);
+  thread_leave (lua_loader, info, &L);
   return success;
 }
 
@@ -375,7 +419,23 @@ static void
 peas_plugin_loader_lua_unload (PeasPluginLoader *loader,
                                PeasPluginInfo   *info)
 {
-  /* Lua always keeps a reference anyways */
+  PeasPluginLoaderLua *lua_loader = PEAS_PLUGIN_LOADER_LUA (loader);
+  PeasPluginLoaderLuaPrivate *priv = GET_PRIV (lua_loader);
+  lua_State *L = priv->L;
+
+  priv->lgi_enter_func (priv->lgi_lock);
+
+  /* The stack should always be empty */
+  g_assert_cmpint (lua_gettop (info->loader_data), ==, 0);
+
+  /* Delete the thread's reference */
+  lua_pushlightuserdata (L, info);
+  lua_pushnil (L);
+  lua_rawset (L, LUA_REGISTRYINDEX);
+
+  priv->lgi_leave_func (priv->lgi_lock);
+
+  info->loader_data = NULL;
 }
 
 static void
@@ -383,9 +443,12 @@ peas_plugin_loader_lua_garbage_collect (PeasPluginLoader *loader)
 {
   PeasPluginLoaderLua *lua_loader = PEAS_PLUGIN_LOADER_LUA (loader);
   PeasPluginLoaderLuaPrivate *priv = GET_PRIV (lua_loader);
+  lua_State *L = priv->L;
 
   priv->lgi_enter_func (priv->lgi_lock);
-  lua_gc (priv->L, LUA_GCCOLLECT, 0);
+
+  lua_gc (L, LUA_GCCOLLECT, 0);
+
   priv->lgi_leave_func (priv->lgi_lock);
 }