[glib] gdbus: Avoid blocking on worker thread in connection initialization

[Colin Walters <walters src gnome org>]

commit 7ed328aaf01acdc2537498c9617129e4c3004608
Author: Colin Walters <walters verbum org>
Date:   Wed Jun 1 15:11:02 2011 -0400

    gdbus: Avoid blocking on worker thread in connection initialization
    
    I can't see a reason to spin until the worker thread runs, so don't.
    This avoids ugly sched_yield() calls that show up in strace and
    annoy me; the code is cleaner now too.
    
    We now grab the types needed for the WebKit workaround in the
    thread creation area, but only release them when the thread itself
    exits.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=651650

 gio/gdbusprivate.c |  182 +++++++++++++++++++---------------------------------
 1 files changed, 65 insertions(+), 117 deletions(-)
---
diff --git a/gio/gdbusprivate.c b/gio/gdbusprivate.c
index 4a668b8..fcc8cf7 100644
--- a/gio/gdbusprivate.c
+++ b/gio/gdbusprivate.c
@@ -55,6 +55,8 @@
 
 #include "glibintl.h"
 
+static gboolean _g_dbus_worker_do_initial_read (gpointer data);
+
 /* ---------------------------------------------------------------------------------------------------- */
 
 gchar *
@@ -240,7 +242,7 @@ ensure_type (GType gtype)
 }
 
 static void
-released_required_types (void)
+release_required_types (void)
 {
   g_ptr_array_foreach (ensured_classes, (GFunc) g_type_class_unref, NULL);
   g_ptr_array_unref (ensured_classes);
@@ -257,131 +259,75 @@ ensure_required_types (void)
 }
 /* ---------------------------------------------------------------------------------------------------- */
 
-G_LOCK_DEFINE_STATIC (shared_thread_lock);
-
 typedef struct
 {
-  gint num_users;
+  volatile gint refcount;
   GThread *thread;
   GMainContext *context;
   GMainLoop *loop;
 } SharedThreadData;
 
-static SharedThreadData *shared_thread_data = NULL;
-
 static gpointer
-gdbus_shared_thread_func (gpointer data)
+gdbus_shared_thread_func (gpointer user_data)
 {
-  g_main_context_push_thread_default (shared_thread_data->context);
-  g_main_loop_run (shared_thread_data->loop);
-  g_main_context_pop_thread_default (shared_thread_data->context);
-  return NULL;
-}
+  SharedThreadData *data = user_data;
 
-typedef void (*GDBusSharedThreadFunc) (gpointer user_data);
+  g_main_context_push_thread_default (data->context);
+  g_main_loop_run (data->loop);
+  g_main_context_pop_thread_default (data->context);
 
-typedef struct
-{
-  GDBusSharedThreadFunc func;
-  gpointer              user_data;
-  gboolean              done;
-} CallerData;
+  release_required_types ();
 
-static gboolean
-invoke_caller (gpointer user_data)
-{
-  CallerData *data = user_data;
-  data->func (data->user_data);
-  data->done = TRUE;
-  return FALSE;
+  return NULL;
 }
 
 /* ---------------------------------------------------------------------------------------------------- */
 
-static void
-_g_dbus_shared_thread_ref (GDBusSharedThreadFunc func,
-                           gpointer              user_data)
+static SharedThreadData *
+_g_dbus_shared_thread_ref (void)
 {
-  GError *error;
-  GSource *idle_source;
-  CallerData *data;
-  gboolean release_types;
-
-  G_LOCK (shared_thread_lock);
+  static gsize shared_thread_data = 0;
+  GError *error = NULL;
+  SharedThreadData *ret;
 
-  release_types = FALSE;
-
-  if (shared_thread_data != NULL)
+  if (g_once_init_enter (&shared_thread_data))
     {
-      shared_thread_data->num_users += 1;
-      goto have_thread;
+      SharedThreadData *data;
+
+      /* Work-around for https://bugzilla.gnome.org/show_bug.cgi?id=627724 */
+      ensure_required_types ();
+
+      data = g_new0 (SharedThreadData, 1);
+      data->refcount = 0;
+      
+      data->context = g_main_context_new ();
+      data->loop = g_main_loop_new (data->context, FALSE);
+      data->thread = g_thread_create (gdbus_shared_thread_func,
+				      data,
+				      TRUE,
+				      &error);
+      g_assert_no_error (error);
+      /* We can cast between gsize and gpointer safely */
+      g_once_init_leave (&shared_thread_data, (gsize) data);
     }
 
-  shared_thread_data = g_new0 (SharedThreadData, 1);
-  shared_thread_data->num_users = 1;
-
-  /* Work-around for https://bugzilla.gnome.org/show_bug.cgi?id=627724 */
-  ensure_required_types ();
-  release_types = TRUE;
-
-  error = NULL;
-  shared_thread_data->context = g_main_context_new ();
-  shared_thread_data->loop = g_main_loop_new (shared_thread_data->context, FALSE);
-  shared_thread_data->thread = g_thread_create (gdbus_shared_thread_func,
-                                                NULL,
-                                                TRUE,
-                                                &error);
-  g_assert_no_error (error);
-
- have_thread:
-
-  data = g_new0 (CallerData, 1);
-  data->func = func;
-  data->user_data = user_data;
-  data->done = FALSE;
-
-  idle_source = g_idle_source_new ();
-  g_source_set_priority (idle_source, G_PRIORITY_DEFAULT);
-  g_source_set_callback (idle_source,
-                         invoke_caller,
-                         data,
-                         NULL);
-  g_source_attach (idle_source, shared_thread_data->context);
-  g_source_unref (idle_source);
-
-  /* wait for the user code to run.. hmm.. probably use a condition variable instead */
-  while (!data->done)
-    g_thread_yield ();
-
-  if (release_types)
-    released_required_types ();
-
-  g_free (data);
-
-  G_UNLOCK (shared_thread_lock);
+  ret = (SharedThreadData*) shared_thread_data;
+  g_atomic_int_inc (&ret->refcount);
+  return ret;
 }
 
 static void
-_g_dbus_shared_thread_unref (void)
+_g_dbus_shared_thread_unref (SharedThreadData *data)
 {
   /* TODO: actually destroy the shared thread here */
 #if 0
-  G_LOCK (shared_thread_lock);
-  g_assert (shared_thread_data != NULL);
-  shared_thread_data->num_users -= 1;
-  if (shared_thread_data->num_users == 0)
-    {
-      g_main_loop_quit (shared_thread_data->loop);
-      //g_thread_join (shared_thread_data->thread);
-      g_main_loop_unref (shared_thread_data->loop);
-      g_main_context_unref (shared_thread_data->context);
-      g_free (shared_thread_data);
-      shared_thread_data = NULL;
-      G_UNLOCK (shared_thread_lock);
-    }
-  else
+  g_assert (data != NULL);
+  if (g_atomic_int_dec_and_test (&data->refcount))
     {
-      G_UNLOCK (shared_thread_lock);
+      g_main_loop_quit (data->loop);
+      //g_thread_join (data->thread);
+      g_main_loop_unref (data->loop);
+      g_main_context_unref (data->context);
     }
 #endif
 }
@@ -392,6 +338,8 @@ struct GDBusWorker
 {
   volatile gint                       ref_count;
 
+  SharedThreadData                   *shared_thread_data;
+
   gboolean                            stopped;
 
   /* TODO: frozen (e.g. G_DBUS_CONNECTION_FLAGS_DELAY_MESSAGE_PROCESSING) currently
@@ -409,8 +357,6 @@ struct GDBusWorker
   GDBusWorkerDisconnectedCallback     disconnected_callback;
   gpointer                            user_data;
 
-  GThread                            *thread;
-
   /* if not NULL, stream is GSocketConnection */
   GSocket *socket;
 
@@ -470,7 +416,7 @@ _g_dbus_worker_unref (GDBusWorker *worker)
     {
       g_assert (worker->write_pending_flushes == NULL);
 
-      _g_dbus_shared_thread_unref ();
+      _g_dbus_shared_thread_unref (worker->shared_thread_data);
 
       g_object_unref (worker->stream);
 
@@ -575,7 +521,7 @@ _g_dbus_worker_unfreeze (GDBusWorker *worker)
                          unfreeze_in_idle_cb,
                          _g_dbus_worker_ref (worker),
                          (GDestroyNotify) _g_dbus_worker_unref);
-  g_source_attach (idle_source, shared_thread_data->context);
+  g_source_attach (idle_source, worker->shared_thread_data->context);
   g_source_unref (idle_source);
 }
 
@@ -850,12 +796,14 @@ _g_dbus_worker_do_read_unlocked (GDBusWorker *worker)
 }
 
 /* called in private thread shared by all GDBusConnection instances (without read-lock held) */
-static void
-_g_dbus_worker_do_read (GDBusWorker *worker)
+static gboolean
+_g_dbus_worker_do_initial_read (gpointer data)
 {
+  GDBusWorker *worker = data;
   g_mutex_lock (worker->read_lock);
   _g_dbus_worker_do_read_unlocked (worker);
   g_mutex_unlock (worker->read_lock);
+  return FALSE;
 }
 
 /* ---------------------------------------------------------------------------------------------------- */
@@ -1389,7 +1337,7 @@ _g_dbus_worker_send_message (GDBusWorker    *worker,
                              write_message_in_idle_cb,
                              _g_dbus_worker_ref (worker),
                              (GDestroyNotify) _g_dbus_worker_unref);
-      g_source_attach (idle_source, shared_thread_data->context);
+      g_source_attach (idle_source, worker->shared_thread_data->context);
       g_source_unref (idle_source);
     }
   g_mutex_unlock (worker->write_lock);
@@ -1397,17 +1345,6 @@ _g_dbus_worker_send_message (GDBusWorker    *worker,
 
 /* ---------------------------------------------------------------------------------------------------- */
 
-static void
-_g_dbus_worker_thread_begin_func (gpointer user_data)
-{
-  GDBusWorker *worker = user_data;
-
-  worker->thread = g_thread_self ();
-
-  /* begin reading */
-  _g_dbus_worker_do_read (worker);
-}
-
 GDBusWorker *
 _g_dbus_worker_new (GIOStream                              *stream,
                     GDBusCapabilityFlags                    capabilities,
@@ -1418,6 +1355,7 @@ _g_dbus_worker_new (GIOStream                              *stream,
                     gpointer                                user_data)
 {
   GDBusWorker *worker;
+  GSource *idle_source;
 
   g_return_val_if_fail (G_IS_IO_STREAM (stream), NULL);
   g_return_val_if_fail (message_received_callback != NULL, NULL);
@@ -1446,7 +1384,17 @@ _g_dbus_worker_new (GIOStream                              *stream,
   if (G_IS_SOCKET_CONNECTION (worker->stream))
     worker->socket = g_socket_connection_get_socket (G_SOCKET_CONNECTION (worker->stream));
 
-  _g_dbus_shared_thread_ref (_g_dbus_worker_thread_begin_func, worker);
+  worker->shared_thread_data = _g_dbus_shared_thread_ref ();
+
+  /* begin reading */
+  idle_source = g_idle_source_new ();
+  g_source_set_priority (idle_source, G_PRIORITY_DEFAULT);
+  g_source_set_callback (idle_source,
+                         _g_dbus_worker_do_initial_read,
+                         worker,
+                         NULL);
+  g_source_attach (idle_source, worker->shared_thread_data->context);
+  g_source_unref (idle_source);
 
   return worker;
 }