registrybackend: make subscribe and unsubscribe thread-safe

The registry backend uses a thread to monitor
registry changes and send notifications.
The state of this thread and structures used
for communicating with it are kept in the watch
variable.
The subscribe and unsubscribe functions might be
concurrently called from multiple threads and
need to communicate with the monitoring thread.
For this reason we need to synchronize the access
to the watch variable.

gio/gregistrysettingsbackend.c

@@ -179,6 +179,10 @@ typedef struct {
   CRITICAL_SECTION *cache_lock;
   GNode *cache_root;
 
+  /* A lock to protect access to the watch variable */
+  CRITICAL_SECTION watch_lock;
+  /* Contains the state of the watching thread. Any access to this variable
+   * must be done while holding the watch_lock critical section. */
   WatchThreadState *watch;
 } GRegistrySettingsBackend;
 
@@ -1901,6 +1905,7 @@ watch_thread_function (LPVOID parameter)
   return -1;
 }
 
+/* This function assumes you hold the watch lock! */
 static gboolean
 watch_start (GRegistrySettingsBackend *self)
 {
@@ -1947,6 +1952,7 @@ fail:
   return FALSE;
 }
 
+/* This function assumes you hold the watch lock! */
 /* This function assumes you hold the message lock! */
 static void
 watch_stop_unlocked (GRegistrySettingsBackend *self)
@@ -1982,6 +1988,7 @@ watch_stop_unlocked (GRegistrySettingsBackend *self)
   self->watch = NULL;
 }
 
+/* This function assumes you hold the watch lock! */
 static gboolean
 watch_add_notify (GRegistrySettingsBackend *self,
                   HANDLE                    event,
@@ -2055,6 +2062,7 @@ watch_add_notify (GRegistrySettingsBackend *self,
   return TRUE;
 }
 
+/* This function assumes you hold the watch lock! */
 static void
 watch_remove_notify (GRegistrySettingsBackend *self,
                      const gchar              *key_name)
@@ -2105,12 +2113,17 @@ g_registry_settings_backend_subscribe (GSettingsBackend *backend,
   HANDLE event;
   LONG result;
 
+  EnterCriticalSection (&self->watch_lock);
   if (self->watch == NULL && !watch_start (self))
-    return;
+    {
+      LeaveCriticalSection (&self->watch_lock);
+      return;
+    }
 
   if (g_atomic_int_dec_and_test (&self->watch->watches_remaining))
     {
       g_atomic_int_inc (&self->watch->watches_remaining);
+      LeaveCriticalSection (&self->watch_lock);
       g_warning ("subscribe() failed: only %i different paths may be watched.", MAX_WATCHES);
       return;
     }
@@ -2139,6 +2152,7 @@ g_registry_settings_backend_subscribe (GSettingsBackend *backend,
     {
       g_message_win32_error (result, "gregistrysettingsbackend: Unable to subscribe to key %s.", key_name);
       g_atomic_int_inc (&self->watch->watches_remaining);
+      LeaveCriticalSection (&self->watch_lock);
       return;
     }
 
@@ -2147,6 +2161,7 @@ g_registry_settings_backend_subscribe (GSettingsBackend *backend,
     {
       g_message_win32_error (result, "gregistrysettingsbackend: CreateEvent failed.");
       g_atomic_int_inc (&self->watch->watches_remaining);
+      LeaveCriticalSection (&self->watch_lock);
       RegCloseKey (hpath);
       return;
     }
@@ -2159,15 +2174,21 @@ g_registry_settings_backend_subscribe (GSettingsBackend *backend,
       RegCloseKey (hpath);
       CloseHandle (event);
     }
+
+  LeaveCriticalSection (&self->watch_lock);
 }
 
 static void
 g_registry_settings_backend_unsubscribe (GSettingsBackend *backend,
                                          const char       *key_name)
 {
+  GRegistrySettingsBackend *self = G_REGISTRY_SETTINGS_BACKEND (backend);
+
   trace ("unsubscribe: %s.\n", key_name);
 
-  watch_remove_notify (G_REGISTRY_SETTINGS_BACKEND (backend), key_name);
+  EnterCriticalSection (&self->watch_lock);
+  watch_remove_notify (self, key_name);
+  LeaveCriticalSection (&self->watch_lock);
 }
 
 /********************************************************************************
@@ -2191,6 +2212,7 @@ g_registry_settings_backend_finalize (GObject *object)
       EnterCriticalSection (self->watch->message_lock);
       watch_stop_unlocked (self);
     }
+  DeleteCriticalSection (&self->watch_lock);
 
   DeleteCriticalSection (self->cache_lock);
   g_slice_free (CRITICAL_SECTION, self->cache_lock);
@@ -2341,6 +2363,7 @@ g_registry_settings_backend_init (GRegistrySettingsBackend *self)
   InitializeCriticalSection (self->cache_lock);
 
   self->watch = NULL;
+  InitializeCriticalSection (&self->watch_lock);
 }
 
 /**