gtask: Add a GTask:completed property

This can be used to query whether the task has completed, in the sense
that it has had a result set on it, and has already – or will soon –
invoke its callback function.

Notifications for this property are emitted immediately after the task’s
main callback, in the same main context as that callback. This allows
for multiple bits of code to listen for completion of the GTask, which
opens the door for blocking on cancellation of the GTask and improved
handling of ‘pending’ behaviour.

https://bugzilla.gnome.org/show_bug.cgi?id=743636

gio/gtask.c

@@ -23,6 +23,8 @@
 #include "gasyncresult.h"
 #include "gcancellable.h"
 
+#include "glibintl.h"
+
 /**
  * SECTION:gtask
  * @short_description: Cancellable synchronous or asynchronous task
@@ -547,6 +549,7 @@ struct _GTask {
 
   GAsyncReadyCallback callback;
   gpointer callback_data;
+  gboolean completed;
 
   GTaskThreadFunc task_func;
   GMutex lock;
@@ -574,6 +577,11 @@ struct _GTaskClass
   GObjectClass parent_class;
 };
 
+typedef enum
+{
+  PROP_COMPLETED = 1,
+} GTaskProperty;
+
 static void g_task_thread_pool_resort (void);
 
 static void g_task_async_result_iface_init (GAsyncResultIface *iface);
@@ -1074,9 +1082,17 @@ static void
 g_task_return_now (GTask *task)
 {
   g_main_context_push_thread_default (task->context);
-  task->callback (task->source_object,
-                  G_ASYNC_RESULT (task),
-                  task->callback_data);
+
+  if (task->callback != NULL)
+    {
+      task->callback (task->source_object,
+                      G_ASYNC_RESULT (task),
+                      task->callback_data);
+    }
+
+  task->completed = TRUE;
+  g_object_notify (G_OBJECT (task), "completed");
+
   g_main_context_pop_thread_default (task->context);
 }
 
@@ -1103,7 +1119,7 @@ g_task_return (GTask           *task,
   if (type == G_TASK_RETURN_SUCCESS)
     task->result_set = TRUE;
 
-  if (task->synchronous || !task->callback)
+  if (task->synchronous)
     return;
 
   /* Normally we want to invoke the task's callback when its return
@@ -1354,6 +1370,7 @@ g_task_run_in_thread (GTask           *task,
  * Normally this is used with tasks created with a %NULL
  * `callback`, but note that even if the task does
  * have a callback, it will not be invoked when @task_func returns.
+ * #GTask:completed will be set to %TRUE just before this function returns.
  *
  * Since: 2.36
  */
@@ -1372,6 +1389,11 @@ g_task_run_in_thread_sync (GTask           *task,
     g_cond_wait (&task->cond, &task->lock);
 
   g_mutex_unlock (&task->lock);
+
+  /* Notify of completion in this thread. */
+  task->completed = TRUE;
+  g_object_notify (G_OBJECT (task), "completed");
+
   g_object_unref (task);
 }
 
@@ -1718,6 +1740,26 @@ g_task_had_error (GTask *task)
   return FALSE;
 }
 
+/**
+ * g_task_get_completed:
+ * @task: a #GTask.
+ *
+ * Gets the value of #GTask:completed. This changes from %FALSE to %TRUE after
+ * the task’s callback is invoked, and will return %FALSE if called from inside
+ * the callback.
+ *
+ * Returns: %TRUE if the task has completed, %FALSE otherwise.
+ *
+ * Since: 2.44
+ */
+gboolean
+g_task_get_completed (GTask *task)
+{
+  g_return_val_if_fail (G_IS_TASK (task), FALSE);
+
+  return task->completed;
+}
+
 /**
  * g_task_is_valid:
  * @result: (type Gio.AsyncResult): A #GAsyncResult
@@ -1790,12 +1832,50 @@ g_task_thread_pool_resort (void)
   g_thread_pool_set_sort_function (task_pool, g_task_compare_priority, NULL);
 }
 
+static void
+g_task_get_property (GObject    *object,
+                     guint       prop_id,
+                     GValue     *value,
+                     GParamSpec *pspec)
+{
+  GTask *task = G_TASK (object);
+
+  switch ((GTaskProperty) prop_id)
+    {
+    case PROP_COMPLETED:
+      g_value_set_boolean (value, task->completed);
+      break;
+    }
+}
+
 static void
 g_task_class_init (GTaskClass *klass)
 {
   GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
 
+  gobject_class->get_property = g_task_get_property;
   gobject_class->finalize = g_task_finalize;
+
+  /**
+   * GTask:completed:
+   *
+   * Whether the task has completed, meaning its callback (if set) has been
+   * invoked. This can only happen after g_task_return_pointer(),
+   * g_task_return_error() or one of the other return functions have been called
+   * on the task.
+   *
+   * This property is guaranteed to change from %FALSE to %TRUE exactly once.
+   *
+   * The #GObject::notify signal for this change is emitted in the same main
+   * context as the task’s callback, immediately after that callback is invoked.
+   *
+   * Since: 2.44
+   */
+  g_object_class_install_property (gobject_class, PROP_COMPLETED,
+    g_param_spec_boolean ("completed",
+                          P_("Task completed"),
+                          P_("Whether the task has completed yet"),
+                          FALSE, G_PARAM_READABLE | G_PARAM_STATIC_STRINGS));
 }
 
 static gpointer