gtask: Track pending GTasks if G_ENABLE_DEBUG is defined

Track the `GTask`s which are still alive (not finalised) in a shared
list, and provide a secret debugging function for printing that list.

Too often when debugging apps, I have found that a ‘leaked’ object is
actually still (validly) referenced by an ongoing `GTask` which hasn’t
completed for whatever reason. Or I have found that an operation has
obviously stalled, but there are no pointers available to the `GTask`
which is stalled, because it’s being tracked as a collection of closure
pointers from some `GSource` which is hard to get to in the debugger.

It will be very useful for debugging apps, if there’s a list of all the
still alive `GTask`s somewhere. This is that list.

The code is disabled if `G_ENABLE_DEBUG` is not defined, to avoid every
`GTask` construction/finalisation imposing a global locking penalty.

To use the new list, break in `gdb` while running your app, and call
`g_task_print_alive_tasks()`, or inspect the `task_list` manually:
```
(gdb) print g_task_print_alive_tasks()
16:44:17:788 GLib-GIO 5 GTasks still alive:
 • GTask 0x6100000ac740, gs_plugin_appstream_setup_async, ref count: 1, ever_returned: 0, completed: 0
 • GTask 0x6100000bf940, [gio] D-Bus read, ref count: 2, ever_returned: 0, completed: 0
 • GTask 0x6100000aac40, gs_plugin_loader_setup_async, ref count: 1, ever_returned: 0, completed: 0
 • GTask 0x61000006d940, gs_plugin_loader_job_process_async GsPluginJobRefine, ref count: 1, ever_returned: 0, completed: 0
 • GTask 0x610000118c40, [gio] D-Bus read, ref count: 2, ever_returned: 0, completed: 0
```

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

gio/gtask.c

@@ -676,6 +676,58 @@ g_task_init (GTask *task)
   task->check_cancellable = TRUE;
 }
 
+#ifdef G_ENABLE_DEBUG
+G_LOCK_DEFINE_STATIC (task_list);
+static GPtrArray *task_list = NULL;
+
+void
+g_task_print_alive_tasks (void)
+{
+  GString *message_str = g_string_new ("");
+
+  G_LOCK (task_list);
+
+  if (task_list != NULL)
+    {
+      g_string_append_printf (message_str, "%u GTasks still alive:", task_list->len);
+      for (guint i = 0; i < task_list->len; i++)
+        {
+          GTask *task = g_ptr_array_index (task_list, i);
+          const gchar *name = g_task_get_name (task);
+          g_string_append_printf (message_str,
+                                  "\n • GTask %p, %s, ref count: %u, ever_returned: %u, completed: %u",
+                                  task, (name != NULL) ? name : "(no name set)",
+                                  ((GObject *) task)->ref_count,
+                                  task->ever_returned, task->completed);
+        }
+    }
+  else
+    {
+      g_string_append (message_str, "No GTasks still alive");
+    }
+
+  G_UNLOCK (task_list);
+
+  g_message ("%s", message_str->str);
+  g_string_free (message_str, TRUE);
+}
+
+static void
+g_task_constructed (GObject *object)
+{
+  GTask *task = G_TASK (object);
+
+  G_OBJECT_CLASS (g_task_parent_class)->constructed (object);
+
+  /* Track pending tasks for debugging purposes */
+  G_LOCK (task_list);
+  if (G_UNLIKELY (task_list == NULL))
+    task_list = g_ptr_array_new ();
+  g_ptr_array_add (task_list, task);
+  G_UNLOCK (task_list);
+}
+#endif  /* G_ENABLE_DEBUG */
+
 static void
 g_task_finalize (GObject *object)
 {
@@ -732,6 +784,16 @@ g_task_finalize (GObject *object)
       g_cond_clear (&task->cond);
     }
 
+  /* Track pending tasks for debugging purposes */
+#ifdef G_ENABLE_DEBUG
+  G_LOCK (task_list);
+  g_assert (task_list != NULL);
+  g_ptr_array_remove_fast (task_list, task);
+  if (G_UNLIKELY (task_list->len == 0))
+    g_clear_pointer (&task_list, g_ptr_array_unref);
+  G_UNLOCK (task_list);
+#endif  /* G_ENABLE_DEBUG */
+
   G_OBJECT_CLASS (g_task_parent_class)->finalize (object);
 }
 
@@ -2316,6 +2378,9 @@ g_task_class_init (GTaskClass *klass)
 {
   GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
 
+#ifdef G_ENABLE_DEBUG
+  gobject_class->constructed = g_task_constructed;
+#endif
   gobject_class->get_property = g_task_get_property;
   gobject_class->finalize = g_task_finalize;
 

gio/gtask.h

@@ -194,6 +194,12 @@ gboolean      g_task_had_error                 (GTask           *task);
 GIO_AVAILABLE_IN_2_44
 gboolean      g_task_get_completed             (GTask           *task);
 
+/*< private >*/
+#ifndef __GTK_DOC_IGNORE__
+/* Debugging API, not part of the public API */
+void g_task_print_alive_tasks (void);
+#endif  /* !__GTK_DOC_IGNORE__ */
+
 G_END_DECLS
 
 #endif /* __G_TASK_H__ */