proxy: fix crash that happens when a client exits very quickly after starting

This is very easy to reproduce when the pipewire-alsa integration
is installed and you do 'arecord -l'; the alsa plugin connects and
disconnects again before the proxy is ready.

In this case we have to skip remote-global-added and we also have
to be careful with the references: the global-removed callback is
called earlier, so the core's reference to the proxy is gone and
the GTask is the only thing holding a reference to the proxy.
When we unref the GTask, the proxy is also unrefed, so we have
to keep an additional reference in order to avoid crashing
when accessing the hash table below.

lib/wp/core.c

@@ -138,6 +138,7 @@ on_proxy_ready (GObject * obj, GAsyncResult * res, gpointer data)
 
   if (!wp_proxy_augment_finish (proxy, res, &error)) {
     g_warning ("Failed to augment WpProxy (%p): %s", obj, error->message);
+    return;
   }
 
   g_signal_emit (self, signals[SIGNAL_REMOTE_GLOBAL_ADDED],

lib/wp/proxy.c

@@ -130,7 +130,9 @@ wp_proxy_find_quark_for_type (guint32 type)
 static void
 proxy_event_destroy (void *data)
 {
-  WpProxy *self = WP_PROXY (data);
+  /* hold a reference to the proxy because unref-ing the tasks might
+    destroy the proxy, in case the core is no longer holding a reference */
+  g_autoptr (WpProxy) self = g_object_ref (WP_PROXY (data));
   WpProxyPrivate *priv = wp_proxy_get_instance_private (self);
   GHashTableIter iter;
   GTask *task;
@@ -144,7 +146,7 @@ proxy_event_destroy (void *data)
   if (priv->task) {
     g_task_return_new_error (priv->task, WP_DOMAIN_LIBRARY,
         WP_LIBRARY_ERROR_OPERATION_FAILED,
-        "pipewire node proxy destroyed before finishing");
+        "pipewire proxy destroyed before finishing");
     g_clear_object (&priv->task);
   }
 
@@ -152,7 +154,7 @@ proxy_event_destroy (void *data)
   while (g_hash_table_iter_next (&iter, NULL, (gpointer *) &task)) {
     g_task_return_new_error (task, WP_DOMAIN_LIBRARY,
         WP_LIBRARY_ERROR_OPERATION_FAILED,
-        "pipewire node proxy destroyed before finishing");
+        "pipewire proxy destroyed before finishing");
     g_hash_table_iter_remove (&iter);
   }
 }
@@ -218,9 +220,9 @@ wp_proxy_finalize (GObject * object)
   g_debug ("%s:%p destroyed (global %u; pw_proxy %p)",
       G_OBJECT_TYPE_NAME (object), object, priv->global_id, priv->pw_proxy);
 
+  g_clear_pointer (&priv->pw_proxy, pw_proxy_destroy);
   g_clear_object (&priv->task);
   g_clear_pointer (&priv->global_props, wp_properties_unref);
-  g_clear_pointer (&priv->pw_proxy, pw_proxy_destroy);
   g_weak_ref_clear (&priv->core);
   g_clear_pointer (&priv->async_tasks, g_hash_table_unref);