[gnome-control-center/wip/rishi/issue-208-gnome-3-28] online-accounts: Track the lifecycle of CcGoaPanel across async calls

[Debarshi Ray <debarshir src gnome org>]

commit 36189e5594b65bfe095f2bb11f83c72b3d2ba855
Author: Debarshi Ray <debarshir gnome org>
Date:   Thu Oct 4 11:28:15 2018 +0200

    online-accounts: Track the lifecycle of CcGoaPanel across async calls
    
    Due to an API bug in GNOME Online Accounts, the asynchronous
    goa_provider_get_all method doesn't accept a GCancellable argument.
    This makes it difficult to cancel an ongoing call when the CcGoaPanel
    gets destroyed.
    
    Prior to commit c26f8ae018900a55, this was hacked around by taking a
    reference on the panel for the duration of the call. Instead of
    cancelling a pending call on destruction, it would keep the panel alive
    until the call was over. However, that was lost during commit
    c26f8ae018900a55.
    
    One thing to bear in mind is that GtkWidgets, CcGoaPanel is one, can
    be destroyed by a gtk_widget_destroy call, which is subtly different
    than a simple sequence of g_object_unref calls. When gtk_widget_destroy
    is used, it invokes the GObject::dispose virtual method of the widget.
    It is expected this will cause anything holding a reference to this
    widget to drop their references, leading to GObject::finalize being
    called. However, there is no guarantee that this will happen in the
    same iteration of the GMainLoop. Therefore, it is possible that when
    the goa_provider_get_all call finishes, CcGoaPanel might be in a
    disposed, but not yet finalized state.
    
    When a GObject is in a disposed-but-not-finalized state, only a very
    limited number of operations can be performed on it. Its reference
    count can be altered, the memory used by the instance struct can be
    accessed, but none of the member GObjects can be assumed to be valid.
    eg., it's definitely illegal to add new rows to the member GtkListBox.
    Hence a boolean flag is used to mark the destroyed state of the panel.
    
    This second part is a small improvement over the earlier hack.
    
    https://gitlab.gnome.org/GNOME/gnome-control-center/issues/208

 panels/online-accounts/cc-online-accounts-panel.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)
---
diff --git a/panels/online-accounts/cc-online-accounts-panel.c 
b/panels/online-accounts/cc-online-accounts-panel.c
index 3177a4d27..5cac1b8ac 100644
--- a/panels/online-accounts/cc-online-accounts-panel.c
+++ b/panels/online-accounts/cc-online-accounts-panel.c
@@ -56,6 +56,7 @@ struct _CcGoaPanel
   GtkWidget *stack;
   GtkWidget *accounts_vbox;
 
+  gboolean   destroyed;
   guint      remove_account_timeout_id;
 };
 
@@ -394,6 +395,8 @@ cc_goa_panel_dispose (GObject *object)
   /* Must be destroyed in dispose, not finalize. */
   g_clear_pointer (&panel->edit_account_dialog, gtk_widget_destroy);
 
+  panel->destroyed = TRUE;
+
   G_OBJECT_CLASS (cc_goa_panel_parent_class)->dispose (object);
 }
 
@@ -489,7 +492,7 @@ cc_goa_panel_init (CcGoaPanel *panel)
                     panel);
 
   fill_accounts_listbox (panel);
-  goa_provider_get_all (get_all_providers_cb, panel);
+  goa_provider_get_all (get_all_providers_cb, g_object_ref_sink (panel));
 
   gtk_widget_show (GTK_WIDGET (panel));
 }
@@ -852,7 +855,7 @@ get_all_providers_cb (GObject      *source,
                       GAsyncResult *res,
                       gpointer      user_data)
 {
-  CcGoaPanel *self = user_data;
+  g_autoptr (CcGoaPanel) self = user_data;
   GList *providers;
   GList *l;
 
@@ -860,6 +863,9 @@ get_all_providers_cb (GObject      *source,
   if (!goa_provider_get_all_finish (&providers, res, NULL))
     return;
 
+  if (self->destroyed)
+    return;
+
   for (l = providers; l != NULL; l = l->next)
     {
       GoaProvider *provider;