[gtk/gtk-3-24: 2/3] printing: Show all Avahi advertised printers

[Matthias Clasen <matthiasc src gnome org>]

commit de3e97309f91fc631a1a4b5ab48cd69dd1421ed5
Author: Marek Kasik <mkasik redhat com>
Date:   Fri Feb 5 18:00:02 2021 +0100

    printing: Show all Avahi advertised printers
    
    This commit unsubscribes CUPS backend from a DBus
    signal in idle when listening for new items on Avahi.
    
    Since GDBus emits gathered signals in idle while
    checking whether the signal has been unsubscribed
    it could happen that a signal was not processed
    because it was removed from hash table of
    subscribed signals.
    This caused the situation where printers advertised
    on Avahi were not listed in CUPS backend sometimes.
    
    We need those signals since this happens when switching
    from a general subscription which listens to signals
    for all Avahi services to a specific one which listens
    to just _ipp._tcp and _ipps._tcp (chicken and egg problem).

 modules/printbackends/cups/gtkprintbackendcups.c | 27 +++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)
---
diff --git a/modules/printbackends/cups/gtkprintbackendcups.c 
b/modules/printbackends/cups/gtkprintbackendcups.c
index b1ee0584d0..6b8c9e02ad 100644
--- a/modules/printbackends/cups/gtkprintbackendcups.c
+++ b/modules/printbackends/cups/gtkprintbackendcups.c
@@ -150,6 +150,7 @@ struct _GtkPrintBackendCups
   guint avahi_service_browser_subscription_ids[2];
   char *avahi_service_browser_paths[2];
   GCancellable *avahi_cancellable;
+  guint unsubscribe_general_subscription_id;
 
   gboolean      secrets_service_available;
   guint         secrets_service_watch_id;
@@ -1009,6 +1010,12 @@ gtk_print_backend_cups_dispose (GObject *object)
       backend_cups->avahi_service_browser_subscription_id = 0;
     }
 
+  if (backend_cups->unsubscribe_general_subscription_id > 0)
+    {
+      g_source_remove (backend_cups->unsubscribe_general_subscription_id);
+      backend_cups->unsubscribe_general_subscription_id = 0;
+    }
+
   backend_parent_class->dispose (object);
 }
 
@@ -3514,6 +3521,19 @@ avahi_service_browser_signal_handler (GDBusConnection *connection,
     }
 }
 
+static gboolean
+unsubscribe_general_subscription_cb (gpointer user_data)
+{
+  GtkPrintBackendCups *cups_backend = user_data;
+
+  g_dbus_connection_signal_unsubscribe (cups_backend->dbus_connection,
+                                        cups_backend->avahi_service_browser_subscription_id);
+  cups_backend->avahi_service_browser_subscription_id = 0;
+  cups_backend->unsubscribe_general_subscription_id = 0;
+
+  return G_SOURCE_REMOVE;
+}
+
 static void
 avahi_service_browser_new_cb (GObject      *source_object,
                               GAsyncResult *res,
@@ -3555,9 +3575,10 @@ avahi_service_browser_new_cb (GObject      *source_object,
           cups_backend->avahi_service_browser_paths[1] &&
           cups_backend->avahi_service_browser_subscription_id > 0)
         {
-          g_dbus_connection_signal_unsubscribe (cups_backend->dbus_connection,
-                                                cups_backend->avahi_service_browser_subscription_id);
-          cups_backend->avahi_service_browser_subscription_id = 0;
+          /* We need to unsubscribe in idle since signals in queue destined for emit
+           * are emitted in idle and check whether the subscriber is still subscribed.
+           */
+          cups_backend->unsubscribe_general_subscription_id = g_idle_add 
(unsubscribe_general_subscription_cb, cups_backend);
         }
 
       g_variant_unref (output);