[gvfs/gnome-3-34] dnssd: Prevent crashes after releasing resolver

[Ondrej Holy <oholy src gnome org>]

commit 4145ef22274deb08a86e78b195cf8eba2d6eb0ee
Author: Ondrej Holy <oholy redhat com>
Date:   Mon Jan 27 13:02:36 2020 +0000

    dnssd: Prevent crashes after releasing resolver
    
    The following error often happens when mounting dav+sd share (e.g. created
    over gnome-user-media) and the mount operation fails for some reason:
    
    dbus[47482]: arguments to dbus_connection_unref() were incorrect,
    assertion "connection->generation == _dbus_current_generation" failed
    in file ../../dbus/dbus-connection.c line 2823. This is normally a bug
    in some application using the D-Bus library.
    
    This is because avahi is not thread-safe and our resolver doesn't use
    mutexes. But it seems to me that mutexes are not necessarily needed if
    everything would work as expected. Unfortunately, changes made by commit
    3384af8 causes that start_avahi_resolver is being called multiple times
    by mistake, which also calls avahi_service_resolver_new multiple times.
    Consequently, it may crash when resolver is beeing freed too early as
    there might still be unexpected pending operations using avahi client.
    
    Fixes: GNOME/gvfs#449
    
    
    (cherry picked from commit c8f8f345a0c88e68dd21f6b548ab1cfba56732e9)

 common/gvfsdnssdresolver.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)
---
diff --git a/common/gvfsdnssdresolver.c b/common/gvfsdnssdresolver.c
index 0f0f5f60..be062227 100644
--- a/common/gvfsdnssdresolver.c
+++ b/common/gvfsdnssdresolver.c
@@ -81,6 +81,7 @@ struct _GVfsDnsSdResolver
   char **txt_records;
 
   AvahiServiceResolver *avahi_resolver;
+  guint start_avahi_resolver_id;
 };
 
 
@@ -241,6 +242,7 @@ start_avahi_resolver (gpointer user_data)
                                                          resolver);
 
 out:
+  resolver->start_avahi_resolver_id = 0;
   g_object_unref (resolver);
   return FALSE;
 }
@@ -248,10 +250,10 @@ out:
 static void
 ensure_avahi_resolver (GVfsDnsSdResolver  *resolver)
 {
-  if (resolver->avahi_resolver != NULL)
+  if (resolver->avahi_resolver != NULL || resolver->start_avahi_resolver_id != 0)
     return;
 
-  g_idle_add (start_avahi_resolver, g_object_ref (resolver));
+  resolver->start_avahi_resolver_id = g_idle_add (start_avahi_resolver, g_object_ref (resolver));
 }
 
 static void
@@ -373,6 +375,8 @@ g_vfs_dns_sd_resolver_finalize (GObject *object)
   if (resolver->avahi_resolver != NULL)
     avahi_service_resolver_free (resolver->avahi_resolver);
 
+  if (resolver->start_avahi_resolver_id != 0)
+    g_source_remove (resolver->start_avahi_resolver_id);
 
   resolvers = g_list_remove (resolvers, resolver);