[gvfs/wip/oholy/dnssd-crashes] dnssd: Prevent crashes after releasing resolver

[Ondrej Holy <oholy src gnome org>]

commit 80d698aa9f579c80e5d1768402a4e52f2a596645
Author: Ondrej Holy <oholy redhat com>
Date:   Mon Jan 27 14:02:36 2020 +0100

    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

 common/gvfsdnssdresolver.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)
---
diff --git a/common/gvfsdnssdresolver.c b/common/gvfsdnssdresolver.c
index 7aa2cfcb..b7f0cd3f 100644
--- a/common/gvfsdnssdresolver.c
+++ b/common/gvfsdnssdresolver.c
@@ -85,6 +85,7 @@ struct _GVfsDnsSdResolver
   char **txt_records;
 
   AvahiServiceResolver *avahi_resolver;
+  guint start_avahi_resolver_id;
 };
 
 
@@ -245,6 +246,7 @@ start_avahi_resolver (gpointer user_data)
                                                          resolver);
 
 out:
+  resolver->start_avahi_resolver_id = 0;
   g_object_unref (resolver);
   return FALSE;
 }
@@ -252,10 +254,10 @@ 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
@@ -382,6 +384,8 @@ 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);