Send events to MQ when hosts are renamed.

  - Captures the old name with pre_save.
  - Makes no assumtion on signal consistency in Django...

Fixes #476

mreg/signals.py

@@ -318,6 +318,25 @@ def send_event_ip_added_to_host(sender, instance, created, **kwargs):
     }
     MQSender().send_event(obj, "host.ipaddress")
 
+# In case of host rename, we need to know the old name, so we
+# capture it here and store it in the instance. Note that we first
+# try to get the object in its original state from the database,
+# and if it does not exist there, we set the old name to None as
+# we are being called as part of an object creation.
+@receiver(pre_save, sender=Host)
+def capture_old_name(sender, instance, **kwargs):
+    try:
+        obj = sender.objects.get(pk=instance.pk)
+        instance._old_name = obj.name
+    except sender.DoesNotExist:
+        instance._old_name = None
+
+# Process host events, and send them to the message queue.
+# If the hostname itself is changed, send a host_updated
+# event with both the old and new hostname.
+# The old hostname is captured in the pre_save signal above.
+# Also note that _old_name is not a field in the model, so it
+# will not be saved to the database.
 @receiver(post_save, sender=Host)
 def send_event_host_created(sender, instance, created, **kwargs):
     if created:
@@ -326,6 +345,28 @@ def send_event_host_created(sender, instance, created, **kwargs):
             'action': 'host_created',
         }
         MQSender().send_event(obj, "host")
+    else:
+        # There are situations in Django where singals do not
+        # complete correctly, or pre_save isn't triggered at
+        # all. In these cases, _old_name will not be set, but
+        # the created boolean sent to post_save will still be
+        # false.
+        # To handle such eventualities, we don't simply assume
+        # that old_name is set during updates. The best we can
+        # do in these cases is to send a host_updated event.
+        old_name = getattr(instance, '_old_name', None)
+        if old_name is not None and old_name != instance.name:
+            obj = {
+                'old_host': old_name,
+                'new_host': instance.name,
+                'action': 'host_updated',
+            }
+        else:
+            obj = {
+                'host': instance.name,
+                'action': 'host_updated',
+            }
+        MQSender().send_event(obj, "host")
 
 @receiver(post_delete, sender=Host)
 def send_event_host_removed(sender, instance, **kwargs):