Qpid: advance thru the list of brokers on reconnect

In Qpid implementation, when using multiple qpid_hosts, we don't want to
immediately retry failed connection for the same failed broker. This was not
the case in existing implementation though, where we've always attempted to
reconnect starting from the first broker in the list of candidates. So if the
first broker failed, we initiated reconnect to the same failed broker.

This change makes reconnect() implementation to select the next broker in the
list. This also means that non-failure reconnect attempts will also switch the
current broker. All in all, users should not rely on any particular order to
use brokers from the list, so this should not constitute an issue.

Change-Id: I257c2ad6d7ebead356c0239134340975da6dbc07
Partial-Bug: 1261631

oslo/messaging/_drivers/impl_qpid.py

@@ -466,6 +466,10 @@ class Connection(object):
         params.update(server_params or {})
 
         self.brokers = params['qpid_hosts']
+
+        brokers_count = len(self.brokers)
+        self.next_broker_indices = itertools.cycle(range(brokers_count))
+
         self.username = params['username']
         self.password = params['password']
         self.reconnect()
@@ -494,7 +498,6 @@ class Connection(object):
 
     def reconnect(self):
         """Handles reconnecting and re-establishing sessions and queues."""
-        attempt = 0
         delay = 1
         while True:
             # Close the session if necessary
@@ -504,8 +507,7 @@ class Connection(object):
                 except qpid_exceptions.ConnectionError:
                     pass
 
-            broker = self.brokers[attempt % len(self.brokers)]
-            attempt += 1
+            broker = self.brokers[next(self.next_broker_indices)]
 
             try:
                 self.connection_create(broker)