Raise TemporaryFailure if no conductors are online

Return 503 Service Unavailable error in case no conductors
are online. This indicate to the caller that it can retry
the operation. (Previously a 400 error was returned,
which is confusing because it is not a user error.)

Story: 2002600
Task:  22214
Change-Id: I2c21baaec2e35c0a63f3f4a95990b12c6a836065

ironic/conductor/rpcapi.py

@@ -128,6 +128,10 @@ class ConductorAPI(object):
         """
         self.ring_manager.reset()
 
+        # There are no conductors, temporary failure - 503 Service Unavailable
+        if not self.ring_manager.ring:
+            raise exception.TemporaryFailure()
+
         try:
             ring = self.ring_manager[node.driver]
             dest = ring.get_nodes(node.uuid.encode('utf-8'),

ironic/tests/unit/conductor/test_rpcapi.py

@@ -98,7 +98,7 @@ class RPCAPITestCase(db_base.DbTestCase):
         CONF.set_override('host', 'fake-host')
 
         rpcapi = conductor_rpcapi.ConductorAPI(topic='fake-topic')
-        self.assertRaises(exception.NoValidHost,
+        self.assertRaises(exception.TemporaryFailure,
                           rpcapi.get_topic_for,
                           self.fake_node_obj)
 

releasenotes/notes/story-2002600-return-503-if-no-conductors-online-ead1512628182ec4.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Ironic API now returns ``503 Service Unavailable`` for action requiring a
+    conductor when no conductors are online.
+    `Bug: 2002600 <https://storyboard.openstack.org/#!/story/2002600>`_.