Only attempt a rebuild claim for an evacuation to a new host

Change I11746d1ea996a0f18b7c54b4c9c21df58cc4714b changed the
behavior of the API and conductor when rebuilding an instance
with a new image such that the image is run through the scheduler
filters again to see if it will work on the existing host that
the instance is running on.

As a result, conductor started passing 'scheduled_node' to the
compute which was using it for logic to tell if a claim should be
attempted. We don't need to do a claim for a rebuild since we're
on the same host.

This removes the scheduled_node logic from the claim code, as we
should only ever attempt a claim if we're evacuating, which we
can determine based on the 'recreate' parameter.

Change-Id: I7fde8ce9dea16679e76b0cb2db1427aeeec0c222
Closes-Bug: #1750618

nova/compute/manager.py

@@ -2849,23 +2849,14 @@ class ComputeManager(manager.Manager):
 
         LOG.info("Rebuilding instance", instance=instance)
 
-        # NOTE(gyee): there are three possible scenarios.
-        #
-        #   1. instance is being rebuilt on the same node. In this case,
-        #      recreate should be False and scheduled_node should be None.
-        #   2. instance is being rebuilt on a node chosen by the
-        #      scheduler (i.e. evacuate). In this case, scheduled_node should
-        #      be specified and recreate should be True.
-        #   3. instance is being rebuilt on a node chosen by the user. (i.e.
-        #      force evacuate). In this case, scheduled_node is not specified
-        #      and recreate is set to True.
-        #
-        # For scenarios #2 and #3, we must do rebuild claim as server is
-        # being evacuated to a different node.
         rt = self._get_resource_tracker()
-        if recreate or scheduled_node is not None:
+        if recreate:
+            # This is an evacuation to a new host, so we need to perform a
+            # resource claim.
             rebuild_claim = rt.rebuild_claim
         else:
+            # This is a rebuild to the same host, so we don't need to make
+            # a claim since the instance is already on this host.
             rebuild_claim = claims.NopClaim
 
         image_meta = {}

nova/tests/functional/test_servers.py

@@ -1230,7 +1230,10 @@ class ServerRebuildTestCase(integrated_helpers._IntegratedTestBase,
             '/servers/%s/action' % server['id'], rebuild_req_body)
         self.assertIn('NoValidHost', six.text_type(ex))
 
-    def test_rebuild_with_new_image(self):
+    # A rebuild to the same host should never attempt a rebuild claim.
+    @mock.patch('nova.compute.resource_tracker.ResourceTracker.rebuild_claim',
+                new_callable=mock.NonCallableMock)
+    def test_rebuild_with_new_image(self, mock_rebuild_claim):
         """Rebuilds a server with a different image which will run it through
         the scheduler to validate the image is still OK with the compute host
         that the instance is running on.