Raise specific exception when swapping migration allocations fails

If we fail to find resource allocations for an instance on the
source compute node when swapping allocations for a cold migrate /
resize operation, we need to raise a specific exception which will
eventually be a 500 internal error from the REST API. Using
InstanceUnacceptable is incorrect since it extends Invalid which is
handled in the resize REST API controller code and returned as a 400
bad request with the message "Instance image invalid" which is not
at all the actual failure.

As for the actual reason why the instance allocation on the source
node is not found, that is a different bug.

Change-Id: Id8e2dcf21776f8237a8b63acb23787bb42c9bd13
Closes-Bug: #1729356

nova/conductor/manager.py

@@ -251,7 +251,7 @@ class ComputeTaskManager(base.Base):
         exception.MigrationPreCheckError,
         exception.MigrationPreCheckClientException,
         exception.LiveMigrationWithOldNovaNotSupported,
-        exception.InstanceUnacceptable,
+        exception.ConsumerAllocationNotFound,
         exception.UnsupportedPolicyException)
     @targets_cell
     @wrap_instance_event(prefix='conductor')

nova/conductor/tasks/migrate.py

@@ -47,13 +47,11 @@ def replace_allocation_with_migration(context, instance, migration):
     orig_alloc = reportclient.get_allocations_for_consumer_by_provider(
         source_cn.uuid, instance.uuid)
     if not orig_alloc:
-        LOG.error('Unable to find existing allocations for instance',
+        LOG.error('Unable to find existing allocations for instance on '
+                  'source compute node: %s', source_cn.uuid,
                   instance=instance)
-        # A generic error like this will just error out the migration
-        # and do any rollback required
-        raise exception.InstanceUnacceptable(
-            instance_id=instance.uuid,
-            reason=_('Instance has no source node allocation'))
+        raise exception.ConsumerAllocationNotFound(
+            consumer_id=instance.uuid, provider_uuid=source_cn.uuid)
 
     # FIXME(danms): Since we don't have an atomic operation to adjust
     # allocations for multiple consumers, we have to have space on the

nova/exception.py

@@ -2128,6 +2128,13 @@ class InvalidAllocationConstraintsViolated(InvalidInventory):
                 "amount would violate inventory constraints.")
 
 
+class ConsumerAllocationNotFound(NovaException):
+    # This is intentionally not extending NotFound because if this
+    # leaks to the REST API it should be considered a 500 internal error.
+    msg_fmt = _('Consumer %(consumer_id)s has no resource allocation '
+                'on provider %(provider_uuid)s.')
+
+
 class UnsupportedPointerModelRequested(Invalid):
     msg_fmt = _("Pointer model '%(model)s' requested is not supported by "
                 "host.")

nova/tests/unit/conductor/tasks/test_migrate.py

@@ -210,7 +210,7 @@ class MigrationTaskAllocationUtils(test.NoDBTestCase):
         instance = objects.Instance(uuid=uuids.instance,
                                     host='host', node='node')
 
-        self.assertRaises(exception.InstanceUnacceptable,
+        self.assertRaises(exception.ConsumerAllocationNotFound,
                           migrate.replace_allocation_with_migration,
                           mock.sentinel.context,
                           instance, migration)