Fix zone migration instance not found issue

When retrieving the list of instances and volumes to propose a
solution, the zone migration strategy can raise an exception for
instance or volume not found, which will make the audit goes to a
failure state. This fix maintains the logic of listing all elements
directly from the client (nova) but now checks if the instance
is already in the model. The storage model check was already fixed
in another patch[1].

[1] cb6fb16097

Closes-Bug: #2098984
Assisted-By: Cursor (claude-3.5-sonnet)

Change-Id: I4c8993f051b797104172047eaae1fe1523eaf7eb
Signed-off-by: Douglas Viroel <viroel@gmail.com>

releasenotes/notes/bug-2098984-zone-migration-instance-not-found-b9ce947dd3e7e020.yaml

@@ -0,0 +1,9 @@
+---
+fixes:
+  - |
+    Fixes a bug in the zone migration strategy where audits would fail due to
+    an unhandled exception when trying to plan instance migration that exist
+    in Nova but not in Watcher's compute model.
+    The strategy now filters out the elements that are not found in the model,
+    allowing the audit to complete successfully. For more details, please see
+    `Bug #2098984 <https://bugs.launchpad.net/watcher/+bug/2098984>`_.

watcher/decision_engine/strategy/strategies/zone_migration.py

@@ -540,7 +540,7 @@ class ZoneMigration(base.ZoneMigrationBaseStrategy):
 
         return [i for i in self.nova.get_instance_list()
                 if getattr(i, 'OS-EXT-SRV-ATTR:host') in src_node_list and
-                self.compute_model.get_instance_by_uuid(i.id)]
+                self.compute_model.has_node(i.id)]
 
     def get_volumes(self):
         """Get migrate target volumes

watcher/tests/decision_engine/strategy/strategies/test_zone_migration.py

@@ -17,7 +17,6 @@ from unittest import mock
 
 import cinderclient
 import novaclient
-from watcher.common import exception as watcher_exception
 from watcher.common import utils
 from watcher.decision_engine.strategy import strategies
 from watcher.tests.decision_engine.model import faker_cluster_state
@@ -168,15 +167,9 @@ class TestZoneMigration(TestBaseStrategy):
         # Mock nova helper to return our test instance
         self.m_n_helper.get_instance_list.return_value = [instance_on_src1]
 
-        # FIXME(dviroel): Fix this test together with bug #2098984
-        # It should return an empty list and not raise an exception
         # Verify that the instance does not exist in the model
-        # instances = self.strategy.get_instances()
-        # self.assertEqual([], instances)
-
-        self.assertRaises(
-            watcher_exception.InstanceNotFound,
-            self.strategy.get_instances)
+        instances = self.strategy.get_instances()
+        self.assertEqual([], instances)
 
     def test_get_volumes_with_volume_not_found(self):
         # Create a test volume without an known id