Skip change_nova_service_state actions in pre_condition phase

This patch is implementing detection of certain conditions and moving
the action to SKIPPED status so that, execution is not started. We will
check that:

- The specified nova-compute service exist
- The spedified nova-compute service is in the desired state

Implements: blueprint skip-actions-in-pre-condition

Change-Id: I0fa0869468b2f4478b592a59bc301abb7618ec74
Signed-off-by: Alfredo Moralejo <amoralej@redhat.com>

doc/source/actions/change_nova_service_state.rst

@@ -30,3 +30,12 @@ parameter                type   required description
                                          ``watcher_disabled`` or
                                          ``watcher_maintaining``
 ======================== ====== ======== ===================================
+
+Skipping conditions
+--------------------
+
+Change nova service state actions will be automatically skipped in the
+pre_condition phase in the following cases:
+
+- nova-compute service does not exist
+- nova-compute service is already in the desired state (enabled or disabled)

releasenotes/notes/change-nova-service-precondition-check-8c87bc04a4fd3d0f.yaml

@@ -0,0 +1,9 @@
+---
+features:
+  - |
+    Added a pre-condition check to the change_nova_service_state action
+    to prevent executing it when required criteria are not met. In the
+    following conditions, the action status will be set to SKIPPED:
+
+    - The specified nova-compute service is not found
+    - The specified nova-compute service is in the desired state

watcher/applier/actions/change_nova_service_state.py

@@ -118,7 +118,22 @@ class ChangeNovaServiceState(base.BaseAction):
             return nova.disable_service_nova_compute(self.host, self.reason)
 
     def pre_condition(self):
-        pass
+        """Check change_nova_service_state preconditions
+
+        Skipping conditions:
+        - nova-compute service does not exist
+        - nova-compute service is already in the desired state
+        """
+        nova = nova_helper.NovaHelper(osc=self.osc)
+        services = nova.get_service_list()
+        service = next((s for s in services if s.host == self.host), None)
+        if service is None:
+            raise exception.ActionSkipped(
+                _("nova-compute service %s not found") % self.host)
+        if service.status == self.state:
+            raise exception.ActionSkipped(
+                _("nova-compute service %s is already in state %s") %
+                (self.host, self.state))
 
     def post_condition(self):
         pass

watcher/tests/unit/applier/actions/test_change_nova_service_state.py

@@ -20,12 +20,14 @@ import jsonschema
 from watcher.applier.actions import base as baction
 from watcher.applier.actions import change_nova_service_state
 from watcher.common import clients
+from watcher.common import exception
 from watcher.common import nova_helper
 from watcher.decision_engine.model import element
 from watcher.tests.unit import base
+from watcher.tests.unit.common import utils as test_utils
 
 
-class TestChangeNovaServiceState(base.TestCase):
+class TestChangeNovaServiceState(test_utils.NovaResourcesMixin, base.TestCase):
 
     def setUp(self):
         super().setUp()
@@ -87,11 +89,53 @@ class TestChangeNovaServiceState(base.TestCase):
         self.assertRaises(jsonschema.ValidationError,
                           self.action.validate_parameters)
 
-    def test_change_service_state_pre_condition(self):
-        try:
-            self.action.pre_condition()
-        except Exception as exc:
-            self.fail(exc)
+    def test_change_service_state_pre_condition_enable(self):
+        svc1 = nova_helper.Service.from_openstacksdk(
+            self.create_openstacksdk_service(
+                host='compute-1',
+                status=element.ServiceState.DISABLED.value)
+        )
+        self.m_helper.get_service_list.return_value = [svc1]
+        self.action.pre_condition()
+        self.m_helper.get_service_list.assert_called_once_with()
+
+    def test_change_service_state_pre_condition_disable(self):
+        self.action.input_parameters["state"] = (
+            element.ServiceState.DISABLED.value)
+        svc1 = nova_helper.Service.from_openstacksdk(
+            self.create_openstacksdk_service(
+                host='compute-1',
+                status=element.ServiceState.ENABLED.value)
+        )
+        self.m_helper.get_service_list.return_value = [svc1]
+        self.action.pre_condition()
+        self.m_helper.get_service_list.assert_called_once_with()
+
+    def test_change_service_state_pre_condition_skipped_not_found(self):
+        svc1 = nova_helper.Service.from_openstacksdk(
+            self.create_openstacksdk_service(
+                host='compute-2',
+                status=element.ServiceState.DISABLED.value)
+        )
+        self.m_helper.get_service_list.return_value = [svc1]
+        self.assertRaisesRegex(
+            exception.ActionSkipped,
+            "nova-compute service compute-1 not found",
+            self.action.pre_condition)
+        self.m_helper.get_service_list.assert_called_once_with()
+
+    def test_change_service_state_pre_condition_skipped_state(self):
+        svc1 = nova_helper.Service.from_openstacksdk(
+            self.create_openstacksdk_service(
+                host='compute-1',
+                status=element.ServiceState.ENABLED.value)
+        )
+        self.m_helper.get_service_list.return_value = [svc1]
+        self.assertRaisesRegex(
+            exception.ActionSkipped,
+            "nova-compute service compute-1 is already in state enabled",
+            self.action.pre_condition)
+        self.m_helper.get_service_list.assert_called_once_with()
 
     def test_change_service_state_post_condition(self):
         try: