Skip resize 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 instance exists
On other contitions the action will be FAILED in the pre_condition
check:
- Destination flavor does not exist

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

Change-Id: I1f6c8d99df3925556ea3ab22c81513dd0d4d7197
Signed-off-by: Alfredo Moralejo <amoralej@redhat.com>

doc/source/actions/resize.rst

@@ -23,3 +23,16 @@ parameter                type   required description
 ``flavor``               string yes      ID or Name of Flavor (Nova accepts
                                          either ID or Name)
 ======================== ====== ======== ===================================
+
+Skipping conditions
+--------------------
+
+Resize actions will be automatically skipped in the pre_condition phase in
+the following case:
+
+- The server does not exist
+
+On other condition the action will be FAILED in the pre_condition check:
+
+- Destination flavor does not exist
+

releasenotes/notes/resize-precondition-check-3d234a9000c3c93f.yaml

@@ -0,0 +1,15 @@
+---
+features:
+  - |
+    Added a pre-condition check to the resize action to prevent executing
+    it when required criteria are not met. In following condition,
+    the action status will be set to SKIPPED:
+
+    - Instance is not found
+
+    On other condition the action will be FAILED in the pre_condition
+    check:
+
+    - Destination flavor does not exist
+
+    This improves safety and correctness of migrations.

watcher/applier/actions/resize.py

@@ -18,7 +18,9 @@
 
 from oslo_log import log
 
+from watcher._i18n import _
 from watcher.applier.actions import base
+from watcher.common import exception
 from watcher.common import nova_helper
 
 LOG = log.getLogger(__name__)
@@ -97,9 +99,27 @@ class Resize(base.BaseAction):
         LOG.warning("revert not supported")
 
     def pre_condition(self):
-        # TODO(jed): check if the instance exists / check if the instance is on
-        # the source_node
-        pass
+        """Check resize preconditions
+
+        Skipping conditions:
+        - Instance does not exist
+        Failing conditions:
+        - Flavor does not exist
+        """
+        nova = nova_helper.NovaHelper(osc=self.osc)
+
+        # Check that the instance exists
+        try:
+            nova.find_instance(self.instance_uuid)
+        except exception.ComputeResourceNotFound:
+            raise exception.ActionSkipped(
+                _("Instance %s not found") % self.instance_uuid)
+
+        try:
+            nova.get_flavor_id(self.flavor)
+        except exception.ComputeResourceNotFound:
+            raise exception.ActionExecutionFailure(
+                _("Flavor %s not found") % self.flavor)
 
     def post_condition(self):
         # TODO(jed): check extra parameters (network response, etc.)

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

@@ -18,6 +18,7 @@ from unittest import mock
 from watcher.applier.actions import base as baction
 from watcher.applier.actions import resize
 from watcher.common import clients
+from watcher.common import exception
 from watcher.common import nova_helper
 from watcher.tests.unit import base
 
@@ -89,3 +90,29 @@ class TestResize(base.TestCase):
         self.action.execute()
         self.r_helper.resize_instance.assert_called_once_with(
             instance_id=self.INSTANCE_UUID, flavor='x1')
+
+    def test_pre_condition_success(self):
+        """Test pre_condition succeeds when instance and flavor exist"""
+        self.r_helper.find_instance.return_value = self.INSTANCE_UUID
+        self.r_helper.get_flavor_id.return_value = '123'
+        self.action.pre_condition()
+
+    def test_pre_condition_instance_not_found(self):
+        """Test pre_condition fails when instance doesn't exist"""
+        err = exception.ComputeResourceNotFound()
+        self.r_helper.find_instance.side_effect = err
+
+        self.assertRaisesRegex(
+            exception.ActionSkipped,
+            f"Instance {self.INSTANCE_UUID} not found",
+            self.action.pre_condition)
+
+    def test_pre_condition_flavor_not_found(self):
+        """Test pre_condition fails when flavor doesn't exist"""
+        err = exception.ComputeResourceNotFound()
+        self.r_helper.get_flavor_id.side_effect = err
+
+        self.assertRaisesRegex(
+            exception.ActionExecutionFailure,
+            "Flavor x1 not found",
+            self.action.pre_condition)