Bypass queries which cause a contradiction

This addresses the cause of the log lines like:

SAWarning: The IN-predicate on "instance_system_metadata.instance_uuid"
was invoked with an empty sequence. This results in a contradiction,
which nonetheless can be expensive to evaluate. Consider alternative
strategies for improved performance.

for instance_metadata, instance_system_metadata, and instance_faults.

It especially helps the case of a call to /servers/detail when there are
no servers to list because each of those tables is scanned despite a
guarantee that there will be now rows.

Bug 1204916

Change-Id: I62e65c4409d3e3ab8373eb66450414565c222547

nova/db/sqlalchemy/api.py

@@ -4186,6 +4186,8 @@ def cell_get_all(context):
 # User-provided metadata
 
 def _instance_metadata_get_multi(context, instance_uuids, session=None):
+    if not instance_uuids:
+        return []
     return model_query(context, models.InstanceMetadata,
                        session=session).\
                     filter(
@@ -4257,6 +4259,8 @@ def instance_metadata_update(context, instance_uuid, metadata, delete,
 
 
 def _instance_system_metadata_get_multi(context, instance_uuids, session=None):
+    if not instance_uuids:
+        return []
     return model_query(context, models.InstanceSystemMetadata,
                        session=session).\
                     filter(
@@ -4885,6 +4889,9 @@ def instance_fault_create(context, values):
 
 def instance_fault_get_by_instance_uuids(context, instance_uuids):
     """Get all instance faults for the provided instance_uuids."""
+    if not instance_uuids:
+        return {}
+
     rows = model_query(context, models.InstanceFault, read_deleted='no').\
                        filter(models.InstanceFault.instance_uuid.in_(
                            instance_uuids)).\

nova/tests/db/test_db_api.py

@@ -1349,6 +1349,29 @@ class InstanceTestCase(test.TestCase, ModelsObjectComparatorMixin):
         filtered_instances = db.instance_get_all_by_filters(self.ctxt, {})
         self._assertEqualListsOfInstances(instances, filtered_instances)
 
+    def test_instance_metadata_get_multi(self):
+        uuids = [self.create_instance_with_args()['uuid'] for i in range(3)]
+        meta = sqlalchemy_api._instance_metadata_get_multi(self.ctxt, uuids)
+        for row in meta:
+            self.assertTrue(row['instance_uuid'] in uuids)
+
+    def test_instance_metadata_get_multi_no_uuids(self):
+        self.mox.StubOutWithMock(query.Query, 'filter')
+        self.mox.ReplayAll()
+        sqlalchemy_api._instance_metadata_get_multi(self.ctxt, [])
+
+    def test_instance_system_system_metadata_get_multi(self):
+        uuids = [self.create_instance_with_args()['uuid'] for i in range(3)]
+        sys_meta = sqlalchemy_api._instance_system_metadata_get_multi(
+                self.ctxt, uuids)
+        for row in sys_meta:
+            self.assertTrue(row['instance_uuid'] in uuids)
+
+    def test_instance_system_metadata_get_multi_no_uuids(self):
+        self.mox.StubOutWithMock(query.Query, 'filter')
+        self.mox.ReplayAll()
+        sqlalchemy_api._instance_system_metadata_get_multi(self.ctxt, [])
+
     def test_instance_get_all_by_filters_regex(self):
         i1 = self.create_instance_with_args(display_name='test1')
         i2 = self.create_instance_with_args(display_name='teeeest2')
@@ -2268,6 +2291,12 @@ class InstanceFaultTestCase(test.TestCase, ModelsObjectComparatorMixin):
         expected = {uuid: []}
         self.assertEqual(expected, faults)
 
+    def test_instance_faults_get_by_instance_uuids_no_uuids(self):
+        self.mox.StubOutWithMock(query.Query, 'filter')
+        self.mox.ReplayAll()
+        faults = db.instance_fault_get_by_instance_uuids(self.ctxt, [])
+        self.assertEqual({}, faults)
+
 
 class InstanceTypeTestCase(BaseInstanceTypeTestCase):