Provide dry-run flag to validate deployment requirements

Nova may want to validate that the setup requirements
needed by the auto_allocate feature are met before
issuing the actual request to provision resources.

This patch adds this validation API.

Partial-implements: blueprint get-me-a-network

Change-Id: I9fb26e752650569fc5692ff8afda82a077647f7a

neutron/services/auto_allocate/db.py

@@ -39,6 +39,7 @@ from neutron.services.auto_allocate import models
 
 LOG = logging.getLogger(__name__)
 IS_DEFAULT = 'is_default'
+CHECK_REQUIREMENTS = 'dry-run'
 
 
 def _extend_external_network_default(self, net_res, net_db):
@@ -91,6 +92,15 @@ class AutoAllocatedTopologyMixin(common_db_mixin.CommonDbMixin):
 
         The topology will be provisioned upon return, if network is missing.
         """
+        if CHECK_REQUIREMENTS in fields:
+            # for dry-run requests, simply validates that subsequent
+            # requests can be fullfilled based on a set of requirements
+            # such as existence of default networks, pools, etc.
+            return self._check_requirements(context)
+        elif fields:
+            raise n_exc.BadRequest(resource='auto_allocate',
+                msg=_("Unrecognized field"))
+
         tenant_id = self._validate(context, tenant_id)
         # Check for an existent topology
         network_id = self._get_auto_allocated_network(context, tenant_id)
@@ -123,6 +133,16 @@ class AutoAllocatedTopologyMixin(common_db_mixin.CommonDbMixin):
                 constants.L3_ROUTER_NAT)
         return self._l3_plugin
 
+    def _check_requirements(self, context):
+        """Raise if requirements are not met."""
+        self._get_default_external_network(context)
+        try:
+            self._get_supported_subnetpools(context)
+        except n_exc.NotFound:
+            raise exceptions.AutoAllocationFailure(
+                reason=_("No default subnetpools defined"))
+        return {'id': 'dry-run=pass'}
+
     def _validate(self, context, tenant_id):
         """Validate and return the tenant to be associated to the topology."""
         if tenant_id == 'None':

neutron/tests/unit/services/auto_allocate/test_db.py

@@ -17,10 +17,10 @@ from neutron.common import exceptions as n_exc
 from neutron import context
 from neutron.services.auto_allocate import db
 from neutron.services.auto_allocate import exceptions
-from neutron.tests import base
+from neutron.tests.unit import testlib_api
 
 
-class AutoAllocateTestCase(base.BaseTestCase):
+class AutoAllocateTestCase(testlib_api.SqlTestCaseLight):
 
     def setUp(self):
         super(AutoAllocateTestCase, self).setUp()
@@ -46,3 +46,35 @@ class AutoAllocateTestCase(base.BaseTestCase):
             mock_cleanup.assert_called_once_with(
                 self.ctx, network_id='network_foo',
                 router_id='router_foo', subnets=[])
+
+    def test_get_auto_allocated_topology_dry_run_happy_path_for_kevin(self):
+        with mock.patch.object(self.mixin, '_check_requirements') as f:
+            self.mixin.get_auto_allocated_topology(
+                self.ctx, mock.ANY, fields=['dry-run'])
+            self.assertEqual(1, f.call_count)
+
+    def test_get_auto_allocated_topology_dry_run_bad_input(self):
+        self.assertRaises(n_exc.BadRequest,
+            self.mixin.get_auto_allocated_topology,
+            self.ctx, mock.ANY, fields=['foo'])
+
+    def test__check_requirements_fail_on_missing_ext_net(self):
+        self.assertRaises(exceptions.AutoAllocationFailure,
+            self.mixin._check_requirements, self.ctx)
+
+    def test__check_requirements_fail_on_missing_pools(self):
+        with mock.patch.object(
+            self.mixin, '_get_default_external_network'),\
+            mock.patch.object(
+                self.mixin, '_get_supported_subnetpools') as g:
+            g.side_effect = n_exc.NotFound()
+            self.assertRaises(exceptions.AutoAllocationFailure,
+                self.mixin._check_requirements, self.ctx)
+
+    def test__check_requirements_happy_path_for_kevin(self):
+        with mock.patch.object(
+            self.mixin, '_get_default_external_network'),\
+            mock.patch.object(
+                self.mixin, '_get_supported_subnetpools'):
+            result = self.mixin._check_requirements(self.ctx)
+            self.assertEqual(list(result.values())[0], 'dry-run=pass')