Allow update request with current segment_id

If the segment_id in the update request equals the segment_id
already associated with the subnet being updated, there is no
need for further update related segment validations.

Closes-Bug: #1793391
Change-Id: I65cd5b92b219c14dd07ae72457af243756c4ed02
(cherry picked from commit 3b5373b4ef)

neutron/db/ipam_backend_mixin.py

@@ -315,7 +315,8 @@ class IpamBackendMixin(db_base_plugin_common.DbBasePluginCommon):
                         pool_2=r_range,
                         subnet_cidr=subnet_cidr)
 
-    def _validate_segment(self, context, network_id, segment_id, action=None):
+    def _validate_segment(self, context, network_id, segment_id, action=None,
+                          old_segment_id=None):
         query = context.session.query(models_v2.Subnet.segment_id)
         query = query.filter(models_v2.Subnet.network_id == network_id)
         associated_segments = set(row.segment_id for row in query)
@@ -323,7 +324,7 @@ class IpamBackendMixin(db_base_plugin_common.DbBasePluginCommon):
             raise segment_exc.SubnetsNotAllAssociatedWithSegments(
                 network_id=network_id)
 
-        if action == 'update':
+        if action == 'update' and old_segment_id != segment_id:
             # Check the current state of segments and subnets on the network
             # before allowing migration from non-routed to routed network.
             if query.count() > 1:

neutron/db/ipam_pluggable_backend.py

@@ -428,9 +428,12 @@ class IpamPluggableBackend(ipam_backend_mixin.IpamBackendMixin):
                                   port['fixed_ips'])
 
     def update_db_subnet(self, context, id, s, old_pools):
+        subnet = obj_subnet.Subnet.get_object(context, id=id)
+        old_segment_id = subnet.segment_id if subnet else None
         if 'segment_id' in s:
-            self._validate_segment(context, s['network_id'], s['segment_id'],
-                                   action='update')
+            self._validate_segment(
+                context, s['network_id'], s['segment_id'], action='update',
+                old_segment_id=old_segment_id)
         # 'allocation_pools' is removed from 's' in
         # _update_subnet_allocation_pools (ipam_backend_mixin),
         # so create unchanged copy for ipam driver

neutron/tests/unit/extensions/test_segment.py

@@ -540,6 +540,27 @@ class TestSegmentSubnetAssociation(SegmentTestCase):
         self.assertEqual(webob.exc.HTTPOk.code, response.status_int)
         self.assertEqual(res['subnet']['segment_id'], segment['id'])
 
+    def test_update_subnet_with_current_segment_id(self):
+        with self.network() as network:
+            net = network['network']
+
+        segment1 = self._test_create_segment(network_id=net['id'],
+                                            physical_network='phys_net1',
+                                            segmentation_id=200)['segment']
+        self._test_create_segment(network_id=net['id'],
+                                  physical_network='phys_net2',
+                                  segmentation_id=200)['segment']
+        with self.subnet(network=network, segment_id=segment1['id']) as subnet:
+            subnet = subnet['subnet']
+
+        data = {'subnet': {'segment_id': segment1['id']}}
+        request = self.new_update_request('subnets', data, subnet['id'])
+        response = request.get_response(self.api)
+        res = self.deserialize(self.fmt, response)
+
+        self.assertEqual(webob.exc.HTTPOk.code, response.status_int)
+        self.assertEqual(res['subnet']['segment_id'], segment1['id'])
+
     def test_associate_existing_subnet_fail_if_multiple_segments(self):
         with self.network() as network:
             net = network['network']