Fix API test for external subnet visibility

The test_external_network_visibility test incorrectly
assumes that subnets will never be visible on external
networks. However, this is not true in the case that a
network is both external and shared, in which case its
subnets will be shared and will be visible globally.

This updates the test to only make the subnet visibility
assertion on the configured public external network which
is stable and won't be in flux like networks created by
other tests.

At some point this test should just create its own external
network and make assertions on that rather than depend on
a specific configuration of the public network.

Related-Bug: #1553595
Change-Id: I0219e17d2ec70939039344826d35cb533b3ce065

tempest/api/network/test_networks.py

@@ -12,10 +12,9 @@
 #    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 #    License for the specific language governing permissions and limitations
 #    under the License.
-import itertools
-
 import netaddr
 import six
+import testtools
 
 from tempest.api.network import base
 from tempest.common import custom_matchers
@@ -376,6 +375,9 @@ def test_create_delete_subnet_all_attributes(self):
 
     @test.attr(type='smoke')
     @test.idempotent_id('af774677-42a9-4e4b-bb58-16fe6a5bc1ec')
+    @test.requires_ext(extension='external-net', service='network')
+    @testtools.skipUnless(CONF.network.public_network_id,
+                          'The public_network_id option must be specified.')
     def test_external_network_visibility(self):
         """Verifies user can see external networks but not subnets."""
         body = self.networks_client.list_networks(**{'router:external': True})
@@ -387,17 +389,12 @@ def test_external_network_visibility(self):
         self.assertEmpty(nonexternal, "Found non-external networks"
                                       " in filtered list (%s)." % nonexternal)
         self.assertIn(CONF.network.public_network_id, networks)
-
-        subnets_iter = (network['subnets']
-                        for network in body['networks']
-                        if not network['shared'])
-        # subnets_iter is a list (iterator) of lists. This flattens it to a
-        # list of UUIDs
-        public_subnets_iter = itertools.chain(*subnets_iter)
-        body = self.subnets_client.list_subnets()
-        subnets = [sub['id'] for sub in body['subnets']
-                   if sub['id'] in public_subnets_iter]
-        self.assertEmpty(subnets, "Public subnets visible")
+        # only check the public network ID because the other networks may
+        # belong to other tests and their state may have changed during this
+        # test
+        body = self.subnets_client.list_subnets(
+            network_id=CONF.network.public_network_id)
+        self.assertEmpty(body['subnets'], "Public subnets visible")
 
 
 class BulkNetworkOpsTestJSON(base.BaseNetworkTest):