Check if namespace exists before getting devices

If a neutron router has no ports defined, its namespace is deleted
by the cronjob /etc/cron.d/neutron-l3-agent-netns-cleanup.
It is required to check if the namespace exists before calling
iproute2 commands against the namespace.
It is not enough for the string of the namespace uuid to be defined,
the namespace must really exist on the network node.
This patch checks if the namespace exists when calling get_devices().
Otherwise the agent log file will be flooded with messages like
Cannot open network namespace "qrouter-<uuid>": No such file or directory
Failed to process compatible router '<uuid>'

Related-bug: 1573073

Change-Id: I744ef11529f9da5cbfdb812de0b35b95f9d078bb
(cherry picked from commit 7675495f26)

neutron/agent/linux/ip_lib.py

@@ -125,12 +125,21 @@ class IPWrapper(SubProcessBase):
             # we call out manually because in order to avoid screen scraping
             # iproute2 we use find to see what is in the sysfs directory, as
             # suggested by Stephen Hemminger (iproute2 dev).
-            output = utils.execute(['ip', 'netns', 'exec', self.namespace,
-                                    'find', SYS_NET_PATH, '-maxdepth', '1',
-                                    '-type', 'l', '-printf', '%f '],
-                                   run_as_root=True,
-                                   log_fail_as_error=self.log_fail_as_error
-                                   ).split()
+            try:
+                cmd = ['ip', 'netns', 'exec', self.namespace,
+                       'find', SYS_NET_PATH, '-maxdepth', '1',
+                       '-type', 'l', '-printf', '%f ']
+                output = utils.execute(
+                    cmd,
+                    run_as_root=True,
+                    log_fail_as_error=self.log_fail_as_error).split()
+            except RuntimeError:
+                # We could be racing with a cron job deleting namespaces.
+                # Just return a empty list if the namespace is deleted.
+                with excutils.save_and_reraise_exception() as ctx:
+                    if not self.netns.exists(self.namespace):
+                        ctx.reraise = False
+                        return []
         else:
             output = (
                 i for i in os.listdir(SYS_NET_PATH)

neutron/tests/unit/agent/linux/test_ip_lib.py

@@ -282,6 +282,24 @@ class TestIpWrapper(base.BaseTestCase):
         self.assertTrue(fake_str.split.called)
         self.assertEqual(retval, [ip_lib.IPDevice('lo', namespace='foo')])
 
+    @mock.patch('neutron.agent.common.utils.execute')
+    def test_get_devices_namespaces_ns_not_exists(self, mocked_execute):
+        mocked_execute.side_effect = RuntimeError(
+            "Cannot open network namespace")
+        with mock.patch.object(ip_lib.IpNetnsCommand, 'exists',
+                               return_value=False):
+            retval = ip_lib.IPWrapper(namespace='foo').get_devices()
+            self.assertEqual([], retval)
+
+    @mock.patch('neutron.agent.common.utils.execute')
+    def test_get_devices_namespaces_ns_exists(self, mocked_execute):
+        mocked_execute.side_effect = RuntimeError(
+            "Cannot open network namespace")
+        with mock.patch.object(ip_lib.IpNetnsCommand, 'exists',
+                               return_value=True):
+            self.assertRaises(RuntimeError,
+                              ip_lib.IPWrapper(namespace='foo').get_devices)
+
     @mock.patch('neutron.agent.common.utils.execute')
     def test_get_devices_exclude_loopback_and_gre(self, mocked_execute):
         device_name = 'somedevice'