TC doesn't rise exception if device doesn't exist

If device doesn't exit, TC doesn't need to raise an exception
during the deletion of the qdisc.

In iproute2 source code [1], the error code used is "1".

[1] https://kernel.googlesource.com/pub/scm/linux/kernel/git/shemminger/iproute2/+/master/tc/tc_qdisc.c#188

Change-Id: I9abecd6ef9d74abff27fb0ed0f5f4535c0622fd6
Closes-Bug: #1697937

neutron/agent/linux/tc_lib.py

@@ -200,7 +200,9 @@ class TcCommand(ip_lib.IPDevice):
         # Return_code=2 is fine because it means
         # "RTNETLINK answers: No such file or directory" what is fine when we
         # are trying to delete qdisc
-        return self._execute_tc_cmd(cmd, extra_ok_codes=[2])
+        # Return_code=1 means "RTNETLINK answers: Cannot find device <device>".
+        # If the device doesn't exist, the qdisc is already deleted.
+        return self._execute_tc_cmd(cmd, extra_ok_codes=[1, 2])
 
     def _get_tbf_burst_value(self, bw_limit, burst_limit):
         min_burst_value = float(bw_limit) / float(self.kernel_hz)

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

@@ -218,7 +218,7 @@ class TestTcCommand(base.BaseTestCase):
                 run_as_root=True,
                 check_exit_code=True,
                 log_fail_as_error=True,
-                extra_ok_codes=[2]
+                extra_ok_codes=[1, 2]
             ),
             mock.call(
                 ['tc', 'qdisc', 'add', 'dev', DEVICE_NAME, "ingress",
@@ -263,7 +263,7 @@ class TestTcCommand(base.BaseTestCase):
             run_as_root=True,
             check_exit_code=True,
             log_fail_as_error=True,
-            extra_ok_codes=[2]
+            extra_ok_codes=[1, 2]
         )
 
     def test_delete_tbf_bw_limit(self):
@@ -273,7 +273,7 @@ class TestTcCommand(base.BaseTestCase):
             run_as_root=True,
             check_exit_code=True,
             log_fail_as_error=True,
-            extra_ok_codes=[2]
+            extra_ok_codes=[1, 2]
         )
 
     def test_get_ingress_qdisc_burst_value_burst_not_none(self):