[OVN] Method `get_port_qos` should always return 2 values

The method ``neutron.agent.ovn.agent.ovsdb.get_port_qos`` should
always return 2 values, even when the LSP is not present. This
patch:
* fixes the output, returning 2 values
* catches the exception raised when the LSP is not present in the
  OVN NB database
* returns the default max_kbps and min_kbps values (0, 0).

Closes-Bug: #2009804
Change-Id: I03ea78fcc5825e35c07dda1aa790769aa918dd8a

neutron/agent/ovn/agent/ovsdb.py

@@ -160,9 +160,12 @@ def get_port_qos(nb_idl, port_id):
     this method is only returning the egress one. The min-bw rule is only
     implemented for egress traffic.
     """
-    lsp = nb_idl.lsp_get(port_id).execute(check_error=True)
-    if not lsp:
-        return {}
+    try:
+        lsp = nb_idl.lsp_get(port_id).execute(check_error=True)
+    except idlutils.RowNotFound:
+        # If the LSP is not present, we can't retrieve any QoS info. The
+        # default values are (0, 0).
+        return 0, 0
 
     net_name = lsp.external_ids[ovn_const.OVN_NETWORK_NAME_EXT_ID_KEY]
     ls = nb_idl.lookup('Logical_Switch', net_name)

neutron/tests/functional/agent/ovn/agent/test_ovsdb.py

@@ -69,9 +69,17 @@ class GetPortQosTestCase(base.TestOVNFunctionalBase):
         max_kbps, min_kbps = agent_ovsdb.get_port_qos(self.nb_api, lsp.name)
         self.assertEqual((max_qos_value, 0), (max_kbps, min_kbps))
 
-        # Remove the max-bw rukle
+        # Remove the max-bw rule
         ext_ids = {ovn_const.OVN_PORT_EXT_ID_KEY: lsp_name}
         self.nb_api.qos_del_ext_ids(
             network_name, ext_ids).execute(check_error=True)
         max_kbps, min_kbps = agent_ovsdb.get_port_qos(self.nb_api, lsp.name)
         self.assertEqual((0, 0), (max_kbps, min_kbps))
+
+        # Remove the port, the default values returned by the method are (0, 0)
+        lsp_name = lsp.name
+        self.nb_api.lsp_del(lsp_name).execute(check_error=True)
+        lsp = self.nb_api.lookup('Logical_Switch_Port', lsp_name, default=None)
+        self.assertIsNone(lsp)
+        max_kbps, min_kbps = agent_ovsdb.get_port_qos(self.nb_api, lsp_name)
+        self.assertEqual((0, 0), (max_kbps, min_kbps))