sonic-mgmt: Fix namespace issues for qos tests on T2 single ASIC

We are seeing UnboundLocalError when running sonic-mgmt tests against
a single-ASIC linecard:

```
UnboundLocalError: local variable 'dst_sys_port_id' referenced before assignment
```

Upon further investigation, this was determined to be happening
because a previous attempt to fix this issue (PR #13700) completely
omitted the ASIC prefix, but the entries in SYSTEM_PORT in config_db
do have an Asic0 prefix even on a single ASIC DUT.

Resolve this by specifically adding the Asic0 prefix in the case of a
single-ASIC T2 DUT, instead of leaving the prefix out.

Tested by manually running qos tests on a T2 single ASIC DUT with
these changes.

tests/qos/qos_sai_base.py

@@ -1083,6 +1083,7 @@ def dutConfig(
             dutPortIps[src_dut_index][src_asic_index] = {}
             sysPortMap[src_dut_index][src_asic_index] = {}
             active_ips = src_asic.get_active_ip_interfaces(tbinfo)
+            src_namespace_prefix = src_asic.namespace + '|' if src_asic.namespace else f'Asic{src_asic.asic_index}|'
             for iface, addr in active_ips.items():
                 if iface.startswith("Ethernet") and ("Ethernet-Rec" not in iface):
                     portIndex = src_mgFacts["minigraph_ptf_indices"][iface]
@@ -1091,7 +1092,7 @@ def dutConfig(
                     # Map port IDs to system port for dnx chassis
                     if 'platform_asic' in get_src_dst_asic_and_duts["src_dut"].facts and \
                             get_src_dst_asic_and_duts["src_dut"].facts['platform_asic'] == 'broadcom-dnx':
-                        sys_key = src_asic.namespace + '|' + iface if src_asic.namespace else iface
+                        sys_key = src_namespace_prefix + iface
                         if sys_key in src_system_port:
                             system_port = src_system_port[sys_key]['system_port_id']
                             sysPort = {'port': iface, 'system_port': system_port, 'port_type': iface}
@@ -1108,7 +1109,7 @@ def dutConfig(
                     if 'platform_asic' in get_src_dst_asic_and_duts["src_dut"].facts and \
                             get_src_dst_asic_and_duts["src_dut"].facts['platform_asic'] == 'broadcom-dnx':
                         for portName in src_mgFacts["minigraph_portchannels"][iface]["members"]:
-                            sys_key = src_asic.namespace + '|' + portName if src_asic.namespace else portName
+                            sys_key = src_namespace_prefix + portName
                             port_Index = src_mgFacts["minigraph_ptf_indices"][portName]
                             if sys_key in src_system_port:
                                 system_port = src_system_port[sys_key]['system_port_id']
@@ -1126,6 +1127,7 @@ def dutConfig(
                     dutPortIps[dst_dut_index] = {}
                     testPortIds[dst_dut_index] = {}
                     sysPortMap[dst_dut_index] = {}
+                    dst_system_port = {}
                     if 'platform_asic' in get_src_dst_asic_and_duts["src_dut"].facts and \
                             get_src_dst_asic_and_duts["src_dut"].facts['platform_asic'] == 'broadcom-dnx':
                         dst_system_port = dst_dut.config_facts(host=dst_dut.hostname, source='running')[
@@ -1135,6 +1137,7 @@ def dutConfig(
                     dst_system_port = src_system_port
                 dutPortIps[dst_dut_index][dst_asic_index] = {}
                 sysPortMap[dst_dut_index][dst_asic_index] = {}
+                dst_namespace_prefix = dst_asic.namespace + '|' if dst_asic.namespace else f'Asic{dst_asic.asic_index}|'
                 active_ips = dst_asic.get_active_ip_interfaces(tbinfo)
                 for iface, addr in active_ips.items():
                     if iface.startswith("Ethernet") and ("Ethernet-Rec" not in iface):
@@ -1144,7 +1147,7 @@ def dutConfig(
                         # Map port IDs to system port IDs
                         if 'platform_asic' in get_src_dst_asic_and_duts["src_dut"].facts and \
                                 get_src_dst_asic_and_duts["src_dut"].facts['platform_asic'] == 'broadcom-dnx':
-                            sys_key = dst_asic.namespace + '|' + iface if dst_asic.namespace else iface
+                            sys_key = dst_namespace_prefix + iface
                             if sys_key in dst_system_port:
                                 system_port = dst_system_port[sys_key]['system_port_id']
                                 sysPort = {'port': iface, 'system_port': system_port, 'port_type': iface}
@@ -1161,7 +1164,7 @@ def dutConfig(
                         if 'platform_asic' in get_src_dst_asic_and_duts["src_dut"].facts and \
                                 get_src_dst_asic_and_duts["src_dut"].facts['platform_asic'] == 'broadcom-dnx':
                             for portName in dst_mgFacts["minigraph_portchannels"][iface]["members"]:
-                                sys_key = dst_asic.namespace + '|' + portName if dst_asic.namespace else portName
+                                sys_key = dst_namespace_prefix + portName
                                 port_Index = dst_mgFacts["minigraph_ptf_indices"][portName]
                                 if sys_key in dst_system_port:
                                     system_port = dst_system_port[sys_key]['system_port_id']

tests/saitests/py3/sai_qos_tests.py

@@ -3965,10 +3965,9 @@ def runTest(self):
             self.dst_client, asic_type, port_list['dst'][dst_port_id])
         # for t2 chassis
         if platform_asic and platform_asic == "broadcom-dnx":
-            if dst_port_id in dst_sys_port_ids:
-                for port_id, sysport in dst_sys_port_ids.items():
-                    if dst_port_id == port_id:
-                        dst_sys_port_id = int(sysport)
+            assert dst_port_id in dst_sys_port_ids, \
+                "dst_port_id does not have a sys port id configured"
+            dst_sys_port_id = int(dst_sys_port_ids[dst_port_id])
             log_message("actual dst_sys_port_id: {}".format(dst_sys_port_id), to_stderr=True)
             voq_list = sai_thrift_get_voq_port_id(self.src_client, dst_sys_port_id)
             voq_queue_counters_base = sai_thrift_read_port_voq_counters(self.src_client, voq_list)