[port_util] Fix issue: port_util.get_vlan_interface_oid_map should no…

…t raise exception when DB has not RIF data (#117)

**Why I did this?**

port_util.get_vlan_interface_oid_map would raise exception when there is no RIF configured, and it would causes snmpagent not work. This PR is to fix it.

```
#012Traceback (most recent call last):
#012File "/usr/local/lib/python3.7/dist-packages/ax_interface/mib.py", line 37, in start
#12    self.reinit_data()#12  File "/usr/local/lib/python3.7/dist-packages/sonic_ax_impl/mibs/ietf/rfc1213.py", line 233, in reinit_data
#12    self.vlan_oid_name_map = Namespace.get_sync_d_from_all_namespace(mibs.init_sync_d_vlan_tables, self.db_conn)
#12  File "/usr/local/lib/python3.7/dist-packages/sonic_ax_impl/mibs/__init__.py", line 651, in get_sync_d_from_all_namespace
#12    ns_tuple = per_namespace_func(db_conn)
#12  File "/usr/local/lib/python3.7/dist-packages/sonic_ax_impl/mibs/__init__.py", line 341, in init_sync_d_vlan_tables
#12    vlan_name_map = port_util.get_vlan_interface_oid_map(db_conn)
#12  File "/usr/local/lib/python3.7/dist-packages/swsssdk/port_util.py", line 167, in get_vlan_interface_oid_map
#12    rif_name_map = db.get_all('COUNTERS_DB', 'COUNTERS_RIF_NAME_MAP', blocking=True)
#12  File "/usr/lib/python3/dist-packages/swsscommon/swsscommon.py", line 1751, in get_all
#12    return dict(super(SonicV2Connector, self).get_all(db_name, _hash, blocking))
#12  File "/usr/lib/python3/dist-packages/swsscommon/swsscommon.py", line 1708, in get_all
#12    return _swsscommon.SonicV2Connector_Native_get_all(self, db_name, _hash, blocking)
#012RuntimeError: Key '{COUNTERS_RIF_NAME_MAP}' unavailable in database '{COUNTERS_DB}'
```

**How I did this:**
check redis key exists first before calling get_all.

src/swsssdk/port_util.py

@@ -164,13 +164,14 @@ def get_rif_port_map(db):
 
     return rif_port_oid_map
 
-def get_vlan_interface_oid_map(db):
+def get_vlan_interface_oid_map(db, blocking=True):
     """
         Get Vlan Interface names and sai oids
     """
     db.connect('COUNTERS_DB')
-    rif_name_map = db.get_all('COUNTERS_DB', 'COUNTERS_RIF_NAME_MAP', blocking=True)
-    rif_type_name_map = db.get_all('COUNTERS_DB', 'COUNTERS_RIF_TYPE_MAP', blocking=True)
+
+    rif_name_map = db.get_all('COUNTERS_DB', 'COUNTERS_RIF_NAME_MAP', blocking=blocking)
+    rif_type_name_map = db.get_all('COUNTERS_DB', 'COUNTERS_RIF_TYPE_MAP', blocking=blocking)
 
     if not rif_name_map or not rif_type_name_map:
         return {}

test/test_port_util.py

@@ -0,0 +1,19 @@
+import os
+import sys
+
+if sys.version_info.major == 3:
+    from unittest import mock
+else:
+    import mock
+
+modules_path = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+sys.path.insert(0, os.path.join(modules_path, 'src'))
+
+class TestPortUtil:
+    def test_get_vlan_interface_oid_map(self):
+        db = mock.MagicMock()
+        db.get_all = mock.MagicMock()
+        db.get_all.return_value = {}
+
+        from swsssdk.port_util import get_vlan_interface_oid_map
+        assert not get_vlan_interface_oid_map(db, True)