[lldp-syncd] Fix unexpected exception in snmp-subagent (#64)

Separate deleted and changed scearios
deleted interfaces, delete their table key
changed interfaces
if lldp_rem_time_mark is only changed key, just use hset its new value. Don't delete interface table key.
If other keys are changed as well, delete interface key and use hmset to set the whole dict into DB, instead of using hset each item one by one. This will avoid interface exists but some of its keys are not added.

Signed-off-by: Zhaohui Sun <zhaohuisun@microsoft.com>

src/lldp_syncd/daemon.py

@@ -349,6 +349,26 @@ def cache_diff(self, cache, update):
 
         return new_keys, changed_keys, deleted_keys
 
+    def is_only_time_mark_modified(self, cached_interface, updated_interface):
+        """
+        Check if only lldp_rem_time_mark is modified in the update
+        :param cached_interface: Local cached interface dict
+        :param updated_interface: Updated interface dict
+        :return: True if only lldp_rem_time_mark is modified, False otherwise
+        """
+        if len(cached_interface) != len(updated_interface):
+            return False
+
+        changed_keys = 0
+
+        for key in cached_interface.keys():
+            if 'lldp_rem_time_mark' == key and cached_interface[key] != updated_interface[key]:
+                changed_keys += 1
+            elif key not in updated_interface or cached_interface[key] != updated_interface[key]:
+                return False
+
+        return True if changed_keys == 1 else False
+
     def sync(self, parsed_update):
         """
         Sync LLDP information to redis DB.
@@ -367,18 +387,32 @@ def sync(self, parsed_update):
                 logger.debug("sync'd: {}".format(json.dumps(chassis_update, indent=3)))
 
         new, changed, deleted = self.cache_diff(self.interfaces_cache, parsed_update)
+
+        # For changed elements, if only lldp_rem_time_mark changed, update its value, otherwise delete and repopulate
+        for interface in changed:
+            if re.match(SONIC_ETHERNET_RE_PATTERN, interface) is None:
+                logger.warning("Ignoring interface '{}'".format(interface))
+                continue
+            table_key = ':'.join([LldpSyncDaemon.LLDP_ENTRY_TABLE, interface])
+            if self.is_only_time_mark_modified(self.interfaces_cache[interface], parsed_update[interface]):
+                self.db_connector.set(self.db_connector.APPL_DB, table_key, 'lldp_rem_time_mark', parsed_update[interface]['lldp_rem_time_mark'], blocking=True)
+                logger.debug("Only sync'd interface {} lldp_rem_time_mark: {}".format(interface, parsed_update[interface]['lldp_rem_time_mark']))
+            else:
+                self.db_connector.delete(self.db_connector.APPL_DB, table_key)
+                self.db_connector.hmset(self.db_connector.APPL_DB, table_key, parsed_update[interface])
+                logger.debug("Sync'd changed interface {} : {}".format(interface, parsed_update[interface]))
         self.interfaces_cache = parsed_update
-        # Delete LLDP_ENTRIES which were modified or are missing
-        for interface in changed + deleted:
+        # Delete LLDP_ENTRIES which are missing
+        for interface in deleted:
             table_key = ':'.join([LldpSyncDaemon.LLDP_ENTRY_TABLE, interface])
             self.db_connector.delete(self.db_connector.APPL_DB, table_key)
-        # Repopulate LLDP_ENTRY_TABLE by adding all changed elements
-        for interface in changed + new:
+            logger.debug("Delete table_key: {}".format(table_key))
+        # Repopulate LLDP_ENTRY_TABLE by adding new elements
+        for interface in new:
             if re.match(SONIC_ETHERNET_RE_PATTERN, interface) is None:
                 logger.warning("Ignoring interface '{}'".format(interface))
                 continue
             # port_table_key = LLDP_ENTRY_TABLE:INTERFACE_NAME;
             table_key = ':'.join([LldpSyncDaemon.LLDP_ENTRY_TABLE, interface])
-            for k, v in parsed_update[interface].items():
-                self.db_connector.set(self.db_connector.APPL_DB, table_key, k, v, blocking=True)
-            logger.debug("sync'd: \n{}".format(json.dumps(parsed_update[interface], indent=3)))
+            self.db_connector.hmset(self.db_connector.APPL_DB, table_key, parsed_update[interface])
+            logger.debug("Add new interface {} : {}".format(interface, parsed_update[interface]))