python: idl: Fix last-id update from a monitor reply.

While sending a reply to the monitor_cond_since request, server
includes the last transaction ID.  And it sends new IDs with each
subsequent update.  Current implementation doesn't use the one
supplied with a monitor reply, and only takes into account IDs
provided with monitor updates.  That may cause various issues:

1. Performance: During initialization, the last-id is set to zero.
   If re-connection will happen after receiving a monitor reply,
   but before any monitor update, the client will send a new
   monitor request with an all-zero last-id and will re-download
   the whole database again.

2. Data inconsistency: Assuming one of the clients sends a
   transaction, but our python client disconnects before receiving
   a monitor update for this transaction.  The last-id will point
   to a database state before this transaction.  On re-connection,
   this last-id will be sent and the monitor reply will contain
   a diff with a new data from that transaction.  But if another
   disconnection happens right after that, on second re-connection
   our python client will send another monitor_cond_since with
   exactly the same last-id.  That will cause receiving the same
   set of updates again.  And since it's an update2 message with
   a diff of the data, the client will remove previously applied
   result of the transaction.  At this point it will have a
   different database view with the server potentially leading
   to all sorts of data inconsistency problems.

Fix that by always updating the last-id from the latest monitor
reply.

Fixes: 46d44cf3be ("python: idl: Add monitor_cond_since support.")
Acked-by: Simon Horman <horms@ovn.org>
Acked-by: Han Zhou <hzhou@ovn.org>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

python/ovs/db/idl.py

@@ -494,6 +494,7 @@ class Idl(object):
                         if not msg.result[0]:
                             self.__clear()
                         self.__parse_update(msg.result[2], OVSDB_UPDATE3)
+                        self.last_id = msg.result[1]
                     elif self.state == self.IDL_S_DATA_MONITOR_COND_REQUESTED:
                         self.__clear()
                         self.__parse_update(msg.result, OVSDB_UPDATE2)