problem with the way the config_string is parsed by memcached()

[Mikhail]

Symptoms:
I was trying to use UDP to communicate with the server and provided the following config_string.

const char *udpConfigString = "--SERVER=localhost:11211 --VERIFY-KEY --SUPPORT-CAS --USE-UDP";

The subsequent calls to memcached() and memcached_set() seem to succeed but server logs show that the opcode is garbage.

Analysis:
I looked through the code and it seems like the socket connection is setup as soon as the yacc parser reads --SERVER. Instead the entire config string should be parsed before the socket connection is established.

/* Line 1806 of yacc.c */
#line 230 "libmemcached/csl/parser.yy"
    {
            if (memcached_failed(context->rc= memcached_server_add_with_weight(context->memc, (yyvsp[(2) - (4)].server).c_str, (yyvsp[(3) - (4)].number), (yyvsp[(4) - (4)].number))))
            {
              char buffer[1024];
              snprintf(buffer, sizeof(buffer), "Failed to add server: %s:%u", (yyvsp[(2) - (4)].server).c_str, uint32_t((yyvsp[(3) - (4)].number)));
              parser_abort(context, buffer);
            }
            context->unset_server();
          }
    break;

Here the call to memcached_server_add_with_weight will create the socket and assume TCP.

Workaround:
In the config string specify --USE-UDP before --SERVER
const char *udpConfigString = "--VERIFY-KEY --SUPPORT-CAS --USE-UDP --SERVER=localhost:11211";

[Brian Aker (brianaker)]

You are right. The behavior was changed a while ago to speed up connections,... side effect is that UDP will not be set quite correctly.

This is fixable by delaying the connection until after parsing like you suggest.