Something seems to be broken once we exceed the back_log count. Once getting drizzleslap to hang, I dumped the active sockets for each process:

eday@orisndriz01:~/bzr/drizzle/eday-dev/tests$ lsof -p 32531 -i4 -a
COMMAND     PID USER   FD   TYPE   DEVICE SIZE NODE NAME
drizzlesl 32531 eday    3u  IPv4 10320113       TCP localhost:54858->localhost:9306 (CLOSE_WAIT)
drizzlesl 32531 eday  185u  IPv4 10324113       TCP localhost:56240->localhost:9306 (ESTABLISHED)
drizzlesl 32531 eday  186u  IPv4 10324115       TCP localhost:56241->localhost:9306 (ESTABLISHED)
drizzlesl 32531 eday  187u  IPv4 10324117       TCP localhost:56242->localhost:9306 (ESTABLISHED)
drizzlesl 32531 eday  188u  IPv4 10324119       TCP localhost:56243->localhost:9306 (ESTABLISHED)
drizzlesl 32531 eday  189u  IPv4 10324121       TCP localhost:56244->localhost:9306 (ESTABLISHED)
drizzlesl 32531 eday  190u  IPv4 10324123       TCP localhost:56245->localhost:9306 (ESTABLISHED)
drizzlesl 32531 eday  191u  IPv4 10324125       TCP localhost:56246->localhost:9306 (ESTABLISHED)
drizzlesl 32531 eday  192u  IPv4 10324127       TCP localhost:56247->localhost:9306 (ESTABLISHED)
drizzlesl 32531 eday  193u  IPv4 10324129       TCP localhost:56248->localhost:9306 (ESTABLISHED)
drizzlesl 32531 eday  194u  IPv4 10324131       TCP localhost:56249->localhost:9306 (ESTABLISHED)
drizzlesl 32531 eday  195u  IPv4 10324133       TCP localhost:56250->localhost:9306 (ESTABLISHED)
drizzlesl 32531 eday  196u  IPv4 10324135       TCP localhost:56251->localhost:9306 (ESTABLISHED)
drizzlesl 32531 eday  197u  IPv4 10324137       TCP localhost:56252->localhost:9306 (ESTABLISHED)
drizzlesl 32531 eday  198u  IPv4 10324139       TCP localhost:56253->localhost:9306 (ESTABLISHED)
drizzlesl 32531 eday  199u  IPv4 10324141       TCP localhost:56254->localhost:9306 (ESTABLISHED)
drizzlesl 32531 eday  200u  IPv4 10324143       TCP localhost:56255->localhost:9306 (ESTABLISHED)
drizzlesl 32531 eday  201u  IPv4 10324145       TCP localhost:56256->localhost:9306 (ESTABLISHED)
drizzlesl 32531 eday  202u  IPv4 10324147       TCP localhost:56257->localhost:9306 (ESTABLISHED)
eday@orisndriz01:~/bzr/drizzle/eday-dev/tests$ lsof -p 20886 -i4 -a
COMMAND    PID USER   FD   TYPE   DEVICE SIZE NODE NAME
drizzled 20886 eday   10u  IPv4 10302592       TCP *:9306 (LISTEN)
eday@orisndriz01:~/bzr/drizzle/eday-dev/tests$ netstat -n|grep 9306
tcp        0      0 127.0.0.1:56243         127.0.0.1:9306          ESTABLISHED
tcp        0      0 127.0.0.1:56255         127.0.0.1:9306          ESTABLISHED
tcp        1      0 127.0.0.1:54858         127.0.0.1:9306          CLOSE_WAIT 
tcp        0      0 127.0.0.1:56247         127.0.0.1:9306          ESTABLISHED
tcp        0      0 127.0.0.1:56252         127.0.0.1:9306          ESTABLISHED
tcp        0      0 127.0.0.1:56249         127.0.0.1:9306          ESTABLISHED
tcp        0      0 127.0.0.1:56256         127.0.0.1:9306          ESTABLISHED
tcp        0      0 127.0.0.1:56251         127.0.0.1:9306          ESTABLISHED
tcp        0      0 127.0.0.1:56240         127.0.0.1:9306          ESTABLISHED
tcp        0      0 127.0.0.1:56257         127.0.0.1:9306          ESTABLISHED
tcp        0      0 127.0.0.1:56254         127.0.0.1:9306          ESTABLISHED
tcp        0      0 127.0.0.1:56245         127.0.0.1:9306          ESTABLISHED
tcp        0      0 127.0.0.1:56241         127.0.0.1:9306          ESTABLISHED
tcp        0      0 127.0.0.1:56246         127.0.0.1:9306          ESTABLISHED
tcp        0      0 127.0.0.1:56242         127.0.0.1:9306          ESTABLISHED
tcp        0      0 127.0.0.1:56248         127.0.0.1:9306          ESTABLISHED
tcp        0      0 127.0.0.1:56250         127.0.0.1:9306          ESTABLISHED
tcp        0      0 127.0.0.1:56244         127.0.0.1:9306          ESTABLISHED
tcp        0      0 127.0.0.1:56253         127.0.0.1:9306          ESTABLISHED


As you can see, drizzleslap thinks it has valid connections and are all blocking in poll(), but the server has no knowledge of the active connections. The OS should be causing poll() to return POLLERR or POLLHUP inside of drizzleslap, rather than have them block indefinitely for POLLIN.