(Note by apenwarr: seems to still work for me.  The reason the
problem occurred is that reassigning 'handlers' doesn't change it in its
parent; it creates a whole new list, and the caller still owns the old one
with all the dead sockets in it.  The problem seems to have been introduced
in commit 84376284 when I factored the
runonce() functionality out of the client and server but didn't notice this
reassignment.)