The intent was to run custom code after the user is done modifying the socket,
for example to update a (not yet existing) port->socket map in SocketSet. However
this wouldn't work, since the SocketRef would have to borrow the SocketSet at
the same time as the Socket to be able to notify the SocketSet.
I believe such indexing can be achieved by setting a "dirty" bit *before* giving
the socket to the user, then on poll() reindexing all dirty sockets. This could
even be faster: if user gets a socket multiple times between polls, it'd be reindexed
only once.
f8cc1eacbe