> Empty TCP messages are perfectly valid, but they should be hidden from
> you. recvfrom returning 0 means the connection has been closed, but
> thatʼs a separate thing.

I suspect there's a missing ifdef in the code which ends up terminating
the process on such a condition, for both TCP and UDP.

> Could you show how youʼre generating the empty packets?

Download the attachments from my previous email, launch the server with
`emacs --batch -l bug-server.el`, then the client with `guile
fixed-client.scm`. Swap out the server with `guile fixed-server.scm`.

> Itʼs allowed by the protocol. I guess it could be useful for people
> wanting to implement their own keep-alive protocol over UDP.

I think so as well.