I think you guys discussed this a bit on the mailing list -- I think only detecting RDHUP when reads are enabled would potentially make the API a bit harder to use.

Imagine you are writing a TCP proxy. The proxy reads bytes from socket A and writes them to socket B. If socket B is no longer writable, it will stop reading from socket A (to create backpressure). However, the server would like to know if socket A is closed by the client (because it will abort both connections).

The proxy can't detect the early EOF while at the same time putting backpressure on the connection.