Hello, I wonder if there would be interest in trying to standardise POLLRDHUP. It originated on Linux, and has spread also to illumos and FreeBSD.
The use case I am aware of: A client has sent a message (say, query), a server (say, a database) is expending large amounts of CPU computing a response, and in the meantime, the TCP stack has learned that the client has gone away, gracefully or not, by shutting down, closing, exiting, crashing, or losing power or network link, which in that last case requires socket-level keepalive to be configured. The server application can't discover this until it eventually completes its work and tries to send a response and learns that the connection is reset. One might think that it could periodically try recv(sock, &buf, 1, MSG_PEEK) to see if EOF is buffered already, but this approach doesn't work if the application-level protocol allows pipelining of requests or for some other reason there is data buffered, including a goodbye message from the client. Arguably, the application protocol could be redesigned to include periodic application-level heartbeat messages during processing, but many applications have discovered the technique of polling for POLLRDHUP periodically, because the kernel in fact already knows about this condition. There is just no standardised way to ask about it. Examples of users: https://github.com/Exim/exim/blob/e4e884faa7f5a04d937282113681d97a355ed2af/src/src/acl.c#L3523 https://github.com/mysql/mysql-server/blob/7ed30a748964c009d4909cb8b4b22036ebdef239/vio/viosocket.cc#L750 https://github.com/postgres/postgres/blob/c30f54ad732ca5c8762bb68bbe0f51de9137dd72/src/backend/libpq/pqcomm.c#L1936 The FreeBSD change for poll(): https://github.com/freebsd/freebsd-src/commit/3aaaa2efde896e19d229ee2cf09fe7e6ab0fbf6e Other systems with shared history probably implement poll() similarly, but some related OSs have re-implemented poll() internally on top of kqueue machinery. Kqueue/kevent() already exposes this condition as EV_EOF, which is reported independently of whether data is buffered and available to read. I am not aware of any other programming interfaces that expose the condition, despite searching. If you know of any, I'd love to hear about those too. Thanks, Thomas Munro
