Re: [Patch][RFC] epoll and half closed TCP connections

[Jamie Lokier <jamie@shareable.org>]

Eric Varsanyi wrote:
> 	- epoll_wait() returns a single EPOLLIN event on the FD representing
> 	  both the 1/2 shutdown state and data available

Correct.

> At this point there is no way the app can tell if there is a half closed
> connection so it may issue a close() back to the client after writing
> results. Normally the server would distinguish these events by assuming
> EOF if it got a read ready indication and the first read returned 0 bytes,
> or would issue read calls until less data was returned than was asked for.
> 
> In a level triggered world this all just works because the read ready
> indication is driven back to the app as long as the socket state is half
> closed. The event driven epoll mechanism folds these two indications
> together and thus loses one 'edge'.

Well then, use epoll's level-triggered mode.  It's quite easy - it's
the default now. :)

If there's an EOF condition pending after you called read(), and then
you call epoll_wait(), you _should_ see another EPOLLIN condition
immediately.

If you aren't seeing epoll_wait() return with EPOLLIN when there's an
EOF pending, *and* you haven't set EPOLLET in the event flags, that's
a bug in epoll.  Is that what you're seeing?

-- Jamie