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    Subject[Patch][RFC] epoll and half closed TCP connections
    I'm proposing adding a new POLL event type (POLLRDHUP) as way to solve
    a new race introduced by having an edge triggered event mechanism
    (epoll). The problem occurs when a client writes data and then does a
    write side shutdown(). The server (using epoll) sees only one event for
    the read data ready and the read EOF condition and has no way to tell
    that an EOF occurred.

    -Eric Varsanyi

    - remote sends data and does a shutdown
    [ we see a data bearing packet and FIN from client on the wire ]

    - user mode server gets around to doing accept() and registers
    for EPOLLIN events (along with HUP and ERR which are forced on)

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

    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'.

    One would be tempted to issue an extra read() after getting back less than
    expected, but this is an extra system call on every read event and you get
    back the same '0' bytes that you get if the buffer is just empty. The only
    sure bet seems to be CTL_MODding the FD to force a re-poll (which would
    cost a syscall and hash-lookup in eventpoll for every read event).

    The POLLHUP indication is specifically not used in this 1/2 closed state
    since it is (by POSIX) not allowed to be masked and would interfere with
    EPOLLOUT back to client if it were set.

    I considered 2 possible ideas:

    1) have epoll return a count of events represented by a single
    appearance in the list after an epoll_wait(); this could be
    used as a hint to make some other (tbd) syscall to find out
    if the socket was in half closed state and normally not have
    to pay an extra syscall on every read; this seems like a lot of
    tap dancing for a work around

    2) add a new 1/2 closed event type that a poll routine can return

    The implementation is trivial, a patch is included below. If this idea sees
    favor I'll fix the other architectures, ipv6, epoll.h, and make a 'real'
    patch. I do not believe any drivers deserve to be modified to return this
    new event.

    This should not break existing programs, it still returns the read ready
    indication as before and unless an app registers for this new event it won't
    be woken up an extra time. Any epoll programs will not get the extra event
    indicated unless they ask for it. I dislike extending such an established
    API, but coopting any of the other event types just seems wrong.

    Other ideas and comments would be appreciated,
    -Eric Varsanyi

    diff -Naur linux-2.4.20/include/asm-i386/poll.h linux-2.4.20_ev/include/asm-i386/poll.h
    --- linux-2.4.20/include/asm-i386/poll.h Thu Jan 23 13:01:28 1997
    +++ linux-2.4.20_ev/include/asm-i386/poll.h Sat Jul 12 12:29:11 2003
    @@ -15,6 +15,7 @@
    #define POLLWRNORM 0x0100
    #define POLLWRBAND 0x0200
    #define POLLMSG 0x0400
    +#define POLLRDHUP 0x0800

    struct pollfd {
    int fd;
    diff -Naur linux-2.4.20/net/ipv4/tcp.c linux-2.4.20_ev/net/ipv4/tcp.c
    --- linux-2.4.20/net/ipv4/tcp.c Tue Jul 8 09:40:42 2003
    +++ linux-2.4.20_ev/net/ipv4/tcp.c Sat Jul 12 12:29:56 2003
    @@ -424,7 +424,7 @@
    if (sk->shutdown == SHUTDOWN_MASK || sk->state == TCP_CLOSE)
    mask |= POLLHUP;
    if (sk->shutdown & RCV_SHUTDOWN)
    - mask |= POLLIN | POLLRDNORM;

    /* Connected? */
    if ((1 << sk->state) & ~(TCPF_SYN_SENT|TCPF_SYN_RECV)) {
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