tcp: select(writefds) don't hang up when a peer close connection

This issue come from ruby language community. Below test program
hang up when only run on Linux.

        % uname -mrsv
        Linux 2.6.26-2-486 #1 Sat Dec 26 08:37:39 UTC 2009 i686
        % ruby -rsocket -ve '
        BasicSocket.do_not_reverse_lookup = true
        serv = TCPServer.open("127.0.0.1", 0)
        s1 = TCPSocket.open("127.0.0.1", serv.addr[1])
        s2 = serv.accept
        s2.close
        s1.write("a") rescue p $!
        s1.write("a") rescue p $!
        Thread.new {
          s1.write("a")
        }.join'
        ruby 1.9.3dev (2010-07-06 trunk 28554) [i686-linux]
        #<Errno::EPIPE: Broken pipe>
        [Hang Here]

FreeBSD, Solaris, Mac doesn't. because Ruby's write() method call
select() internally. and tcp_poll has a bug.

SUS defined 'ready for writing' of select() as following.

|  A descriptor shall be considered ready for writing when a call to an output
|  function with O_NONBLOCK clear would not block, whether or not the function
|  would transfer data successfully.

That said, EPIPE situation is clearly one of 'ready for writing'.

We don't have read-side issue because tcp_poll() already has read side
shutdown care.

|        if (sk->sk_shutdown & RCV_SHUTDOWN)
|                mask |= POLLIN | POLLRDNORM | POLLRDHUP;

So, Let's insert same logic in write side.

- reference url
  http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/31065
  http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/31068

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 1 deletions

diff --git a/pfinet/linux-src/net/ipv4/tcp.c b/pfinet/linux-src/net/ipv4/tcp.c
index 1a057439..13e57954 100644
--- a/pfinet/linux-src/net/ipv4/tcp.c
+++ b/pfinet/linux-src/net/ipv4/tcp.c
@@ -595,7 +595,8 @@ unsigned int tcp_poll(struct file * file, struct socket *sock, poll_table *wait)
 			} else {  /* send SIGIO later */
 				sk->socket->flags |= SO_NOSPACE;
 			}
-		}
+		} else
+			mask |= POLLOUT | POLLWRNORM;
 
 		if (tp->urg_data & URG_VALID)
 			mask |= POLLPRI;