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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/tcp.c

···
 				if (sk_stream_wspace(sk) >= sk_stream_min_wspace(sk))
 					mask |= POLLOUT | POLLWRNORM;
 			}
-		}
+		} else
+			mask |= POLLOUT | POLLWRNORM;
 
 		if (tp->urg_data & TCP_URG_VALID)
 			mask |= POLLPRI;