netdev - Re: [PATCH] tcp: select(writefds) don't hang up when a peer close connection

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Date:	Thu, 26 Aug 2010 12:55:12 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	kosaki.motohiro@...fujitsu.com, David Miller <davem@...emloft.net>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	yoshfuji@...ux-ipv6.org
Subject: Re: [PATCH] tcp: select(writefds) don't hang up when a peer close connection

> On Wed, 2010-08-25 at 15:34 -0700, David Miller wrote:
> > From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> > Date: Wed, 25 Aug 2010 11:05:48 +0900 (JST)
> > 
> > > 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]
> [...]
> > And in this case here, I call into question the behavior of Ruby and
> > the application from two perspectives:
> > 
> > 1) Unlike all of the other conditions signalled by poll() this is
> >    one the application explicitly created and therefore knows about.
> >
> >    If the application calls close() or shutdown() with the send flag
> >    set, IT KNOWS what is going to happen on a write() attempt.
> [...]
> 
> This example seems to have both server (serv, s2) and client (s1) in the
> same process for simplicity.  The server socket (s2) is closed and the
> client cannot be expected to know that.  Of course the client ought to
> drop the connection after the first EPIPE, but it's reasonable to expect
> that this is a sticky condition just as it would be for a pipe.
> 
> Here's a similar test case in C:
> 
> #include <assert.h>
> #include <signal.h>
> #include <stdio.h>
> 
> #include <sys/select.h>
> #include <sys/socket.h>
> #include <sys/types.h>
> #include <unistd.h>
> 
> int main(void)
> {
>     struct sockaddr sa;
>     struct timeval tv;
>     int serv, s1, s2;
>     socklen_t len;
>     fd_set fds;
> 
>     signal(SIGPIPE, SIG_IGN);
> 
>     serv = socket(AF_INET, SOCK_STREAM, 0);
>     assert(serv >= 0);
>     assert(!listen(serv, 1));
>     len = sizeof(sa);
>     assert(!getsockname(serv, &sa, &len));
> 
>     s1 = socket(AF_INET, SOCK_STREAM, 0);
>     assert(s1 >= 0);
>     assert(!connect(s1, &sa, len));
>     len = sizeof(sa);
> 
>     s2 = accept(serv, &sa, &len);
>     assert(s2 >= 0);
>     close(s2);
> 
>     for (;;) {
> 	printf("write: %d\n", write(s1, "a", 1));
> 	FD_ZERO(&fds);
> 	FD_SET(s1, &fds);
> 	tv.tv_sec = 1;
> 	tv.tv_usec = 0;
> 	printf("select: %d\n", select(s1 + 1, NULL, &fds, NULL, &tv));
>     }
>     return 0;
> }

Cool!

Ben, I think your code is cleaner than mine. If you allow me, I hope to
include this one into my patch description.

Thanks.


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