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Date: Wed, 8 Aug 2001 09:03:06 +0200
From: Corinna Vinschen <cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: data in socketpair() channel lost if writer closes or exits without shutting down
Message-ID: <20010808090306.G28586@cygbert.vinschen.de>
Mail-Followup-To: cygwin AT cygwin DOT com
References: <20010713181435 DOT 3695 DOT qmail AT lizard DOT curl DOT com> <20010713202146 DOT B11377 AT redhat DOT com> <20010807150603 DOT B30140 AT redhat DOT com>
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In-Reply-To: <20010807150603.B30140@redhat.com>; from cgf@redhat.com on Tue, Aug 07, 2001 at 03:06:03PM -0400

On Tue, Aug 07, 2001 at 03:06:03PM -0400, Christopher Faylor wrote:
> I'm getting ready to make a new release.  Was there ever any movement
> on a fix for this?

No, I'm afraid.

Corinna

> 
> cgf
> 
> On Fri, Jul 13, 2001 at 08:21:46PM -0400, Christopher Faylor wrote:
> >Thanks for the diagnosis.  Would you be willing to look at the
> >Cygwin socket code and suggest a fix?
> >
> >cgf
> >
> >On Fri, Jul 13, 2001 at 02:14:35PM -0400, Jonathan Kamens wrote:
> >>The test program below creates a socketpair and then forks.  The child
> >>writes some data to the socketpair and then exits, and the parent
> >>tries to read that data.
> >>
> >>Instead of printing "PARENT: foo (exiting)" as it should, the parent
> >>prints "PARENT: read: Connection reset by peer".  This is also true if
> >>you compile the program with "-DUSE_CLOSE" to tell the child to close
> >>the write end of the socketpair before exiting.  However, it works
> >>properly if you compile the program with "-DUSE_SHUTDOWN" to tell the
> >>child to shutdown its end of the socketpair before exiting.
> >>
> >>There are two different bugs here:
> >>
> >>1) close() on an end of a socketpair should behave the same as
> >>   shutdown(..., 2), in terms of flushing data to the other end of the
> >>   socketpair.
> >>
> >>2) Data sent by the writer should be flushed to the reader, not lost,
> >>   when the writer exits.
> >>
> >>This problem causes recent versions of rsync, when they are compiled
> >>to use socketpairs (which is the default), to report a bogus "read
> >>error: connection reset by peer" at the end of every rsync, even when
> >>it in fact copied everything successfully.  I'm going to submit a
> >>patch to the rsync maintainers suggesting that they work around this
> >>problem by calling shutdown() before exiting.
> >>
> >>  jik
> >>
> >>                      *************************
> >>
> >>#include <unistd.h>
> >>#include <stdio.h>
> >>#include <sys/types.h>
> >>#include <signal.h>
> >>#include <sys/socket.h>
> >>
> >>main()
> >>{
> >>  int pipefds[2];
> >>  int readfd, writefd;
> >>  char inbuf[4];
> >>  char outbuf[4] = "foo";
> >>  int pid;
> >>  fd_set readfds, exceptfds;
> >>
> >>  socketpair(AF_UNIX, SOCK_STREAM, 0, pipefds);
> >>  /* pipe(pipefds); */
> >>  
> >>  readfd = pipefds[0];
> >>  writefd = pipefds[1];
> >>
> >>  if (pid = fork()) {
> >>    fprintf(stderr, "PARENT: close(writefd)\n");
> >>    close(writefd);
> >>
> >>    FD_ZERO(&readfds);
> >>    FD_SET(readfd, &readfds);
> >>    exceptfds = readfds;
> >>
> >>    fprintf(stderr, "PARENT: selecting\n");
> >>    if (select(readfd + 1, &readfds, NULL, &exceptfds, NULL) <= 0) {
> >>      perror("PARENT: select");
> >>      exit(1);
> >>    }
> >>
> >>    if (FD_ISSET(readfd, &exceptfds)) {
> >>      fprintf(stderr, "PARENT: exception is set\n");
> >>    }
> >>
> >>    if (FD_ISSET(readfd, &readfds)) {
> >>      fprintf(stderr, "PARENT: read is set\n");
> >>    }
> >>
> >>    fprintf(stderr, "PARENT: reading\n");
> >>    if (read(readfd, inbuf, sizeof(inbuf)) != sizeof(inbuf)) {
> >>      perror("PARENT: read");
> >>      exit(1);
> >>    }
> >>
> >>    printf("PARENT: %s (exiting)\n", inbuf);
> >>    exit(0);
> >>  }
> >>  else {
> >>    fprintf(stderr, "CHILD: close(readfd)\n");
> >>    close(readfd);
> >>
> >>    fprintf(stderr, "CHILD: writing\n");
> >>    if (write(writefd, outbuf, sizeof(outbuf)) != sizeof(outbuf)) {
> >>      perror("CHILD: write");
> >>      exit(1);
> >>    }
> >>#ifdef USE_SHUTDOWN
> >>    shutdown(writefd, 2);
> >>#endif
> >>#ifdef USE_CLOSE
> >>    close(writefd);
> >>#endif
> >>
> >>    fprintf(stderr, "CHILD: exiting\n");
> >>    exit(0);
> >>  }
> >>}
> 
> --
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-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Developer                                mailto:cygwin AT cygwin DOT com
Red Hat, Inc.

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