Re: data in socketpair() channel lost if writer closes or exits without shutting down

[Christopher Faylor <cgf at redhat dot com>]

I'm getting ready to make a new release.  Was there ever any movement
on a fix for this?

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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