Cygwin Subprocesses on XEmacs

[Vin Shelton]

I spent some time debugging M-x shell in XEmacs on 32-bit Cygwin.
Here's what I found out.

In the child after fork() but before exec(), the setsid() call in
disconnect_controlling_terminal() is causing the subprocess not to
function after it gets spawned.

Here is a patch which works around the problem, enabling M-x shell to
work for bash and zsh (at least):

diff -r 00f2705e2cb3 src/sysdep.c
--- a/src/sysdep.c Mon Jan 26 08:53:07 2015 -0500
+++ b/src/sysdep.c Tue Jan 27 22:15:16 2015 -0500
@@ -1319,7 +1319,7 @@
 void
 disconnect_controlling_terminal (void)
 {
-#  ifdef HAVE_SETSID
+#  if defined(HAVE_SETSID) && !defined(CYGWIN)
   /* Controlling terminals are attached to a session.
      Create a new session for us; it will have no controlling
      terminal.  This also, of course, puts us in our own


HOWEVER - I don't understand why this should be necessary.


I here reproduce all of disconnect_controlling_terminal() for your
reading pleasure.

void
disconnect_controlling_terminal (void)
{
#  if defined(HAVE_SETSID) && !defined(CYGWIN)
  /* Controlling terminals are attached to a session.
     Create a new session for us; it will have no controlling
     terminal.  This also, of course, puts us in our own
     process group. */
  setsid ();
#  else
  /* Put us in our own process group. */
  EMACS_SEPARATE_PROCESS_GROUP ();
#    if defined (TIOCNOTTY)
  /* This is the older way of disconnecting the controlling
     terminal, on 4.3 BSD.  We must open /dev/tty; using
     filedesc 0 is not sufficient because it could be
     something else (e.g. our stdin was redirected to
     another terminal).
     */
  {
    int j = open ("/dev/tty", O_RDWR, 0);
    ioctl (j, TIOCNOTTY, 0);
    close (j);
  }
#    endif /* TIOCNOTTY */
  /*
     On systems without TIOCNOTTY and without
     setsid(), we don't need to do anything more to
     disconnect our controlling terminal.  Here is
     what the man page for termio(7) from a SYSV 3.2
     system says:

     "The first terminal file opened by the process group leader
     of a terminal file not already associated with a process
     group becomes the control terminal for that process group.
     The control terminal plays a special role in handling quit
     and interrupt signals, as discussed below.  The control
     terminal is inherited by a child process during a fork(2).
     A process can break this association by changing its process
     group using setpgrp(2)."

     */
#  endif /* not HAVE_SETSID */
}


Since Cygwin doesn't seem to have TIOCNOTTY, commenting out the
setsid() call reduces disconnect_controlling_terminal to:

void
disconnect_controlling_terminal (void)
{
# 1330 "sysdep.c"
  setpgid (0, 0);
# 1362 "sysdep.c"
}

Incidentally, that setpgid() call causes bash to complain at startup:

bash: cannot set terminal process group (-1): Inappropriate ioctl for device
bash: no job control in this shell

Thanks for any insight you can offer.

  - Vin