[PATCH] make clone try to acquire a controlling tty

[Philippe Troin <phil@xxxxxxxx>]

The enclosed patch makes clone try to acquire a controlling tty. This
makes clone more useful because it enables job control in cloned
sessions.

The patch applies on current cvs head.

Notes about the patch:

 - We don't call TIOCNOTTY if HAVE_SETSID: setsid() automatically
   detaches from the current controlling tty.

 - The logic for dup'ing the newly opened tty is changed for a better
   one.

 - Once the dup'ing of the newly opened tty is done, we try to acquire
   the newly opened tty as a controlling tty. We print a message if
   this fails.

 - We have to reset mypgrp to zero so that init_io() can get its job
   done correctly.

Notes about controlling ttys:

 - Due to the way job control works, it is impossible for a process to
   acquire a controlling tty if another process already acquired it
   since:

     * POSIX mandates that if a session leader dies, all the processes
       in the session loose their controlling tty.

     * Most terminal emulator programs (including screen, but see note
       about screen) acquire a controlling tty before exec'ing their
       target process.

 - This means that the following won't work (the cloned zsh will run,
   but will not have a controlling tty (-> no job control)):

     * Open a new xterm.

     * Run tty in xterm.

     * Run in the xterm:

         exec zsh -c 'trap "" INT QUIT TSTP; while :; do sleep 100000; done'

     * Do a clone in the xterm's tty.

 - However this will work (the cloned zsh will have a controlling tty
   and job control will work):

     * clone /dev/tty<n> or clone /dev/vc/<n> on linux

     * zsh will be able to open cloned shells with job control in a
       screen session as long as the screen "setsid off" command is in
       effect in the screen:

        + Run screen. You're now in screen #0.

        + Enter "setsid off": this is done by typing in:

            ^A : setsid off <RETURN>

        + Create a new screen. You're now in screen #1.

        + The shell in screen 1 has job control disabled (it has no
          controlling tty).
 
        + In screen #1, run:

            tty && exec sleep 1000000

        + Switch back to screen #0: ^A 0

        + Run clone on the tty that was printed in screen #1.

        + Switch back to screen #1: ^A 1

        + You now have a cloned zsh with job control active.

Tested to work OK on Linux (with TIOCNOTTY/TIOCSCTTY controlling tty
acquiring method) and Solaris (with SvR4's O_NOCTTY controlling tty
acquiring method).

Phil.

Changelog entry:

2003-07-22  Philippe Troin  <phil@xxxxxxxx>

	* Src/Modules/clone.c (bin_clone): Try to acquire a controlling
	tty when possible. Report failures. Cleanup.

Index: clone.c
===================================================================
RCS file: /cvsroot/zsh/zsh/Src/Modules/clone.c,v
retrieving revision 1.3
diff -b -u -r1.3 clone.c
--- clone.c	27 Aug 2002 21:10:34 -0000	1.3
+++ clone.c	23 Jul 2003 01:07:48 -0000
@@ -43,7 +43,7 @@
 static int
 bin_clone(char *nam, char **args, Options ops, int func)
 {
-    int ttyfd, pid;
+    int ttyfd, pid, cttyfd;
 
     unmetafy(*args, NULL);
     ttyfd = open(*args, O_RDWR|O_NOCTTY);
@@ -57,29 +57,42 @@
 	ppid = getppid();
 	mypid = getpid();
 #ifdef HAVE_SETSID
-	if (setsid() != mypid) {
+	if (setsid() != mypid)
 	    zwarnnam(nam, "failed to create new session: %e", NULL, errno);
-#endif
-#ifdef TIOCNOTTY
+#elif defined(TIOCNOTTY)
 	    if (ioctl(SHTTY, TIOCNOTTY, 0))
-		zwarnnam(nam, "%e", NULL, errno);
+	    zwarnnam(*args, "%e", NULL, errno);
 	    setpgrp(0L, mypid);
 #endif
-#ifdef HAVE_SETSID
-	}
-#endif
-	if (ttyfd) {
-	    close(0);
-	    dup(ttyfd);
-	} else
-	    ttyfd = -1;
-	close(1);
-	close(2);
-	dup(0);
-	dup(0);
+	dup2(ttyfd,0);
+	dup2(ttyfd,1);
+	dup2(ttyfd,2);
+	if (ttyfd > 2)
+	    close(ttyfd);
 	closem(0);
 	close(coprocin);
 	close(coprocout);
+	/* Acquire a controlling terminal */
+	cttyfd = open(*args, O_RDWR);
+	if (cttyfd == -1)
+	    zwarnnam(nam, "%e", NULL, errno);
+	else {
+#ifdef TIOCSCTTY
+	    ioctl(cttyfd, TIOCSCTTY, 0);
+#endif
+	    close(cttyfd);
+	}
+	/* check if we acquired the tty successfully */
+	cttyfd = open("/dev/tty", O_RDWR);
+	if (cttyfd == -1)
+	    zwarnnam(nam, "could not make %s my controlling tty, job control "
+		     "disabled", *args, 0);
+	else
+	    close(cttyfd);
+
+	/* Clear mygrp so that acquire_pgrp() gets the new process group.
+	 * (acquire_pgrp() is called from init_io()) */
+	mypgrp = 0;
 	init_io();
 	setsparam("TTY", ztrdup(ttystrname));
     }