[PATCH] Cygwin: pinfo: Align CTTY behavior to the statement of POSIX.

[Takashi Yano <takashi.yano@nifty.ne.jp>]

POSIX states "A terminal may be the controlling terminal for at most
one session."
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap11.html

However, in cygwin, multiple sessions could be associated with the
same TTY. This patch aligns CTTY behavior to the statement of POSIX.

Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>
---
 winsup/cygwin/fhandler/termios.cc | 6 +++++-
 winsup/cygwin/mm/cygheap.cc       | 2 ++
 winsup/cygwin/pinfo.cc            | 4 +++-
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/winsup/cygwin/fhandler/termios.cc b/winsup/cygwin/fhandler/termios.cc
index fe4dfd13e..f94e20ff6 100644
--- a/winsup/cygwin/fhandler/termios.cc
+++ b/winsup/cygwin/fhandler/termios.cc
@@ -737,7 +737,11 @@ fhandler_termios::ioctl (int cmd, void *varg)
     }
 
   myself->ctty = -1;
-  myself->set_ctty (this, 0);
+  if (!myself->set_ctty (this, 0))
+    {
+      set_errno (EPERM);
+      return -1;
+    }
   return 0;
 }
 
diff --git a/winsup/cygwin/mm/cygheap.cc b/winsup/cygwin/mm/cygheap.cc
index a305570df..72861d8d7 100644
--- a/winsup/cygwin/mm/cygheap.cc
+++ b/winsup/cygwin/mm/cygheap.cc
@@ -127,6 +127,8 @@ void
 init_cygheap::close_ctty ()
 {
   debug_printf ("closing cygheap->ctty %p", cygheap->ctty);
+  if (cygheap->ctty->tc ()->getsid () == pid)
+    cygheap->ctty->tc ()->setsid (0); /* Release CTTY ownership */
   cygheap->ctty->close_with_arch ();
   cygheap->ctty = NULL;
 }
diff --git a/winsup/cygwin/pinfo.cc b/winsup/cygwin/pinfo.cc
index e086ab9a8..749a4064c 100644
--- a/winsup/cygwin/pinfo.cc
+++ b/winsup/cygwin/pinfo.cc
@@ -528,7 +528,9 @@ _pinfo::set_ctty (fhandler_termios *fh, int flags)
 {
   tty_min& tc = *fh->tc ();
   debug_printf ("old %s, ctty device number %y, tc.ntty device number %y flags & O_NOCTTY %y", __ctty (), ctty, tc.ntty, flags & O_NOCTTY);
-  if (fh && (ctty <= 0 || ctty == tc.ntty) && !(flags & O_NOCTTY))
+  if (tc.getsid () && tc.getsid () != pid)
+    ; /* Do not attach if another session already attached to the CTTY. */
+  else if (fh && (ctty <= 0 || ctty == tc.ntty) && !(flags & O_NOCTTY))
     {
       ctty = tc.ntty;
       if (cygheap->ctty != fh->archetype)
-- 
2.39.0