Cygwin: pty: setup new pty on opening the master, not in constructor

Setting up the pty in the master constructor ends up creating a new pty
on every stat(2) call on /dev/ptmx.  Only do this when actually opening
the device, not when using the device class in another, non-opening
context.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fhandler_tty.cc

@@ -1470,17 +1470,14 @@ fhandler_pty_master::fhandler_pty_master (int unit)
 {
   if (unit >= 0)
     dev ().parse (DEV_PTYM_MAJOR, unit);
-  else if (!setup ())
-    {
-      dev ().parse (FH_ERROR);
-      return;
-    }
   set_name ("/dev/ptmx");
 }
 
 int
 fhandler_pty_master::open (int flags, mode_t)
 {
+  if (!setup ())
+      return 0;
   set_open_status ();
   dwProcessId = GetCurrentProcessId ();
   return 1;

winsup/cygwin/release/3.2.0

@@ -36,3 +36,6 @@ Bug Fixes
 
 - Fix assertion failure on an invalid path under /proc/<pid>/fd/.
   Addresses: https://cygwin.com/pipermail/cygwin/2020-September/246160.html
+
+- Fix crash on stat(2)'ing /dev/ptmx on 32 bit
+  Addresses: https://cygwin.com/pipermail/cygwin/2020-September/246218.html