[newlib-cygwin/cygwin-3_6-branch] Cygwin: pty: Drop nat_fg() check from to_be_read_from_nat_pipe()

[Takashi Yano]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=6f31acbbc40a25c2d50c9894e9409bd100287f32

commit 6f31acbbc40a25c2d50c9894e9409bd100287f32
Author: Takashi Yano <takashi.yano@nifty.ne.jp>
Date:   Tue Mar 24 11:08:52 2026 +0900

    Cygwin: pty: Drop nat_fg() check from to_be_read_from_nat_pipe()
    
    While a non-cygwin app has exited but the stub process has not yet
    terminated, `nat_fg()` returns false because no non-cygwin app is
    running. In this window, pty input goes to the cyg pipe. Due to
    this, the keystroke order is swapped unexpectedly:
    
    1) start non-cygwin app
    2) press 'a' ('a' goes to nat pipe)
    3) non-cygwin app exits
    4) press 'b' ('b' goes to cyg pipe)
    5) the stub process for non-cygwin app transfers input in nat pipe
       to cyg pipe ('a' goes to cyg pipe)
    6) the result in the cyg pipe is "ba"
    
    Fix this by dropping the `nat_fg()` check from
    `to_be_read_from_nat_pipe()`. The function now returns true when
    `!pcon_start && switch_to_nat_pipe && !masked`. Each component has
    a specific purpose:
    
    - `!pcon_start`: keystrokes go through the CSI6n response handler
      during pseudo console initialization rather than the fast path.
    - `switch_to_nat_pipe`: this session-level flag stays true from
      `setup_for_non_cygwin_app()` through `cleanup_for_non_cygwin_app()`,
      spanning the entire native process lifetime including the post-exit
      cleanup window.
    - `!masked` (`TTY_SLAVE_READING` event does not exist): keystrokes
      go to the Cygwin pipe when a Cygwin process is actively reading
      from the slave, since that process expects POSIX-processed input.
    
    Removing `nat_fg()` is safe because conhost's input buffer
    accumulates keystrokes as INPUT_RECORDs during the post-exit
    window, and `transfer_input(to_cyg)` in `cleanup_for_non_cygwin_app()`
    reads them back via `ReadConsoleInputA()` and writes them to the
    cyg pipe. Those transferred bytes then go through `line_edit()` in
    the master's forward thread (via `input_transferred_to_cyg` from an
    earlier patch in this series), ensuring proper POSIX line discipline
    processing.
    
    Additionally, add a `nat_fg()` check to the disable_pcon transfer
    path in `master::write()`. That transfer moves cyg pipe data to
    the nat pipe when a Cygwin child exits and a native process
    regains the foreground with pcon disabled. Without pcon, there is
    no conhost buffer to accumulate keystrokes (the nat pipe is a raw
    pipe), so keystrokes must only go there when a native process is
    genuinely in the foreground and ready to read them. The `nat_fg()`
    guard prevents the transfer from stealing readline's data during
    the post-exit window.
    
    Fixes: f20641789427 ("Cygwin: pty: Reduce unecessary input transfer.")
    Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>
    Reviewed-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
    (cherry picked from commit 6413c19fd85bd7d6dc42368e22cebe86b730b3b6)

Diff:
---
 winsup/cygwin/fhandler/pty.cc | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/winsup/cygwin/fhandler/pty.cc b/winsup/cygwin/fhandler/pty.cc
index cd19d6399..2a6f072e7 100644
--- a/winsup/cygwin/fhandler/pty.cc
+++ b/winsup/cygwin/fhandler/pty.cc
@@ -1308,14 +1308,8 @@ fhandler_pty_common::to_be_read_from_nat_pipe (void)
       goto out;
   }
 
-  if (!pinfo (get_ttyp ()->getpgid ()))
-    /* GDB may set invalid process group for non-cygwin process. */
-    {
-      ret = true;
-      goto out;
-    }
+  ret = true; /* !pcon_start && switch_to_nat_pipe && !masked */
 
-  ret = get_ttyp ()->nat_fg (get_ttyp ()->getpgid ());
 out:
   ReleaseMutex (pipe_sw_mutex);
   return ret;
@@ -2364,6 +2358,7 @@ fhandler_pty_master::write (const void *ptr, size_t len)
   /* This input transfer is needed when cygwin-app which is started from
      non-cygwin app is terminated if pseudo console is disabled. */
   if (to_be_read_from_nat_pipe () && !get_ttyp ()->pcon_activated
+      && get_ttyp ()->nat_fg (get_ttyp ()->getpgid ())
       && get_ttyp ()->pty_input_state == tty::to_cyg)
     {
       acquire_attach_mutex (mutex_timeout);