Re: [PATCH 1/3] Cygwin: console: Ensure the master thread runs only when it is supposed to

Hi Takashi & Mark,

An AutoHotKey-based test I added to
https://github.com/git-for-windows/msys2-runtime/pull/131 bisected a
Ctrl-C regression to this commit after it landed on master: Ctrl-C stopped
interrupting `cat` in the pipeline `cat | ping`. The diagnosis and the fix
I would propose are below.

On Thu, 11 Jun 2026, Takashi Yano wrote:

> @@ -1190,8 +1191,8 @@ fhandler_console::bg_check (int sig, bool dontsignal)
>       in the same process group. */
>    if (sig == SIGTTIN && con.curr_input_mode != tty::cygwin)
>      {
> -      set_disable_master_thread (false, this);
>        set_input_mode (tty::cygwin, &tc ()->ti, get_handle_set ());
> +      set_disable_master_thread (false, this);
>      }
>    if (sig == SIGTTOU && con.curr_output_mode != tty::cygwin)
>      set_output_mode (tty::cygwin, &tc ()->ti, get_handle_set ());
> @@ -2087,8 +2088,8 @@ fhandler_console::post_open_setup (int fd)
>    /* Setting-up console mode for cygwin app started from non-cygwin app. */
>    if (fd == 0)
>      {
> -      set_disable_master_thread (false, this);
>        set_input_mode (tty::cygwin, &get_ttyp ()->ti, &handle_set);
> +      set_disable_master_thread (false, this);
>      }
>    else if (fd == 1 || fd == 2)
>      set_output_mode (tty::cygwin, &get_ttyp ()->ti, &handle_set);

The console only delivers Ctrl-C as a raw `0x03` byte (which the console
master thread then reads and turns into a `SIGINT` for the foreground
process group) while that thread is live. When the master thread is
suspended or disabled, `set_input_mode (tty::cygwin)` instead requests
`ENABLE_PROCESSED_INPUT`, so the console raises a `CTRL_C_EVENT` and the
`0x03` byte never reaches the master thread.

The reorder above has `set_input_mode (tty::cygwin)` run while
`disable_master_thread` is still set, so `ENABLE_PROCESSED_INPUT` stays on
and a cygwin program sharing a foreground pgrp with a non-cygwin program
(e.g. the pipeline `cat | ping`) never receives its `SIGINT`. Clearing
`disable_master_thread` first, so the mode is configured with the master
thread already live, restores the previous behavior in both `bg_check ()`
and `post_open_setup ()`. The disable paths and the synchronous suspension
this commit added are load-bearing for non-cygwin programs and are left
untouched, so the master thread is still reliably suspended for them.

The fix I would propose is in
https://github.com/git-for-windows/msys2-runtime/pull/131/commits/73aae37a62d8246e1abaac6e52d6c6bb89bc4c5d:

-- snip --
From 73aae37a62d8246e1abaac6e52d6c6bb89bc4c5d Mon Sep 17 00:00:00 2001
From: Johannes Schindelin <[email protected]>
Date: Fri, 26 Jun 2026 09:16:49 +0200
Subject: [PATCH] Cygwin: console: re-enable the master thread before selecting
 cygwin input mode

When a cygwin program and a non-cygwin program run in the same foreground
process group (for example the pipeline `cat | ping`), Ctrl-C stopped
interrupting the cygwin program after "Cygwin: console: Ensure the master
thread runs only when it is supposed to".

The console only delivers Ctrl-C as a raw 0x03 byte (which the console
master thread reads and turns into a SIGINT for the foreground process
group) while that thread is live. When it is suspended or disabled,
set_input_mode (tty::cygwin) instead requests ENABLE_PROCESSED_INPUT, so
the console raises a CTRL_C_EVENT and the 0x03 byte never reaches the
master thread. The referenced commit reordered the two enable paths,
bg_check () and post_open_setup (), so that set_input_mode (tty::cygwin)
runs while disable_master_thread is still set; that leaves
ENABLE_PROCESSED_INPUT on and the cygwin program never receives its SIGINT.

Clear disable_master_thread before selecting cygwin input mode in those two
paths, so the mode is configured with the master thread already live and
ENABLE_PROCESSED_INPUT stays off. The disable paths and the synchronous
suspension that the referenced commit added are left unchanged, so
non-cygwin programs still get the master thread reliably suspended.

Fixes: 733d5a953fa9 ("Cygwin: console: Ensure the master thread runs only when it is supposed to")
Assisted-by: Opus 4.8
Signed-off-by: Johannes Schindelin <[email protected]>
---
 winsup/cygwin/fhandler/console.cc | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/winsup/cygwin/fhandler/console.cc b/winsup/cygwin/fhandler/console.cc
index 0136652878..685e99d62c 100644
--- a/winsup/cygwin/fhandler/console.cc
+++ b/winsup/cygwin/fhandler/console.cc
@@ -1147,8 +1147,8 @@ fhandler_console::bg_check (int sig, bool dontsignal)
      in the same process group. */
   if (sig == SIGTTIN && con.curr_input_mode != tty::cygwin)
     {
-      set_input_mode (tty::cygwin, &tc ()->ti, get_handle_set ());
       set_disable_master_thread (false, this);
+      set_input_mode (tty::cygwin, &tc ()->ti, get_handle_set ());
     }
   if (sig == SIGTTOU && con.curr_output_mode != tty::cygwin)
     set_output_mode (tty::cygwin, &tc ()->ti, get_handle_set ());
@@ -2035,8 +2035,8 @@ fhandler_console::post_open_setup (int fd)
   /* Setting-up console mode for cygwin app started from non-cygwin app. */
   if (fd == 0)
     {
-      set_input_mode (tty::cygwin, &get_ttyp ()->ti, &handle_set);
       set_disable_master_thread (false, this);
+      set_input_mode (tty::cygwin, &get_ttyp ()->ti, &handle_set);
     }
   else if (fd == 1 || fd == 2)
     set_output_mode (tty::cygwin, &get_ttyp ()->ti, &handle_set);
-- snap --

Ciao,
Johannes