[PATCH] Cygwin: pty: detect pcon-backed pty for non-Cygwin-spawned children

[Takashi Yano]

On Sat, 27 Jun 2026 10:18:27 +0200 (CEST)
Johannes Schindelin wrote:
> Hi Takashi & Mark,
> 
> On Wed, 24 Jun 2026, Takashi Yano wrote:
> 
> > On Thu, 30 Apr 2026 15:04:04 +0000
> > "Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com> wrote:
> > > From: Johannes Schindelin <johannes.schindelin@gmx.de>
> > > 
> > > When a Cygwin process (e.g. `bash` under MinTTY) spawns a native
> > > Win32 child (e.g. `git.exe`) with pseudo console support enabled,
> > > the child gets a pseudo console that bridges the pty. If that native
> > > child then spawns a Cygwin grandchild (e.g. `vim`, `less`), the
> > > grandchild inherits the pseudo console's console handles. In
> > > `init_std_file_from_handle()`, the grandchild's msys2-runtime sees
> > > `GetConsoleScreenBufferInfo()` succeed on those handles and, with
> > > no valid `ctty` set, falls back to `FH_CONSOLE` and gives the
> > > process `cons0` instead of connecting to the pty.
> > > 
> > [...]
> > 
> > Pushed to master branch with my fixup patches.
> 
> I had a buffer-grow follow-up to this patch sitting in
> https://github.com/git-for-windows/msys2-runtime/pull/131/commits/77e01abd83836b4b7488328ca899aea3a8e4ffbe
> that I should have sent before you picked up the original and pushed it to
> master; sorry for the delay.
> 
> -- snip --
> From 77e01abd83836b4b7488328ca899aea3a8e4ffbe Mon Sep 17 00:00:00 2001
> From: Johannes Schindelin <johannes.schindelin@gmx.de>
> Date: Fri, 29 May 2026 19:07:26 +0200
> Subject: [PATCH] Cygwin: pty: grow GetConsoleProcessList buffer in
>  find_pcon_pty()
> 
> find_pcon_pty() was passing a fixed 128-DWORD stack array to
> GetConsoleProcessList(). If the calling Cygwin process happens to be
> attached to a console with more than 128 processes, the Win32
> function returns the required size and the buffer contents are
> undefined; the existing if-zero check did not catch that case, so
> the subsequent loop walked uninitialised data and could either miss
> the candidate pty or, worse, match against junk PIDs and return the
> wrong tty index.
> 
> Adopt the buffer-too-small dance from
> fhandler_termios::get_console_process_id() in
> winsup/cygwin/fhandler/termios.cc, which already had to solve this
> problem and which also notes that the new condrv does not accept
> oversized first-call buffers
> (https://github.com/microsoft/terminal/issues/18264#issuecomment-2515448548).
> The buffer comes from tmp_pathbuf so the same NT_MAX_PATH cap
> (currently 1024 DWORDs, i.e. 4096 processes) applies; we bail out
> with -1 if even that is not enough rather than allocate unbounded
> memory or guess. Bumping the start-with size from 1 would defeat the
> condrv work-around mentioned above, so we keep the same one-element
> initial probe as termios.cc and let the loop grow.
> 
> Suggested-by: Takashi Yano <takashi.yano@nifty.ne.jp>
> Assisted-by: Opus 4.7
> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> (cherry picked from commit b65e1544d45567f0033c57a0aa1543c5e654950a)
> ---
>  winsup/cygwin/tty.cc | 19 ++++++++++++++++---
>  1 file changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/winsup/cygwin/tty.cc b/winsup/cygwin/tty.cc
> index 5cce05de34..9bc2a084fb 100644
> --- a/winsup/cygwin/tty.cc
> +++ b/winsup/cygwin/tty.cc
> @@ -19,6 +19,7 @@ details. */
>  #include "cygheap.h"
>  #include "pinfo.h"
>  #include "shared_info.h"
> +#include "tls_pbuf.h"
>  
>  HANDLE NO_COPY tty_list::mutex = NULL;
>  
> @@ -135,7 +136,9 @@ tty_list::init ()
>  int
>  tty_list::find_pcon_pty ()
>  {
> -  DWORD pids[128];
> +  tmp_pathbuf tp;
> +  DWORD *pids = (DWORD *) tp.c_get ();
> +  const DWORD buf_size = NT_MAX_PATH / sizeof (DWORD);
>    DWORD count = 0;
>    bool got_pids = false;
>  
> @@ -144,10 +147,20 @@ tty_list::find_pcon_pty ()
>        if (!ttys[i].has_active_pcon ())
>  	continue;
>  
> -      /* Fetch the console process list lazily, only on first candidate. */
> +      /* Fetch the console process list lazily, only on first candidate.
> +	 The buffer-too-large dance mirrors the one in termios.cc's
> +	 get_console_process_id() and works around new condrv's dislike
> +	 of oversized first-call buffers, see
> +	 https://github.com/microsoft/terminal/issues/18264#issuecomment-2515448548 */
>        if (!got_pids)
>  	{
> -	  count = GetConsoleProcessList (pids, 128);
> +	  DWORD buf_size1 = 1;
> +	  while ((count = GetConsoleProcessList (pids, buf_size1)) > buf_size1)
> +	    {
> +	      if (count > buf_size)
> +		return -1;
> +	      buf_size1 = count;
> +	    }
>  	  if (!count)
>  	    return -1;
>  	  got_pids = true;
> -- snap --
> 
> Ciao,
> Johannes
> 

LGTM. Thanks! Pushed to master.

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>