[PATCH] Cygwin: pty: Restore nat handles in all PTY-slave instances in GDB

[Takashi Yano]

v2: Fixup handles in ctty as well if ctty is a PTY-slave

On Sun,  8 Mar 2026 20:19:21 +0900
Takashi Yano wrote:
> If non-cygwin app is started in GDB and terminating it normally,
> re-running the non-cygwin app might fail in setup_pseudoconsole().
> 
> The error is something like:
> 
> $ gdb ./winsleep
> GNU gdb (GDB) (Cygwin 15.2-1) 15.2
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> This GDB was configured as "x86_64-pc-cygwin".
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> For bug reporting instructions, please see:
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> Reading symbols from ./winsleep...
> (gdb) run
> Starting program: /home/yano/winsleep
> [New Thread 49324.0x14178]
> [Thread 49324.0x14178 exited with code 0]
> [Inferior 1 (process 49324) exited normally]
> (gdb) run
> Starting program: /home/yano/winsleep
>       0 [] gdb 294 fhandler_pty_slave::setup_pseudoconsole: CreatePseudoConsole() failed. 00000057 80070057
>                            [New Thread 86480.0xfd4]
> [Thread 86480.0xfd4 exited with code 0]
> [Inferior 1 (process 86480) exited normally]
> (gdb)
> 
> The essential problem is lack of restoring nat handles for *ALL* the
> PTY-slave instances after closing pseudo console in GDB.
> 
> Restoring handles from pseudo console handles to simple pipe handles
> is not necessary in normal non-cygwin apps because pseudo console is
> setup in the stub process for the non-cygwin app and the stub process
> exits after the app is terminated.
> 
> However, for GDB, pseudo console is setup in GDB process in hooked
> CreateProcess() because GDB does not use exec() to run an inferior
> (debuggee). Therefore, after the inferior exits, nat handle must be
> restored to simple pipe handles.
> 
> The current code restores only handles in the PTY-slave instance
> that has called fhandler_pty_slave::reset_switch_to_nat_pipe(). If
> this instance is different from the instance that will setup pseudo
> console, the nat handles are not restored correctly, then call to
> CreatePseudoConsole() causes error.
> 
> To solves this issue, restore nat handles in all the PTY-slave
> instances to simple pipe handles when the inferior exits with this
> patch.
> 
> Fixes: 8aeb3f3e5037 ("Cygwin: pty: Make apps using console APIs be able to debug with gdb.")
> Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>
> Reviewed-by:
> ---
>  winsup/cygwin/fhandler/pty.cc | 32 ++++++++++++++++++++++++--------
>  1 file changed, 24 insertions(+), 8 deletions(-)
> 
> diff --git a/winsup/cygwin/fhandler/pty.cc b/winsup/cygwin/fhandler/pty.cc
> index 2c2917525..781273688 100644
> --- a/winsup/cygwin/fhandler/pty.cc
> +++ b/winsup/cygwin/fhandler/pty.cc
> @@ -1277,6 +1277,8 @@ fhandler_pty_slave::reset_switch_to_nat_pipe (void)
>  	      else
>  		hand_over_only (get_ttyp ());
>  	      ReleaseMutex (pipe_sw_mutex);
> +
> +	      HANDLE input_handle_nat, output_handle_nat;
>  	      if (need_restore_handles)
>  		{
>  		  pinfo p (get_ttyp ()->master_pid);
> @@ -1284,16 +1286,15 @@ fhandler_pty_slave::reset_switch_to_nat_pipe (void)
>  		    OpenProcess (PROCESS_DUP_HANDLE, FALSE, p->dwProcessId);
>  		  if (pty_owner)
>  		    {
> -		      CloseHandle (get_handle_nat ());
>  		      DuplicateHandle (pty_owner,
>  				       get_ttyp ()->from_master_nat (),
> -				       GetCurrentProcess (), &get_handle_nat (),
> +				       GetCurrentProcess (),
> +				       &input_handle_nat,
>  				       0, TRUE, DUPLICATE_SAME_ACCESS);
> -		      CloseHandle (get_output_handle_nat ());
>  		      DuplicateHandle (pty_owner,
>  				       get_ttyp ()->to_master_nat (),
>  				       GetCurrentProcess (),
> -				       &get_output_handle_nat (),
> +				       &output_handle_nat,
>  				       0, TRUE, DUPLICATE_SAME_ACCESS);
>  		      CloseHandle (pty_owner);
>  		    }
> @@ -1313,11 +1314,26 @@ fhandler_pty_slave::reset_switch_to_nat_pipe (void)
>  		      CloseHandle (repl.to_master); /* not used. */
>  		      CloseHandle (repl.to_slave_nat); /* not used. */
>  		      CloseHandle (repl.to_slave); /* not used. */
> -		      CloseHandle (get_handle_nat ());
> -		      set_handle_nat (repl.from_master_nat);
> -		      CloseHandle (get_output_handle_nat ());
> -		      set_output_handle_nat (repl.to_master_nat);
> +		      input_handle_nat = repl.from_master_nat;
> +		      output_handle_nat = repl.to_master_nat;
>  		    }
> +
> +		  /* Restore nat handles in all pty slave instances */
> +		  HANDLE orig_input_handle_nat = get_handle_nat();
> +		  HANDLE orig_output_handle_nat = get_output_handle_nat();
> +		  cygheap_fdenum cfd (false);
> +		  while (cfd.next () >= 0)
> +		    if (cfd->get_device () == get_device ())
> +		      {
> +			fhandler_base *fh = cfd;
> +			fhandler_pty_slave *ptys = (fhandler_pty_slave *) fh;
> +			if (ptys->get_handle_nat () == orig_input_handle_nat)
> +			  ptys->set_handle_nat (input_handle_nat);
> +			if (ptys->get_output_handle_nat () == orig_output_handle_nat)
> +			  ptys->set_output_handle_nat (output_handle_nat);
> +		      }
> +		  CloseHandle (orig_input_handle_nat);
> +		  CloseHandle (orig_output_handle_nat);
>  		}
>  	      myself->exec_dwProcessId = 0;
>  	      isHybrid = false;
> -- 
> 2.51.0
> 


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