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Date: Mon, 18 Aug 2025 14:38:12 +0200
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Subject: Re: pty/terminfo dependency when running WSL
To: cygwin@cygwin.com
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Precedence: list
From: Thomas Wolff via Cygwin <cygwin@cygwin.com>
Reply-To: Thomas Wolff <towo@towo.net>
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Am 16.08.2025 um 18:32 schrieb Brian Inglis via Cygwin:
> On 2025-08-16 07:29, Thomas Wolff via Cygwin wrote:
>> Hi Takashi,
>> thanks for the background...
>>
>> Am 16.08.2025 um 14:45 schrieb Takashi Yano via Cygwin:
>>> Hi Thomas,
>>>
>>> On Sat, 16 Aug 2025 13:22:35 +0200
>>> Thomas Wolff wrote:
>>>> There is an weird interworking issue when launching WSL from a minimal
>>>> standalone installation of cygwin.
>>>> (It can be reproced with a full installation, though, see below).
>>>> When calling Windows-native wsl.exe, as implicitly done via mintty
>>>> --WSL, and there is no terminfo database installed, something fails
>>>> about pty setup, and the WSL shell will be run in pty-less mode (no
>>>> prompt, no stty signal handling, ^C terminates the shell). When 
>>>> terminfo
>>>> is present, everything works as expected.
>>>> Test case:
>>>> remove /usr/share/terminfo/78/$TERM (xterm or xterm-256color, or the
>>>> whole subdirectory, or the 6d or 76 subdirectories when using
>>>> TERM=mintty* or vt settings), i.e. move it to some saving place.
>>>> Run mintty --WSL, observe the issue described above.
>>>> Interestingly, cygwin shells are not affected by the issue, so some
>>>> obscure interworking of the dreadful Windows ConPTY layer or so might
>>>> contribute to the issue, but in any case I doubt that the presence of
>>>> terminfo should affect the effective pty behaviour.
>>> ConPTY assumes xterm sequences unconditionally and use CSI 6n at the
>>> startup. If the terminal does not support CSI 6n and no responce from
>>> the terminal, CreatePseudoConsole() hangs. So pty checks terminfo to
>>> determine whethter ConPTY can be used.
>>>
>>> If the terminal does not support CSI 6n, pty does not use ConPTY for
>>> non-cygwin app like WSL. In this case, the stdin/stdout is just a
>>> pipe for WSL, so shell does not shows prompt. However, the shell
>>> itself is running, so the command should be accepted. Please try
>>> to type ls, ps and so on.
>>>
>>> I understand the behaviour of cygwin1.dll should not depends on
>>> the terminal aspects, however, ConPTY forcibly uses xterm sequences.
>>> So this is a desperate measure.
>> The drawback is that standalone packages that use a terminal also 
>> have to bundle terminfo or will run into mysterious failure. I have 
>> solved this for my wsltty package now but maybe an even more tricky 
>> approach could further optimise the scenarios: If pty does not find 
>> terminfo, it could try to propagate CSI6n to the host and only after 
>> a timeout switch to pipe mode. Just an idea.
>
> If mintty does not find terminfo, could it invoke conpty in legacy 
> winpty mode?
Mintty does not look up terminfo, neither does it invoke conpty. This is 
all done "under the hood" in the cygwin pty layer, as I understood.
Or maybe, in the case of launching a WSL session, in the respective WSL 
gateway (wsl.exe or wslbridge2.exe).

>
> $ regtool list -v /proc/registry/HKEY_CURRENT_USER/Console/ | grep 
> ForceV2
> ForceV2 (REG_DWORD) = 0x00000001 (1)
>                     0 = legacy winpty console
>                     1 = updated conpty console
>
>     https://learn.microsoft.com/en-gb/windows/console/legacymode
>
> "ⓘ Note
> This setting is globally applied to all sessions that start after the 
> preference is changed. Sessions that are already open will not be 
> changed."
>
> So it looks like the reg entry could be saved, and if terminfo is not 
> found, restored after the console session is active.
>


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