pty/terminfo dependency when running WSL

[Thomas Wolff]

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.
Thomas