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Date:	Thu, 28 Aug 2025 10:33:43 +0200
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Subject:	Re: pty read loop losing data in wsl context
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From:	Thomas Wolff via Cygwin <cygwin AT cygwin DOT com>
Reply-To:	Thomas Wolff <towo AT towo DOT net>
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Am 28.08.2025 um 05:36 schrieb Thomas Wolff via Cygwin:
> When running wsl.exe from mintty, typing ^O corrupts terminal display.
> This was reported in
> https://github.com/mintty/mintty/issues/1332
> and I later reported it towards Windows conpty
> https://github.com/microsoft/terminal/issues/19285
> but conpty is unlikely the cause, as testing suggests.
>
> Test case, in mintty:
> wsl
> ls -1 /bin
> ^O # enter the control character
> ls -1 /bin # repeat
>
> The same happens in xterm and some other terminals, but not all.
>
> Mintty uses a loop to read terminal output from the pty and fill its 
> buffer:
>         do {
>           int ret = read(pty_fd, buf + len, sizeof buf - len);
>           if (ret > 0)
>             len += ret;
>           else
>             break;
>         } while (len < sizeof buf);
> Tracing the loop shows that, while running wsl, after having entered 
> ^O, every subsequent read returns -1.
> Taking out the loop (by patching it, or currently by using option Baud 
> with some high value) fixes the problem.
> I suspect some very obscure interworking problem between wsl/conpty 
> and cygwin pty handling. As noted in the latter issue, it does not 
> happen with terminals running remotely, so the suspicion goes more to 
> the cygwin side.
> What’s the role of ^O in this puzzle? Does it trigger something in 
> cygwin?
> Is there anything peculiar that would cause pty chunks not be 
> recognised and then skipped in such a loop?
> Thomas
>
I've tested this behaviour does not occur with cygwin 3.3.4 but it does 
with cygwin 3.3.5.

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