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Date: Sat, 30 Aug 2025 10:31:11 +0200
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Subject: Re: pty read loop losing data in wsl context
To: cygwin AT cygwin DOT com
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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 30.08.2025 um 05:11 schrieb Takashi Yano via Cygwin:
> On Thu, 28 Aug 2025 05:36:06 +0200
> Thomas Wolff wrote:
>> 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?
> Thanks for the report. I fixed the Ctrl^O (FLUSHO) handling.
> Please try cygwin-3.7.0-dev-286-g7b5fb35e.
Thanks a lot! That works. I'll aks the OP to retest when the next test 
release is available for setup.

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