Subject: Re: Pipes truncating data in cygwin from main and cygwin-3_4-branch
From: Takashi Yano via Cygwin <[email-masked]>
To: [email-masked]
Cc: キャロウ マーク <[email-masked]>
Date: Tue, 15 Aug 2023 15:42:19 +0900
List-Id: General Cygwin discussions and problem reports <cygwin.cygwin.com>

On Tue, 15 Aug 2023 09:53:16 +0900
キャロウ　マーク wrote:
> > On Aug 15, 2023, at 9:30, Takashi Yano <[email-masked]> wrote:
> > 
> > Your test case does not work in command prompt as well.
> > 
> > Try
> > type testfile | test-pipe sizeoftestfile
> > in command prompt. It will fail.
> 
> Interesting.
> 
> > 
> > New pipe implementation since cygwin 3.4.x provides the pipes
> > more similar to pipe in command prompt for non-cygwin apps.
> 
> What are the differences between these pipes? What changed?

Many changes. But, the change which triggers this behaviour
is setting FILE_SYNCHRONOUS_IO_NONALERT create option.

With this option, seekg() of Microsoft library gets success
for pipes, despite it should not I suppose.

In Linux, man page states that fseek() on pipes will fail.

> > Since your test case is compiled with cl.exe, it is non-cygwin
> > apps.
> 
> When the failure first appeared the bash shell included with Git for Windows was being used to run the pipe (`bash -c "cat foo | bar”`). The shell was started by ctest which had been run from PowerShell. Is this cygwin or non-cygwin?

It depends on "bar". If bar is compiled with cl.exe or gcc/g++ in
MSYS2 mingw environment, it is non-cygwin app.

> in https://github.com/git-for-windows/git/issues/4464#issuecomment-1671137446 the author provided minimal adaption to the reproducer to compile it with g++ and reproduced the failure on Cygwin. It looks like the failure happens for both cygwin and non-cygwin. It is probably related to the new pipe implementation you mentioned.

Is there any failure case where the pipe reader is cygwin (or MSYS2)
binary?

-- 
Takashi Yano <[email-masked]>

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