Re: [PATCH] mingw: enable atomic O_APPEND

> The Windows CRT implements O_APPEND "manually": on write() calls, the
> file pointer is set to EOF before the data is written. Clearly, this is
> not atomic. And in fact, this is the root cause of failures observed in
> t5552-skipping-fetch-negotiator.sh and t5503-tagfollow.sh, where
> different processes write to the same trace file simultanously; it also
> occurred in t5400-send-pack.sh, but there it was worked around in
> 71406ed4d6 ("t5400: avoid concurrent writes into a trace file",
> 2017-05-18).
> 
> Fortunately, Windows does support atomic O_APPEND semantics using the
> file access mode FILE_APPEND_DATA. Provide an implementation that does.
> 
> This implementation is minimal in such a way that it only implements
> the open modes that are actually used in the Git code base. Emulation
> for other modes can be added as necessary later. To become aware of
> the necessity early, the unusal error ENOSYS is reported if an
> unsupported mode is encountered.
> 
> Diagnosed-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
> Helped-by: Jeff Hostetler <git@jeffhostetler.com>
> Signed-off-by: Johannes Sixt <j6t@kdbg.org>
> ---
>   compat/mingw.c | 41 +++++++++++++++++++++++++++++++++++++++--
>   1 file changed, 39 insertions(+), 2 deletions(-)
>