Re: [PATCH 1/3] compat/mingw: share file handles created via `CreateFileW()`

On Wed, Oct 23, 2024 at 05:04:58PM +0200, Patrick Steinhardt wrote:

Unless told otherwise, Windows will keep other processes from reading,
writing and deleting files when one has an open handle that was created
via `CreateFileW()`. This behaviour can be altered via `FILE_SHARE_*`
flags such that other processes _can_ read and/or modify such a file.
This sharing mechanism is quite important in the context of Git, as we
assume POSIX semantics all over the place.

There are two calls where we don't set up those flags though:

  - We don't set `FILE_SHARE_DELETE` when creating a file for appending
    via `mingw_open_append()`. This makes it impossible to delete the
    file from another process or to replace it via an atomic rename.

  - When opening a file such that we can change its access/modification
    times. This makes it impossible to perform any kind of operation
    on this file at all from another process. While we only open the
    file for a short amount of time to update its timestamps, this still
    opens us up for a race condition with another process.

Adapt both of these callsites to pass all three sharing flags.

Interesting, and especially so noting that we *do* call CreateFileW()
with the FILE_SHARE_DELETE flag in other functions like create_watch(),
mingw_open_existing(), mingw_getcwd(), etc.

Why didn't we call CreateFileW() with FILE_SHARE_DELETE in the below two
functions? I assume Johannes Schindelin and/or Johannes Sixt (both CC'd)
would know the answer. Regardless, it would be interesting and useful
(IMHO) to include such a detail in the commit message.

Thanks,
Taylor