[PATCH v3 1/3] compat/mingw: share file handles created via `CreateFileW()`

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:

  - `FILE_SHARE_READ` allows a concurrent process to open the file for
    reading.

  - `FILE_SHARE_WRITE` allows a concurrent process to open the file for
    writing.

  - `FILE_SHARE_DELETE` allows a concurrent process to delete the file
    or to replace it via an atomic rename.

This sharing mechanism is quite important in the context of Git, as we assume POSIX semantics all over the place. But there are two callsites where we don't pass all three of these flags:

  - 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. The
    function was introduced via d641097589 (mingw: enable atomic
    O_APPEND, 2018-08-13) and has been using `FILE_SHARE_READ |
    FILE_SHARE_WRITE` since the inception. There aren't any indicators
    that the omission of `FILE_SHARE_DELETE` was intentional.

  - We don't set any sharing flags in `mingw_utime()`, which changes the
    access and modification of a file. 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.

    `mingw_utime()` was originally implemented via `_wopen()`, which
    doesn't give you full control over the sharing mode. Instead, it
    calls `_wsopen()` with `_SH_DENYNO`, which ultimately translates to
    `FILE_SHARE_READ | FILE_SHARE_WRITE`. It was then refactored via
    090a3085bc (t/helper/test-chmtime: update mingw to support chmtime
    on directories, 2022-03-02) to use `CreateFileW()`, but we stopped
    setting any sharing flags at all, which seems like an unintentional
    side effect. By restoring `FILE_SHARE_READ | FILE_SHARE_WRITE` we
    thus fix this and get back the old behaviour of `_wopen()`.

    The fact that we didn't set the equivalent of `FILE_SHARE_DELETE`
    can be explained, as well: neither `_wopen()` nor `_wsopen()` allow
    you to do so. So overall, it doesn't seem intentional that we didn't
    allow deletions here, either.

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

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 compat/mingw.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)