[PATCH v2 0/2] compat/mingw: fix EACCESS when opening files with `O_CREAT | O_EXCL`

Hi,

I finally found some time to have a look at why t0610 is failing regularly in MinGW. As it turns out the root cause is our emulation of open(3p): when trying to open a file with `_wopen(..., O_CREAT|O_EXCL)` the call fails in case another process has marked the same file for deletion via `DeleteFileW()`. This gets triggered by t0610 because we race around locking the reftable stack and thus causes the failure.

The fix is simple: we get `ERROR_ACCESS_DENIED` in this situation, so instead of translating that error to `EACCESS` we translate it to `EEXIST`. This fixes the flake on my machine, but as usual when it comes to Windows I would very much like to ask those in the know to point out any obvious mistakes I did.

The other patch is a while-at-it patch that I was wondering about while debugging the issue. It's not needed and I'm happy to drop it if you don't think we should include it.

Changes in v2:
  - Make the workaround more specific by also paying attention to the
    NtStatus code. Like this, we only translate the error when we see
    that the error code was `STATUS_DELETE_PENDING`, which should rule
    out that the translation triggers in unintended cases.
  - A new patch for Meson that makes us pull in "compat/msvc.c" instead
    of "compat/mingw.c". This is more of a while-at-it fix that I
    spotted while working on this patch series. It doesn't have any
    ramifications for what I'm doing.
  - Drop the patch that makes us handle O_CLOEXEC. It's not needed, and
    I'd rather focus on changes that actually improve the situation.
  - Link to v1: https://lore.kernel.org/r/20250313-b4-pks-mingw-lockfile-flake-v1-0-bc5d3e70f516@pks.im

Thanks!

Patrick

---
Patrick Steinhardt (2):
      meson: fix compat sources when compiling with MSVC
      compat/mingw: fix EACCESS when opening files with `O_CREAT | O_EXCL`

 compat/mingw.c | 20 ++++++++++++++++++++
 meson.build    |  4 +++-
 2 files changed, 23 insertions(+), 1 deletion(-)

Range-diff versus v1:

1: c2c1f988729 < -: ----------- compat/mingw: handle O_CLOEXEC in `mingw_open_existing()` 2: fd698866034 < -: ----------- compat/mingw: fix EACCESS when opening files with `O_CREAT | O_EXCL` -: ----------- > 1: 9a2798b1b63 meson: fix compat sources when compiling with MSVC -: ----------- > 2: ff5bf477747 compat/mingw: fix EACCESS when opening files with `O_CREAT | O_EXCL`

--- base-commit: 4b68faf6b93311254efad80e554780e372deb42f change-id: 20250313-b4-pks-mingw-lockfile-flake-49dfcce8e7c2