[PATCH 0/3] compat/mingw: implement POSIX-style atomic renames

Hi,

so after chatting with Johannes recently I decided to have another stab at the failures we've seen on Windows with the reftable backend. A quick refresher: reftables use a central "tables.lock" file that gets updated via renames such that the update itself is atomic. This causes issues on Windows when there are many concurrent readers/writers because Windows does not allow you to rename a file over another file that is held open by another process by default. This issue is surfaced by t0610, where we spawn a 100 concurrent git-update-ref(1) processes.

This patch series aims to fix the issue on Windows by implementing POSIX-style atomic renames. The first two patches refactor the code such that we use `FILE_SHARE_DELETE` to open files, which allows other processes to delete the file even while we have an open handle. This sharing mode is also important in the context of renames, which are treated like a deletion of the replaced file. The third patch then reimplements renames via `SetFileAttributesByHandle(FileRenameInfoEx)`, which has a flag `FILE_RENAME_FLAG_POSIX_SEMANTICS`. When set, Windows allows us to replace files during a rename which still have an open handle.

There's one catch: this is only supported in Windows 10 and newer. On one hand this means that we have to provide the required definitions ourselves, as we cannot bump the Windows SDK version. On the other hand we also have to support the case where older Windows versions now fail at runtime due to `FileRenameInfoEx` being unsupported. But overall this isn't too bad, I'd say. Johannes has also kindly tested the fallback logic for me on Windows 8.1.

With these changes t0610 now passes on my Windows 10 machine, but this may also have a positive impact on other parts of Git where we might have previously failed.

Thanks!

Patrick

Patrick Steinhardt (3):
  compat/mingw: share file handles created via `CreateFileW()`
  compat/mingw: allow deletion of most opened files
  compat/mingw: support POSIX semantics for atomic renames

 compat/mingw.c             | 146 +++++++++++++++++++++++++++++++++++--
 t/t0610-reftable-basics.sh |   8 +-
 2 files changed, 145 insertions(+), 9 deletions(-)