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

In our CI systems we can observe that t0610 fails rather frequently. This testcase races a bunch of git-update-ref(1) processes with one another which are all trying to update a unique reference, where we expect that all processes succeed and end up updating the reftable stack. The error message in this case looks like the following:

    fatal: update_ref failed for ref 'refs/heads/branch-88': reftable: transaction prepare: I/O error

Instrumenting the code with a couple of calls to `BUG()` in relevant sites where we return `REFTABLE_IO_ERROR` quickly leads one to discover that this error is caused when calling `flock_acquire()`, which is a thin wrapper around our lockfile API. Curiously, the error code we get in such cases is `EACCESS`, indicating that we are not allowed to access the file.

The root cause of this is an oddity of `CreateFileW()`, which is what `_wopen()` uses internally. Quoting its documentation [1]:

    If you call CreateFile on a file that is pending deletion as a
    result of a previous call to DeleteFile, the function fails. The
    operating system delays file deletion until all handles to the file
    are closed. GetLastError returns ERROR_ACCESS_DENIED.

This behaviour is triggered quite often in the above testcase because all the processes race with one another trying to acquire the lock for the "tables.list" file. This is due to how locking works in the reftable library when compacting a stack:

    1. Lock the "tables.list" file and reads its contents.

    2. Decide which tables to compact.

    3. Lock each of the individual tables that we are about to compact.

    4. Unlock the "tables.list" file.

    5. Compact the individual tables into one large table.

    6. Re-lock the "tables.list" file.

    7. Write the new list of tables into it.

    8. Commit the "tables.list" file.

The important step is (4): we don't commit the file directly by renaming it into place, but instead we delete the lockfile so that concurrent processes can continue to append to the reftable stack while we compact the tables. And because we use `DeleteFileW()` to do so, we may now race with another process that wants to acquire that lockfile. So if we are unlucky, we would now see `ERROR_ACCESS_DENIED` instead of the expected `ERROR_FILE_EXISTS`, which the lockfile subsystem isn't prepared to handle and thus it will bail out without retrying to acquire the lock.

In theory, the issue is not limited to the reftable library and can be triggered by every other user of the lockfile subsystem, as well. My gut feeling tells me it's rather unlikely to surface elsewhere though.

Fix the issue by translating the error to `EEXIST`. This makes the lockfile subsystem handle the error correctly: in case a timeout is set it will now retry acquiring the lockfile until the timeout has expired.

With this, t0610 is now always passing on my machine whereas it was previously failing in around 20-30% of all test runs.

[1]: https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilew

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 compat/mingw.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)