Since 183ea3ea (Merge branch 'ps/mingw-rename', 2024-11-13),
a new technique is used on Windows to rename files, where supported.
The first step of this technique is to open the file with
`CreateFileW`. At that time, `FILE_ATTRIBUTE_NORMAL` was passed as
the value of the `dwFlagsAndAttributes` argument. In b30404df [2], this
was improved by passing `FILE_FLAG_BACKUP_SEMANTICS`, to support
directories as well as regular files.
However, neither value of `dwFlagsAndAttributes` is sufficient to open
a symbolic link with the correct semantics to rename it. Symlinks on
Windows are reparse points. Attempting to open a reparse point with
`CreateFileW` dereferences the reparse point and opens the target
instead, unless `FILE_FLAG_OPEN_REPARSE_POINT` is included in
`dwFlagsAndAttributes`. This is documented for that flag and in the
"Symbolic Link Behavior" section of the `CreateFileW` docs [3].
This produces a regression where attempting to rename a symlink on
Windows renames its target to the intended new name and location of the
symlink. For example, if `symlink` points to `file`, then running
git mv symlink symlink-renamed
leaves `symlink` in place and unchanged, but renames `file` to
`symlink-renamed` [4].
This regression is detectable by existing tests in `t7001-mv.sh`, but
the tests must be run by a Windows user with the ability to create
symlinks, and the `ln -s` command used to create the initial symlink
must also be able to create a real symlink (such as by setting the
`MSYS` environment variable to `winsymlinks:nativestrict`). Then
these two tests fail if the regression is present, and pass otherwise:
38 - git mv should overwrite file with a symlink
39 - check moved symlink
Let's fix this, so that renaming a symlink again renames the symlink
itself and leaves the target unchanged, by passing
FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OPEN_REPARSE_POINT
as the `dwFlagsAndAttributes` argument. This is sufficient (and safe)
because including `FILE_FLAG_OPEN_REPARSE_POINT` causes no harm even
when used to open a file or directory that is not a reparse point. In
that case, as noted in [3], this flag is simply ignored.
[1]: 183ea3eabf
[2]: b30404dfc0
[3]: https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilew
[4]: https://github.com/git-for-windows/git/issues/5436
Signed-off-by: Eliah Kagan <eliah.kagan@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
975fc0471a