[PATCH] compat/mingw: rename the symlink, not the target

From: Eliah Kagan <eliah.kagan@gmail.com>

Since 183ea3e [1], 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]: https://github.com/git-for-windows/git/commit/183ea3eabf81822506d2cd3aa1dc0727099ebccd [2]: https://github.com/git-for-windows/git/commit/b30404dfc04a4b087b630aea4ab88a51cd3a7459 [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>
---
    compat/mingw: rename the symlink, not the target
    
    This contribution just came in as a Git for Windows Pull Request.
    
    Granted, I have not yet managed to find time to upstream support for
    symbolic links (it is in the pipeline:
    https://github.com/dscho/git/tree/support-symlinks-on-windows), but this
    patch still should be in upstream Git because there are other ways to
    create symbolic links than by using Git.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1864%2Fdscho%2Ffix-renaming-symlinks-on-windows-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1864/dscho/fix-renaming-symlinks-on-windows-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1864

 compat/mingw.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)