mingw: align symlinks-related rmdir() behavior with Linux

When performing a rebase, rmdir() is called on the folder .git/logs. On
Unix rmdir() exits without deleting anything in case .git/logs is a
symbolic link but the equivalent functions on Windows (_rmdir, _wrmdir
and RemoveDirectoryW) do not behave the same and remove the folder if it
is symlinked even if it is not empty.

This creates issues when folders in .git/ are symlinks which is
especially the case when git-repo[1] is used: It replaces `.git/logs/`
with a symlink.

One such issue is that the _target_ of that symlink is removed e.g.
during a `git rebase`, where `delete_reflog("REBASE_HEAD")` will not
only try to remove `.git/logs/REBASE_HEAD` but then recursively try to
remove the parent directories until an error occurs, a technique that
obviously relies on `rmdir()` refusing to remove a symlink.

This was reported in https://github.com/git-for-windows/git/issues/2967.

This commit updates mingw_rmdir() so that its behavior is the same as
Linux rmdir() in case of symbolic links.

To verify that Git does not regress on the reported issue, this patch
adds a regression test for the `git rebase` symptom, even if the same
`rmdir()` behavior is quite likely to cause potential problems in other
Git commands as well.

[1]: git-repo is a python tool built on top of Git which helps manage
many Git repositories. It stores all the .git/ folders in a central
place by taking advantage of symbolic links.
More information: https://gerrit.googlesource.com/git-repo/

Signed-off-by: Thomas Bétous <tomspycell@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/compat/mingw.c b/compat/mingw.c
index aa647b367b..9e0cd1e097 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -341,6 +341,27 @@ int mingw_rmdir(const char *pathname)
 {
 	int ret, tries = 0;
 	wchar_t wpathname[MAX_PATH];
+	struct stat st;
+
+	/*
+	 * Contrary to Linux' `rmdir()`, Windows' _wrmdir() and _rmdir()
+	 * (and `RemoveDirectoryW()`) will attempt to remove the target of a
+	 * symbolic link (if it points to a directory).
+	 *
+	 * This behavior breaks the assumption of e.g. `remove_path()` which
+	 * upon successful deletion of a file will attempt to remove its parent
+	 * directories recursively until failure (which usually happens when
+	 * the directory is not empty).
+	 *
+	 * Therefore, before calling `_wrmdir()`, we first check if the path is
+	 * a symbolic link. If it is, we exit and return the same error as
+	 * Linux' `rmdir()` would, i.e. `ENOTDIR`.
+	 */
+	if (!mingw_lstat(pathname, &st) && S_ISLNK(st.st_mode)) {
+		errno = ENOTDIR;
+		return -1;
+	}
+
 	if (xutftowcs_path(wpathname, pathname) < 0)
 		return -1;