[PATCH v2] mingw: align symlinks-related rmdir() behavior with Linux

From: =?UTF-8?q?Thomas=20B=C3=A9tous?= <tomspycell@gmail.com>

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>
---
    mingw: support the git-repo tool better
    
    This addresses an issue, originally reported at
    https://github.com/git-for-windows/git/issues/2967, where the git-repo
    tool [https://gerrit.googlesource.com/git-repo/] replaces folders in
    .git/ with symlinks and mingw_rmdir() erroneously removes the symlink
    target directory's contents.
    
    Changes since v1:
    
     * Fixed the authorship
     * Augmented the commit message to elaborate on what the issues are,
       concretely, with the current behavior of mingw_rmdir()
     * Added an explanation to the commit message as to what happens during
       a git rebase that would trigger the rmdir() code path to misbehave
     * Adjusted the code comment to talk specifically about the
       remove_path() caller and why we want to align with Linux' rmdir()
       behavior

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1008%2Fdscho%2Ffix-rmdir-with-symlinks-on-windows-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1008/dscho/fix-rmdir-with-symlinks-on-windows-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1008

Range-diff vs v1:

 1:  c5a7fe0007f ! 1:  0a9cb1bcc14 mingw: align symlinks-related rmdir() behavior with Linux
     @@
       ## Metadata ##
     -Author: Johannes Schindelin <Johannes.Schindelin@gmx.de>
     +Author: Thomas Bétous <tomspycell@gmail.com>
      
       ## Commit message ##
          mingw: align symlinks-related rmdir() behavior with Linux
     @@ Commit message
          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.
     +    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.
      
     -    This fixes https://github.com/git-for-windows/git/issues/2967
     +    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
     @@ compat/mingw.c: int mingw_rmdir(const char *pathname)
      +	struct stat st;
      +
      +	/*
     -+	* Contrary to Linux rmdir(), Windows' _wrmdir() and _rmdir()
     -+	* will remove the directory at the path if it is a symbolic link
     -+	* which leads to issues when symlinks are used in the .git folder
     -+	* (in the context of git-repo for instance). So 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() in this case (ENOTDIR).
     -+	*/
     ++	 * 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;

 compat/mingw.c    | 21 +++++++++++++++++++++
 t/t3400-rebase.sh | 10 ++++++++++
 t/test-lib.sh     |  6 ++++++
 3 files changed, 37 insertions(+)