[RFC] setup: fail if .git is not a file or directory

[Tian Yuchen <a3205153416@gmail.com>]

Currently, `setup_git_directory_gently_1()` checks if `.git` is a
regular file (handling submodules/worktrees) or a directory. If it is
neither (e.g., a FIFO), the code hits a NEEDSWORK comment and simply
ignores the entity, continuing the discovery process in the parent
directory.

This behavior can be very dangerous. If a user is inside a subdirectory
containing a melformed/broken `.git` entity, the Git will traverse up,
attach to a parent repository and might execute destructive commands.

I tried to resolve the NEEDSWORK by using `lstat()` to explicitly check
the entity's mode. If it is neither a regular file nor a directory, we
kill the discovery process.

But I still have questions:
1. Is failing hard the desired behavior here? Should skipping it and
   continuing discovery be an option for the user, which might seem
   more fault-tolerant?
2. Should we die() immediately here, or return GIT_DIR_INVALID_GITFILE
   and let the caller decide?

Signed-off-by: Tian Yuchen <a3205153416@gmail.com>
---
 setup.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/setup.c b/setup.c
index 3a6a048620..a1b56de67a 100644
--- a/setup.c
+++ b/setup.c
@@ -1581,7 +1581,17 @@ static enum discovery_result setup_git_directory_gently_1(struct strbuf *dir,
 		if (!gitdirenv) {
 			if (die_on_error ||
 			    error_code == READ_GITFILE_ERR_NOT_A_FILE) {
-				/* NEEDSWORK: fail if .git is not file nor dir */
+				struct stat st;
+				if (!lstat(dir->buf, &st) &&
+					!S_ISREG(st.st_mode) &&
+					!S_ISDIR(st.st_mode)){
+
+					if (die_on_error)
+						die(_("Invalid %s: not a regular file or directory"), dir->buf);
+					else
+						return GIT_DIR_INVALID_GITFILE;
+				}
+
 				if (is_git_directory(dir->buf)) {
 					gitdirenv = DEFAULT_GIT_DIR_ENVIRONMENT;
 					gitdir_path = xstrdup(dir->buf);
-- 
2.43.0