[PATCH v2] 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.

Failing instead of ignoring here makes it easier for the users to notice
anomalies and take action, particularly if this non-file non-directory
entity was created by mistake or by file system corruption.

Resolve the NEEDSWORK by using `lstat()` to check the entity's mode. To
avoid performance penalties on the happy path, this check is only
performed after	`is_git_directory()` fails.

Signed-off-by: Tian Yuchen <a3205153416@gmail.com>
---
Change in v2:

- Moved the `lstat` check into an `else` block after `is_git_directory()`
to avoid any extra system call overhead on the normal happy path, as
suggested by Junio.

 setup.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/setup.c b/setup.c
index 3a6a048620..95658f2547 100644
--- a/setup.c
+++ b/setup.c
@@ -1580,11 +1580,23 @@ static enum discovery_result setup_git_directory_gently_1(struct strbuf *dir,
 						NULL : &error_code);
 		if (!gitdirenv) {
 			if (die_on_error ||
-			    error_code == READ_GITFILE_ERR_NOT_A_FILE) {
-				/* NEEDSWORK: fail if .git is not file nor dir */
-				if (is_git_directory(dir->buf)) {
+				error_code == READ_GITFILE_ERR_NOT_A_FILE) {
+				if (is_git_directory(dir->buf)){
 					gitdirenv = DEFAULT_GIT_DIR_ENVIRONMENT;
 					gitdir_path = xstrdup(dir->buf);
+				} else {
+					/*
+					 * It is neither a gitfile nor a git directory.
+					 */
+					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;
+					}
 				}
 			} else if (error_code != READ_GITFILE_ERR_STAT_FAILED)
 				return GIT_DIR_INVALID_GITFILE;
-- 
2.43.0