setup: improve error diagnosis for invalid .git files

'read_gitfile_gently()' treats any non-regular file as
'READ_GITFILE_ERR_NOT_A_FILE' and fails to discern between 'ENOENT'
and other stat failures. This flawed error reporting is noted by two
'NEEDSWORK' comments.

Address these comments by introducing two new error codes:
'READ_GITFILE_ERR_MISSING'(which groups the "file missing" scenarios
together) and 'READ_GITFILE_ERR_IS_A_DIR':

1. Update 'read_gitfile_error_die()' to treat 'IS_A_DIR', 'MISSING',
'NOT_A_FILE' and 'STAT_FAILED' as non-fatal no-ops. This accommodates
intentional non-repo scenarios (e.g., GIT_DIR=/dev/null).

2. Explicitly catch 'NOT_A_FILE' and 'STAT_FAILED' during
discovery and call 'die()' if 'die_on_error' is set.

3. Unconditionally pass '&error_code' to 'read_gitfile_gently()'.

4. Only invoke 'is_git_directory()' when we explicitly receive
   'READ_GITFILE_ERR_IS_A_DIR', avoiding redundant checks.

Additionally, audit external callers of 'read_gitfile_gently()' in
'submodule.c' and 'worktree.c' to accommodate the refined error codes.

Signed-off-by: Tian Yuchen <a3205153416@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 36 insertions, 11 deletions

diff --git a/setup.c b/setup.c
index b723f8b339..0ae9c88466 100644
--- a/setup.c
+++ b/setup.c
@@ -895,8 +895,10 @@ int verify_repository_format(const struct repository_format *format,
 void read_gitfile_error_die(int error_code, const char *path, const char *dir)
 {
 	switch (error_code) {
-	case READ_GITFILE_ERR_STAT_FAILED:
 	case READ_GITFILE_ERR_NOT_A_FILE:
+	case READ_GITFILE_ERR_STAT_FAILED:
+	case READ_GITFILE_ERR_MISSING:
+	case READ_GITFILE_ERR_IS_A_DIR:
 		/* non-fatal; follow return path */
 		break;
 	case READ_GITFILE_ERR_OPEN_FAILED:
@@ -939,8 +941,14 @@ const char *read_gitfile_gently(const char *path, int *return_error_code)
 	static struct strbuf realpath = STRBUF_INIT;
 
 	if (stat(path, &st)) {
-		/* NEEDSWORK: discern between ENOENT vs other errors */
-		error_code = READ_GITFILE_ERR_STAT_FAILED;
+		if (errno == ENOENT || errno == ENOTDIR)
+			error_code = READ_GITFILE_ERR_MISSING;
+		else
+			error_code = READ_GITFILE_ERR_STAT_FAILED;
+		goto cleanup_return;
+	}
+	if (S_ISDIR(st.st_mode)) {
+		error_code = READ_GITFILE_ERR_IS_A_DIR;
 		goto cleanup_return;
 	}
 	if (!S_ISREG(st.st_mode)) {
@@ -1576,20 +1584,37 @@ static enum discovery_result setup_git_directory_gently_1(struct strbuf *dir,
 		if (offset > min_offset)
 			strbuf_addch(dir, '/');
 		strbuf_addstr(dir, DEFAULT_GIT_DIR_ENVIRONMENT);
-		gitdirenv = read_gitfile_gently(dir->buf, die_on_error ?
-						NULL : &error_code);
+		gitdirenv = read_gitfile_gently(dir->buf, &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 */
+			switch (error_code) {
+			case READ_GITFILE_ERR_MISSING:
+				/* no .git in this directory, move on */
+				break;
+			case READ_GITFILE_ERR_IS_A_DIR:
 				if (is_git_directory(dir->buf)) {
 					gitdirenv = DEFAULT_GIT_DIR_ENVIRONMENT;
 					gitdir_path = xstrdup(dir->buf);
 				}
-			} else if (error_code != READ_GITFILE_ERR_STAT_FAILED)
-				return GIT_DIR_INVALID_GITFILE;
-		} else
+				break;
+			case READ_GITFILE_ERR_STAT_FAILED:
+				if (die_on_error)
+					die(_("error reading '%s'"), dir->buf);
+				else
+					return GIT_DIR_INVALID_GITFILE;
+			case READ_GITFILE_ERR_NOT_A_FILE:
+				if (die_on_error)
+					die(_("not a regular file: '%s'"), dir->buf);
+				else
+					return GIT_DIR_INVALID_GITFILE;
+			default:
+				if (die_on_error)
+					read_gitfile_error_die(error_code, dir->buf, NULL);
+				else
+					return GIT_DIR_INVALID_GITFILE;
+			}
+		} else {
 			gitfile = xstrdup(dir->buf);
+		}
 		/*
 		 * Earlier, we tentatively added DEFAULT_GIT_DIR_ENVIRONMENT
 		 * to check that directory for a repository.