setup: add gentle version of read_gitfile

read_gitfile will die on most error cases. This makes it unsuitable
for speculative calls. Extract the core logic and provide a gentle
version that returns NULL on failure.

The first usecase of the new gentle version will be to probe for
submodules during git clean.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Erik Elfström <erik.elfstrom@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -446,7 +446,16 @@ extern int get_common_dir(struct strbuf *sb, const char *gitdir);
 extern const char *get_git_namespace(void);
 extern const char *strip_namespace(const char *namespaced_ref);
 extern const char *get_git_work_tree(void);
-extern const char *read_gitfile(const char *path);
+
+#define READ_GITFILE_ERR_STAT_FAILED 1
+#define READ_GITFILE_ERR_NOT_A_FILE 2
+#define READ_GITFILE_ERR_OPEN_FAILED 3
+#define READ_GITFILE_ERR_READ_FAILED 4
+#define READ_GITFILE_ERR_INVALID_FORMAT 5
+#define READ_GITFILE_ERR_NO_PATH 6
+#define READ_GITFILE_ERR_NOT_A_REPO 7
+extern const char *read_gitfile_gently(const char *path, int *return_error_code);
+#define read_gitfile(path) read_gitfile_gently((path), NULL)
 extern const char *resolve_gitdir(const char *suspect);
 extern void set_git_work_tree(const char *tree);
 

setup.c

@@ -406,35 +406,53 @@ static void update_linked_gitdir(const char *gitfile, const char *gitdir)
 /*
  * Try to read the location of the git directory from the .git file,
  * return path to git directory if found.
+ *
+ * On failure, if return_error_code is not NULL, return_error_code
+ * will be set to an error code and NULL will be returned. If
+ * return_error_code is NULL the function will die instead (for most
+ * cases).
  */
-const char *read_gitfile(const char *path)
+const char *read_gitfile_gently(const char *path, int *return_error_code)
 {
-	char *buf;
-	char *dir;
+	int error_code = 0;
+	char *buf = NULL;
+	char *dir = NULL;
 	const char *slash;
 	struct stat st;
 	int fd;
 	ssize_t len;
 
-	if (stat(path, &st))
-		return NULL;
-	if (!S_ISREG(st.st_mode))
-		return NULL;
+	if (stat(path, &st)) {
+		error_code = READ_GITFILE_ERR_STAT_FAILED;
+		goto cleanup_return;
+	}
+	if (!S_ISREG(st.st_mode)) {
+		error_code = READ_GITFILE_ERR_NOT_A_FILE;
+		goto cleanup_return;
+	}
 	fd = open(path, O_RDONLY);
-	if (fd < 0)
-		die_errno("Error opening '%s'", path);
+	if (fd < 0) {
+		error_code = READ_GITFILE_ERR_OPEN_FAILED;
+		goto cleanup_return;
+	}
 	buf = xmalloc(st.st_size + 1);
 	len = read_in_full(fd, buf, st.st_size);
 	close(fd);
-	if (len != st.st_size)
-		die("Error reading %s", path);
+	if (len != st.st_size) {
+		error_code = READ_GITFILE_ERR_READ_FAILED;
+		goto cleanup_return;
+	}
 	buf[len] = '\0';
-	if (!starts_with(buf, "gitdir: "))
-		die("Invalid gitfile format: %s", path);
+	if (!starts_with(buf, "gitdir: ")) {
+		error_code = READ_GITFILE_ERR_INVALID_FORMAT;
+		goto cleanup_return;
+	}
 	while (buf[len - 1] == '\n' || buf[len - 1] == '\r')
 		len--;
-	if (len < 9)
-		die("No path in gitfile: %s", path);
+	if (len < 9) {
+		error_code = READ_GITFILE_ERR_NO_PATH;
+		goto cleanup_return;
+	}
 	buf[len] = '\0';
 	dir = buf + 8;
 
@@ -448,14 +466,42 @@ const char *read_gitfile(const char *path)
 		free(buf);
 		buf = dir;
 	}
-
-	if (!is_git_directory(dir))
-		die("Not a git repository: %s", dir);
-
+	if (!is_git_directory(dir)) {
+		error_code = READ_GITFILE_ERR_NOT_A_REPO;
+		goto cleanup_return;
+	}
 	update_linked_gitdir(path, dir);
 	path = real_path(dir);
 
+cleanup_return:
 	free(buf);
+
+	if (return_error_code)
+		*return_error_code = error_code;
+
+	if (error_code) {
+		if (return_error_code)
+			return NULL;
+
+		switch (error_code) {
+		case READ_GITFILE_ERR_STAT_FAILED:
+		case READ_GITFILE_ERR_NOT_A_FILE:
+			return NULL;
+		case READ_GITFILE_ERR_OPEN_FAILED:
+			die_errno("Error opening '%s'", path);
+		case READ_GITFILE_ERR_READ_FAILED:
+			die("Error reading %s", path);
+		case READ_GITFILE_ERR_INVALID_FORMAT:
+			die("Invalid gitfile format: %s", path);
+		case READ_GITFILE_ERR_NO_PATH:
+			die("No path in gitfile: %s", path);
+		case READ_GITFILE_ERR_NOT_A_REPO:
+			die("Not a git repository: %s", dir);
+		default:
+			assert(0);
+		}
+	}
+
 	return path;
 }