clone: factor out checking for an alternate path

In a later patch we want to determine if a path is suitable as an
alternate from other commands than builtin/clone. Move the checking
functionality of `add_one_reference` to `compute_alternate_path` that is
defined in cache.h.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/clone.c

@@ -282,44 +282,19 @@ static void strip_trailing_slashes(char *dir)
 
 static int add_one_reference(struct string_list_item *item, void *cb_data)
 {
-	char *ref_git;
+	struct strbuf err = STRBUF_INIT;
-	const char *repo;
+	struct strbuf sb = STRBUF_INIT;
-	struct strbuf alternate = STRBUF_INIT;
+	char *ref_git = compute_alternate_path(item->string, &err);
 
-	/* Beware: read_gitfile(), real_path() and mkpath() return static buffer */
+	if (!ref_git)
-	ref_git = xstrdup(real_path(item->string));
+		die("%s", err.buf);
 
-	repo = read_gitfile(ref_git);
+	strbuf_addf(&sb, "%s/objects", ref_git);
-	if (!repo)
+	add_to_alternates_file(sb.buf);
-		repo = read_gitfile(mkpath("%s/.git", ref_git));
-	if (repo) {
-		free(ref_git);
-		ref_git = xstrdup(repo);
-	}
 
-	if (!repo && is_directory(mkpath("%s/.git/objects", ref_git))) {
-		char *ref_git_git = mkpathdup("%s/.git", ref_git);
-		free(ref_git);
-		ref_git = ref_git_git;
-	} else if (!is_directory(mkpath("%s/objects", ref_git))) {
-		struct strbuf sb = STRBUF_INIT;
-		if (get_common_dir(&sb, ref_git))
-			die(_("reference repository '%s' as a linked checkout is not supported yet."),
-			    item->string);
-		die(_("reference repository '%s' is not a local repository."),
-		    item->string);
-	}
-
-	if (!access(mkpath("%s/shallow", ref_git), F_OK))
-		die(_("reference repository '%s' is shallow"), item->string);
-
-	if (!access(mkpath("%s/info/grafts", ref_git), F_OK))
-		die(_("reference repository '%s' is grafted"), item->string);
-
-	strbuf_addf(&alternate, "%s/objects", ref_git);
-	add_to_alternates_file(alternate.buf);
-	strbuf_release(&alternate);
 	free(ref_git);
+	strbuf_release(&err);
+	strbuf_release(&sb);
 	return 0;
 }
 

cache.h

@@ -1342,6 +1342,7 @@ extern struct alternate_object_database {
 } *alt_odb_list;
 extern void prepare_alt_odb(void);
 extern void read_info_alternates(const char * relative_base, int depth);
+extern char *compute_alternate_path(const char *path, struct strbuf *err);
 extern void add_to_alternates_file(const char *reference);
 typedef int alt_odb_fn(struct alternate_object_database *, void *);
 extern int foreach_alt_odb(alt_odb_fn, void*);

sha1_file.c

@@ -425,6 +425,82 @@ void add_to_alternates_file(const char *reference)
 	free(alts);
 }
 
+/*
+ * Compute the exact path an alternate is at and returns it. In case of
+ * error NULL is returned and the human readable error is added to `err`
+ * `path` may be relative and should point to $GITDIR.
+ * `err` must not be null.
+ */
+char *compute_alternate_path(const char *path, struct strbuf *err)
+{
+	char *ref_git = NULL;
+	const char *repo, *ref_git_s;
+	int seen_error = 0;
+
+	ref_git_s = real_path_if_valid(path);
+	if (!ref_git_s) {
+		seen_error = 1;
+		strbuf_addf(err, _("path '%s' does not exist"), path);
+		goto out;
+	} else
+		/*
+		 * Beware: read_gitfile(), real_path() and mkpath()
+		 * return static buffer
+		 */
+		ref_git = xstrdup(ref_git_s);
+
+	repo = read_gitfile(ref_git);
+	if (!repo)
+		repo = read_gitfile(mkpath("%s/.git", ref_git));
+	if (repo) {
+		free(ref_git);
+		ref_git = xstrdup(repo);
+	}
+
+	if (!repo && is_directory(mkpath("%s/.git/objects", ref_git))) {
+		char *ref_git_git = mkpathdup("%s/.git", ref_git);
+		free(ref_git);
+		ref_git = ref_git_git;
+	} else if (!is_directory(mkpath("%s/objects", ref_git))) {
+		struct strbuf sb = STRBUF_INIT;
+		seen_error = 1;
+		if (get_common_dir(&sb, ref_git)) {
+			strbuf_addf(err,
+				    _("reference repository '%s' as a linked "
+				      "checkout is not supported yet."),
+				    path);
+			goto out;
+		}
+
+		strbuf_addf(err, _("reference repository '%s' is not a "
+					"local repository."), path);
+		goto out;
+	}
+
+	if (!access(mkpath("%s/shallow", ref_git), F_OK)) {
+		strbuf_addf(err, _("reference repository '%s' is shallow"),
+			    path);
+		seen_error = 1;
+		goto out;
+	}
+
+	if (!access(mkpath("%s/info/grafts", ref_git), F_OK)) {
+		strbuf_addf(err,
+			    _("reference repository '%s' is grafted"),
+			    path);
+		seen_error = 1;
+		goto out;
+	}
+
+out:
+	if (seen_error) {
+		free(ref_git);
+		ref_git = NULL;
+	}
+
+	return ref_git;
+}
+
 int foreach_alt_odb(alt_odb_fn fn, void *cb)
 {
 	struct alternate_object_database *ent;