Honor alternate entries in the ODB

The alternates file is now parsed, and the alternate ODB folders are
added as separate backends. This allows the library to efficiently query
the alternate folders.

Signed-off-by: Vicent Marti <tanoku@gmail.com>

src/odb.c

@@ -33,6 +33,8 @@
 
 #include "git2/odb_backend.h"
 
+#define GIT_ALTERNATES_FILE "info/alternates"
+
 static int format_object_header(char *hdr, size_t n, git_rawobj *obj)
 {
 	const char *type_str = git_object_type2string(obj->type);
@@ -171,32 +173,77 @@ int git_odb_add_backend(git_odb *odb, git_odb_backend *backend)
 	return GIT_SUCCESS;
 }
 
+static int add_default_backends(git_odb *db, const char *objects_dir)
+{
+	git_odb_backend *loose, *packed;
+	int error;
+
+	/* add the loose object backend */
+	error = git_odb_backend_loose(&loose, objects_dir);
+	if (error < GIT_SUCCESS)
+		return error;
+
+	error = git_odb_add_backend(db, loose);
+	if (error < GIT_SUCCESS)
+		return error;
+
+	/* add the packed file backend */
+	error = git_odb_backend_pack(&packed, objects_dir);
+	if (error < GIT_SUCCESS)
+		return error;
+
+	error = git_odb_add_backend(db, packed);
+	if (error < GIT_SUCCESS)
+		return error;
+
+	return GIT_SUCCESS;
+}
+
+static int load_alternates(git_odb *odb, const char *objects_dir)
+{
+	char alternates_path[GIT_PATH_MAX];
+	char alternate[GIT_PATH_MAX];
+	char *buffer;
+
+	gitfo_buf alternates_buf = GITFO_BUF_INIT;
+	int error;
+
+	git__joinpath(alternates_path, objects_dir, GIT_ALTERNATES_FILE);
+
+	if (gitfo_exists(alternates_path) < GIT_SUCCESS)
+		return GIT_SUCCESS;
+
+	if (gitfo_read_file(&alternates_buf, alternates_path) < GIT_SUCCESS)
+		return GIT_EOSERR;
+
+	buffer = (char *)alternates_buf.data;
+	error = GIT_SUCCESS;
+
+	/* add each alternate as a new backend; one alternate per line */
+	while ((error == GIT_SUCCESS) && (buffer = git__strtok(alternate, buffer, "\r\n")) != NULL)
+		error = add_default_backends(odb, alternate);
+
+	gitfo_free_buf(&alternates_buf);
+	return error;
+}
 
 int git_odb_open(git_odb **out, const char *objects_dir)
 {
 	git_odb *db;
-	git_odb_backend *loose, *packed;
 	int error;
 
+	assert(out && objects_dir);
+
+	*out = NULL;
+
 	if ((error = git_odb_new(&db)) < 0)
 		return error;
 
-	/* add the loose object backend */
-	if (git_odb_backend_loose(&loose, objects_dir) == 0) {
-		error = git_odb_add_backend(db, loose);
-		if (error < 0)
-			goto cleanup;
-	}
-
-	/* add the packed file backend */
-	if (git_odb_backend_pack(&packed, objects_dir) == 0) {
-		error = git_odb_add_backend(db, packed);
-		if (error < 0)
-			goto cleanup;
-	}
+	if ((error = add_default_backends(db, objects_dir)) < GIT_SUCCESS)
+		goto cleanup;
 
-	/* TODO: add altenernates as new backends;
-	 * how elevant is that? very elegant. */
+	if ((error = load_alternates(db, objects_dir)) < GIT_SUCCESS)
+		goto cleanup;
 
 	*out = db;
 	return GIT_SUCCESS;