Constrain mkdir calls to avoid extra mkdirs

This updates the calls that make the subdirectories for objects
to use a base directory above which git_futils_mkdir won't walk
any higher.  This prevents attempts to mkdir all the way up to
the root of the filesystem.

Also, this moves the objects_dir into the loose backend structure
and removes the separate allocation, plus does some preformatting
of the objects_dir value to guarantee a trailing slash, etc.

src/odb_loose.c

@@ -33,7 +33,9 @@ typedef struct loose_backend {
 
 	int object_zlib_level; /** loose object zlib compression level. */
 	int fsync_object_files; /** loose object file fsync flag. */
-	char *objects_dir;
+
+	size_t objects_dirlen;
+	char objects_dir[GIT_FLEX_ARRAY];
 } loose_backend;
 
 /* State structure for exploring directories,
@@ -56,24 +58,30 @@ typedef struct {
  *
  ***********************************************************/
 
-static int object_file_name(git_buf *name, const char *dir, const git_oid *id)
+static int object_file_name(
+	git_buf *name, const loose_backend *be, const git_oid *id)
 {
-	git_buf_sets(name, dir);
-
-	/* expand length for 40 hex sha1 chars + 2 * '/' + '\0' */
-	if (git_buf_grow(name, git_buf_len(name) + GIT_OID_HEXSZ + 3) < 0)
+	/* expand length for object root + 40 hex sha1 chars + 2 * '/' + '\0' */
+	if (git_buf_grow(name, be->objects_dirlen + GIT_OID_HEXSZ + 3) < 0)
 		return -1;
 
+	git_buf_set(name, be->objects_dir, be->objects_dirlen);
 	git_path_to_dir(name);
 
 	/* loose object filename: aa/aaa... (41 bytes) */
-	git_oid_pathfmt(name->ptr + git_buf_len(name), id);
+	git_oid_pathfmt(name->ptr + name->size, id);
 	name->size += GIT_OID_HEXSZ + 1;
 	name->ptr[name->size] = '\0';
 
 	return 0;
 }
 
+static int object_mkdir(const git_buf *name, const loose_backend *be)
+{
+	return git_futils_mkdir(
+		name->ptr + be->objects_dirlen, be->objects_dir, GIT_OBJECT_DIR_MODE,
+		GIT_MKDIR_PATH | GIT_MKDIR_SKIP_LAST | GIT_MKDIR_VERIFY_DIR);
+}
 
 static size_t get_binary_object_header(obj_hdr *hdr, git_buf *obj)
 {
@@ -457,7 +465,7 @@ static int locate_object(
 	loose_backend *backend,
 	const git_oid *oid)
 {
-	int error = object_file_name(object_location, backend->objects_dir, oid);
+	int error = object_file_name(object_location, backend, oid);
 
 	if (!error && !git_path_exists(object_location->ptr))
 		return GIT_ENOTFOUND;
@@ -769,8 +777,8 @@ static int loose_backend__stream_fwrite(git_oid *oid, git_odb_stream *_stream)
 	int error = 0;
 
 	if (git_filebuf_hash(oid, &stream->fbuf) < 0 ||
-		object_file_name(&final_path, backend->objects_dir, oid) < 0 ||
-		git_futils_mkpath2file(final_path.ptr, GIT_OBJECT_DIR_MODE) < 0)
+		object_file_name(&final_path, backend, oid) < 0 ||
+		object_mkdir(&final_path, backend) < 0)
 		error = -1;
 	/*
 	 * Don't try to add an existing object to the repository. This
@@ -880,8 +888,8 @@ static int loose_backend__write(git_oid *oid, git_odb_backend *_backend, const v
 	git_filebuf_write(&fbuf, header, header_len);
 	git_filebuf_write(&fbuf, data, len);
 
-	if (object_file_name(&final_path, backend->objects_dir, oid) < 0 ||
-		git_futils_mkpath2file(final_path.ptr, GIT_OBJECT_DIR_MODE) < 0 ||
+	if (object_file_name(&final_path, backend, oid) < 0 ||
+		object_mkdir(&final_path, backend) < 0 ||
 		git_filebuf_commit_at(&fbuf, final_path.ptr, GIT_OBJECT_FILE_MODE) < 0)
 		error = -1;
 
@@ -898,7 +906,6 @@ static void loose_backend__free(git_odb_backend *_backend)
 	assert(_backend);
 	backend = (loose_backend *)_backend;
 
-	git__free(backend->objects_dir);
 	git__free(backend);
 }
 
@@ -909,13 +916,20 @@ int git_odb_backend_loose(
 	int do_fsync)
 {
 	loose_backend *backend;
+	size_t objects_dirlen;
+
+	assert(backend_out && objects_dir);
+
+	objects_dirlen = strlen(objects_dir);
 
-	backend = git__calloc(1, sizeof(loose_backend));
+	backend = git__calloc(1, sizeof(loose_backend) + objects_dirlen + 2);
 	GITERR_CHECK_ALLOC(backend);
 
 	backend->parent.version = GIT_ODB_BACKEND_VERSION;
-	backend->objects_dir = git__strdup(objects_dir);
-	GITERR_CHECK_ALLOC(backend->objects_dir);
+	backend->objects_dirlen = objects_dirlen;
+	memcpy(backend->objects_dir, objects_dir, objects_dirlen);
+	if (backend->objects_dir[backend->objects_dirlen - 1] != '/')
+		backend->objects_dir[backend->objects_dirlen++] = '/';
 
 	if (compression_level < 0)
 		compression_level = Z_BEST_SPEED;