game/scripts/save_store.gd

@@ -25,7 +25,8 @@ func player_key() -> PackedByteArray:
 	var key := FileAccess.get_file_as_bytes(path)
 	if key.size() != KEY_BYTES:
 		key = Crypto.new().generate_random_bytes(KEY_BYTES)
-		_write(path, key)
+		if not _write(path, key):
+			push_error("player key could not be stored at %s" % path)
 	return key
 
 
@@ -37,24 +38,28 @@ func read_save() -> PackedByteArray:
 	return FileAccess.get_file_as_bytes(base_dir.path_join(SAVE_FILE))
 
 
-## Writes the save through a temporary file, so that a crash while writing
-## never destroys the previous save. Returns false if nothing was written.
+## Writes the save through a temporary file, so that a crash or a failed
+## write never destroys the previous save. Returns false if the new save was
+## not stored completely; the previous save then stays as it was.
 func write_save(bytes: PackedByteArray) -> bool:
 	if bytes.is_empty():
 		return false
 	var path := base_dir.path_join(SAVE_FILE)
 	var temporary := path + ".tmp"
-	if not _write(temporary, bytes):
+	if not _write(temporary, bytes) or FileAccess.get_file_as_bytes(temporary) != bytes:
+		DirAccess.remove_absolute(temporary)
 		return false
 	return DirAccess.rename_absolute(temporary, path) == OK
 
 
 ## Moves an unusable save out of the way, so that the next save does not
-## overwrite it. An older set-aside save is replaced. Returns the new path.
+## overwrite it. An older set-aside save is replaced. Returns the new path,
+## or an empty text if there was no save or it could not be moved.
 func set_aside_save() -> String:
 	var path := base_dir.path_join(SAVE_FILE)
 	var aside := path + ".broken"
-	DirAccess.rename_absolute(path, aside)
+	if DirAccess.rename_absolute(path, aside) != OK:
+		return ""
 	return aside
 
 
@@ -88,6 +93,6 @@ func _write(path: String, bytes: PackedByteArray) -> bool:
 	var file := FileAccess.open(path, FileAccess.WRITE)
 	if file == null:
 		return false
-	file.store_buffer(bytes)
+	var stored := file.store_buffer(bytes)
 	file.close()
-	return true
+	return stored

game/tests/test_save_store.gd

@@ -30,6 +30,25 @@ func test_an_empty_save_is_not_written() -> void:
 	TestFiles.remove_dir(directory)
 
 
+func test_a_save_that_cannot_be_written_keeps_the_previous_one() -> void:
+	var directory := TestFiles.temporary_dir("blocked")
+	var store := SaveStore.new(directory)
+	store.write_save(PackedByteArray([1, 2, 3]))
+	# A folder in place of the temporary file makes the write fail.
+	DirAccess.make_dir_absolute(directory.path_join(SaveStore.SAVE_FILE + ".tmp"))
+	check(not store.write_save(PackedByteArray([4, 5])), "the failed save is reported")
+	check_eq(store.read_save(), PackedByteArray([1, 2, 3]), "the previous save is untouched")
+	check(not store.write_save(PackedByteArray()), "an empty save is refused")
+	check_eq(store.read_save(), PackedByteArray([1, 2, 3]), "still untouched")
+	TestFiles.remove_dir(directory)
+
+
+func test_setting_aside_without_a_save_reports_nothing_moved() -> void:
+	var directory := TestFiles.temporary_dir("nothing")
+	check_eq(SaveStore.new(directory).set_aside_save(), "", "no save, nothing moved")
+	TestFiles.remove_dir(directory)
+
+
 func test_an_unusable_save_can_be_set_aside() -> void:
 	var directory := TestFiles.temporary_dir("aside")
 	var store := SaveStore.new(directory)