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// Copyright 2019 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
// pubsublib specifies an interface around pubsub.Message for testing purposes, implements
// a wrapper type which implements that interface, and defines a ReceiveToChannel function which
// transmits pubsub messages to a channel where they can be subscribed to in a data-hiding way.
package pubsublib
import (
// Message describes the behaviors of a pubsub message.
type Message interface {
Attributes() map[string]string
Body() []byte
ID() string
// Receiver is an interface wrapping pubsub.Subscription's Receive method.
type Receiver interface {
Receive(context.Context, func(context.Context, *pubsub.Message)) error
var _ Receiver = &pubsub.Subscription{}
// realMessage wraps pubsub messages as a Message.
type realMessage struct {
message *pubsub.Message
var _ Message = &realMessage{}
// Attributes of the message.
func (m *realMessage) Attributes() map[string]string {
return m.message.Attributes
// Body of message, as unformatted bytes.
func (m *realMessage) Body() []byte {
return m.message.Data
// ID is a unique message identifier to identify messages sent more than once.
func (m *realMessage) ID() string {
return m.message.ID
func (m *realMessage) Ack() {
// MessageOrError is a wrapper type for channels which need to transmit both messages and errors.
type MessageOrError struct {
Message Message
Error error
// ReceiveToChannel pulls messages from a subscription sub until it gets an error or its context is closed.
// It returns the channel for messages and/or errors and is never a blocking call.
func ReceiveToChannel(ctx context.Context, sub Receiver) <-chan MessageOrError {
ch := make(chan MessageOrError)
handler := func(c context.Context, m *pubsub.Message) {
select {
case <-c.Done():
case ch <- MessageOrError{Message: &realMessage{message: m}}:
go func() {
err := sub.Receive(ctx, handler)
if err != nil {
ch <- MessageOrError{Error: err}
return ch