Use native Pub/Sub library primitives.

common/gcloud/pubsub/subscriber/source.go

-// Copyright 2016 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.
-
-package subscriber
-
-import (
-        "github.com/luci/luci-go/common/gcloud/pubsub"
-        "golang.org/x/net/context"
-)
-
-// Source is used to pull Pub/Sub messages in batches.
-type Source interface {
-        // Pull retrieves up to the specified number of of Pub/Sub messages to
-        // process.
-        Pull(context.Context, int) ([]*pubsub.Message, error)
-}
-
-// PubSubSource is a Source implementation built on top of a pubsub.PubSub.
-type pubSubSource struct {
-        ps  pubsub.Connection
-        sub pubsub.Subscription
-}
-
-// NewSource generates a new Source by wrapping a pubsub.Connection
-// implementation. This Source is bound to a single subscription.
-func NewSource(ps pubsub.Connection, s pubsub.Subscription) Source {
-        return &pubSubSource{
-                ps:  ps,
-                sub: s,
-        }
-}
-
-func (s *pubSubSource) Pull(c context.Context, batchSize int) (msgs []*pubsub.Message, err error) {
-        return s.ps.Pull(c, s.sub, batchSize)
-}