notification: add Go CDK pubsub support

Add the gocdk_pub_sub package, which supports the Go Cloud Development
Kit pubsub API.

Link in all current providers.

Update the notification scaffold.

weed/notification/gocdk_pub_sub/gocdk_pub_sub.go

@@ -0,0 +1,71 @@
+// Package gocdk_pub_sub supports the Go CDK (Cloud Development Kit) PubSub API,
+// which in turn supports many providers, including Amazon SNS/SQS, Azure Service Bus,
+// Google Cloud PubSub, and RabbitMQ.
+//
+// In the config, select a provider and topic using a URL. See
+// https://godoc.org/gocloud.dev/pubsub and its sub-packages for details.
+//
+// The Go CDK PubSub API does not support administrative operations like topic
+// creation. Create the topic using a UI, CLI or provider-specific API before running
+// weed.
+//
+// The Go CDK obtains credentials via environment variables and other
+// provider-specific default mechanisms. See the provider's documentation for
+// details.
+package gocdk_pub_sub
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/chrislusf/seaweedfs/weed/glog"
+	"github.com/chrislusf/seaweedfs/weed/notification"
+	"github.com/chrislusf/seaweedfs/weed/util"
+	"github.com/golang/protobuf/proto"
+	"gocloud.dev/pubsub"
+	_ "gocloud.dev/pubsub/awssnssqs"
+	_ "gocloud.dev/pubsub/azuresb"
+	_ "gocloud.dev/pubsub/gcppubsub"
+	_ "gocloud.dev/pubsub/natspubsub"
+	_ "gocloud.dev/pubsub/rabbitpubsub"
+)
+
+func init() {
+	notification.MessageQueues = append(notification.MessageQueues, &GoCDKPubSub{})
+}
+
+type GoCDKPubSub struct {
+	topicURL string
+	topic    *pubsub.Topic
+}
+
+func (k *GoCDKPubSub) GetName() string {
+	return "gocdk_pub_sub"
+}
+
+func (k *GoCDKPubSub) Initialize(config util.Configuration) error {
+	k.topicURL = config.GetString("topic_url")
+	glog.V(0).Infof("notification.gocdk_pub_sub.topic_url: %v", k.topicURL)
+	topic, err := pubsub.OpenTopic(context.Background(), k.topicURL)
+	if err != nil {
+		glog.Fatalf("Failed to open topic: %v", err)
+	}
+	k.topic = topic
+	return nil
+}
+
+func (k *GoCDKPubSub) SendMessage(key string, message proto.Message) error {
+	bytes, err := proto.Marshal(message)
+	if err != nil {
+		return err
+	}
+	ctx := context.Background()
+	err = k.topic.Send(ctx, &pubsub.Message{
+		Body:     bytes,
+		Metadata: map[string]string{"key": key},
+	})
+	if err != nil {
+		return fmt.Errorf("send message via Go CDK pubsub %s: %v", k.topicURL, err)
+	}
+	return nil
+}