Tool to replay a pull subscription to a topic (#350)

This tool uses the local gcloud credentials to listen to a pull
subscription (intended for the pull subscription we created by default
for each dead letter topic), and send it back to a topic (the original
topic).

Example usage: 
```
❯ go run . --source=nghia-...-dlq-pjxu --dest=nghia-...-us-central1 --projectID nghia-...
Listening for messages.
Found message: { "content": "this is a test" }
Found message: { "content": "this is a test" }
Found message: { "content": "this is a test" }
Found message: { "content": "this is a test" }
Found message: { "content": "this is a test" }
Found message: { "content": "this is a test" }
Found message: { "content": "this is a test" }
Found message: { "content": "this is a test" }
Found message: { "content": "this is a test" }
Replayed message: { "content": "this is a test" }
Replayed message: { "content": "this is a test" }
Replayed message: { "content": "this is a test" }
Replayed message: { "content": "this is a test" }
Replayed message: { "content": "this is a test" }
Replayed message: { "content": "this is a test" }
Replayed message: { "content": "this is a test" }
Replayed message: { "content": "this is a test" }
Replayed message: { "content": "this is a test" }
Found message: { "content": "this is a test" }
Replayed message: { "content": "this is a test" }
No messages received in the last 10s . Exiting.
```

Signed-off-by: Nghia Tran <tcnghia@gmail.com>

cmd/replayer/main.go

@@ -0,0 +1,95 @@
+/*
+Copyright 2024 Chainguard, Inc.
+SPDX-License-Identifier: Apache-2.0
+*/
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"log"
+	"os"
+	"time"
+
+	"cloud.google.com/go/pubsub"
+)
+
+const PollTimeout = 10 * time.Second
+
+// Pulls messages from a pull subscription and replays them to a topic.
+// This is useful for replaying messages from a pull subscription of a dead-letter topic
+// to the original topic.
+//
+// Usage:
+//
+//	replayer --source=dead-letter-pull-sub --dest=original-topic --projectID=project-id
+func main() {
+	var srcSub, dstTop, prjID string
+	flag.StringVar(&srcSub, "source", "", "source subscription")
+	flag.StringVar(&dstTop, "dest", "", "destination topic")
+	flag.StringVar(&prjID, "projectID", "", "project id")
+
+	flag.Parse()
+	if srcSub == "" {
+		log.Fatal("--source is required")
+	}
+	if dstTop == "" {
+		log.Fatal("--dest is required")
+	}
+
+	if prjID == "" {
+		log.Fatal("--projectID is required")
+	}
+	ctx := context.Background()
+	client, err := pubsub.NewClient(ctx, prjID)
+	if err != nil {
+		log.Fatalf("pubsub.NewClient: %v", err)
+	}
+	defer client.Close()
+
+	sub := client.Subscription(srcSub)
+	top := client.Topic(dstTop)
+
+	fmt.Println("Listening for messages.")
+
+	lastReceived := time.Now()
+	go exitOnIdling(ctx, &lastReceived)
+
+	// Receive blocks until the context is cancelled or an error occurs.
+	_ = sub.Receive(ctx, func(_ context.Context, msg *pubsub.Message) {
+		lastReceived = time.Now()
+		fmt.Println("Found message:", string(msg.Data))
+
+		// TODO: supporting a filter, either based on message content or attributes.
+		// if filter(msg) {
+		//     msg.Nack()
+		// 	   return
+		// }
+		result := top.Publish(ctx, msg)
+		if _, err := result.Get(ctx); err == nil {
+			fmt.Printf("Replayed message: %s\n", string(msg.Data))
+			msg.Ack()
+		} else {
+			fmt.Printf("Failed to publish message: %v\n", err)
+			msg.Nack()
+		}
+	})
+}
+
+// exitOnIdling exits the program if no messages are received in the last PollTimeout.
+func exitOnIdling(_ context.Context, lastReceived *time.Time) {
+	ticker := time.NewTicker(time.Second)
+	defer ticker.Stop()
+	// nolint:all for { select {} } is the recommended way.
+	for {
+		select {
+		case <-ticker.C:
+			if time.Since(*lastReceived) > PollTimeout {
+				fmt.Println("No messages received in the last", PollTimeout, ". Exiting.")
+				// nolint:all We can exit without running ticker.Stop()
+				os.Exit(0)
+			}
+		}
+	}
+}