Implement SQS rendezvous in client and broker

This features adds an additional rendezvous method to send client offers
and receive proxy answers through the use of Amazon SQS queues.

https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/issues/26151

client/lib/rendezvous_sqs.go

@@ -0,0 +1,135 @@
+package snowflake_client
+
+import (
+	"context"
+	"crypto/rand"
+	"encoding/hex"
+	"log"
+	"net/http"
+	"net/url"
+	"regexp"
+	"time"
+
+	"github.com/aws/aws-sdk-go-v2/aws"
+	"github.com/aws/aws-sdk-go-v2/config"
+	"github.com/aws/aws-sdk-go-v2/credentials"
+	"github.com/aws/aws-sdk-go-v2/service/sqs"
+	"github.com/aws/aws-sdk-go-v2/service/sqs/types"
+)
+
+type sqsRendezvous struct {
+	transport   http.RoundTripper
+	sqsClientID string
+	sqsClient   *sqs.Client
+	sqsURL      *url.URL
+}
+
+func newSQSRendezvous(sqsQueue string, sqsAccessKeyId string, sqsSecretKey string, transport http.RoundTripper) (*sqsRendezvous, error) {
+	sqsURL, err := url.Parse(sqsQueue)
+	if err != nil {
+		return nil, err
+	}
+
+	var id [8]byte
+	_, err = rand.Read(id[:])
+	if err != nil {
+		log.Fatal(err)
+	}
+	clientID := hex.EncodeToString(id[:])
+
+	queueURL := sqsURL.String()
+	hostName := sqsURL.Hostname()
+
+	regionRegex, _ := regexp.Compile(`^sqs\.([\w-]+)\.amazonaws\.com$`)
+	res := regionRegex.FindStringSubmatch(hostName)
+	if len(res) < 2 {
+		log.Fatal("Could not extract AWS region from SQS URL. Ensure that the SQS Queue URL provided is valid.")
+	}
+	region := res[1]
+	cfg, err := config.LoadDefaultConfig(context.TODO(),
+		config.WithCredentialsProvider(
+			credentials.NewStaticCredentialsProvider(sqsAccessKeyId, sqsSecretKey, ""),
+		),
+		config.WithRegion(region),
+	)
+	if err != nil {
+		log.Fatal(err)
+	}
+	client := sqs.NewFromConfig(cfg)
+
+	log.Println("Queue URL: ", queueURL)
+	log.Println("SQS Client ID: ", clientID)
+
+	return &sqsRendezvous{
+		transport:   transport,
+		sqsClientID: clientID,
+		sqsClient:   client,
+		sqsURL:      sqsURL,
+	}, nil
+}
+
+func (r *sqsRendezvous) Exchange(encPollReq []byte) ([]byte, error) {
+	log.Println("Negotiating via SQS Queue rendezvous...")
+
+	_, err := r.sqsClient.SendMessage(context.TODO(), &sqs.SendMessageInput{
+		MessageAttributes: map[string]types.MessageAttributeValue{
+			"ClientID": {
+				DataType:    aws.String("String"),
+				StringValue: aws.String(r.sqsClientID),
+			},
+		},
+		MessageBody: aws.String(string(encPollReq)),
+		QueueUrl:    aws.String(r.sqsURL.String()),
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	time.Sleep(time.Second) // wait for client queue to be created by the broker
+
+	numRetries := 5
+	var responseQueueURL *string
+	for i := 0; i < numRetries; i++ {
+		// The SQS queue corresponding to the client where the SDP Answer will be placed
+		// may not be created yet. We will retry up to 5 times before we error out.
+		var res *sqs.GetQueueUrlOutput
+		res, err = r.sqsClient.GetQueueUrl(context.TODO(), &sqs.GetQueueUrlInput{
+			QueueName: aws.String("snowflake-client-" + r.sqsClientID),
+		})
+		if err != nil {
+			log.Println(err)
+			log.Printf("Attempt %d of %d to retrieve URL of response SQS queue failed.\n", i+1, numRetries)
+			time.Sleep(time.Second)
+		} else {
+			responseQueueURL = res.QueueUrl
+			break
+		}
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	var answer string
+	for i := 0; i < numRetries; i++ {
+		// Waiting for SDP Answer from proxy to be placed in SQS queue.
+		// We will retry upt to 5 times before we error out.
+		res, err := r.sqsClient.ReceiveMessage(context.TODO(), &sqs.ReceiveMessageInput{
+			QueueUrl:            responseQueueURL,
+			MaxNumberOfMessages: 1,
+			WaitTimeSeconds:     20,
+		})
+		if err != nil {
+			return nil, err
+		}
+		if len(res.Messages) == 0 {
+			log.Printf("Attempt %d of %d to receive message from response SQS queue failed. No message found in queue.\n", i+1, numRetries)
+			delay := float64(i)/2.0 + 1
+			time.Sleep(time.Duration(delay*1000) * time.Millisecond)
+		} else {
+			answer = *res.Messages[0].Body
+			break
+		}
+	}
+
+	return []byte(answer), nil
+}