Fix data races in the example

Some users rely on this example as a starting point to their
applications. This commit fixes a data race that could cause issues in
any code that relied on the example as base.

Related to #72

Signed-off-by: Aitor Perez Cedres <aitor.perez@broadcom.com>

example_client_test.go

@@ -8,9 +8,9 @@ package amqp091_test
 import (
 	"context"
 	"errors"
-	"fmt"
 	"log"
 	"os"
+	"sync"
 	"time"
 
 	amqp "github.com/rabbitmq/amqp091-go"
@@ -23,9 +23,10 @@ import (
 // It doesn't automatically ack each message, but leaves that
 // to the parent process, since it is usage-dependent.
 //
-// Try running this in one terminal, and `rabbitmq-server` in another.
+// Try running this in one terminal, and rabbitmq-server in another.
+//
 // Stop & restart RabbitMQ to see how the queue reacts.
-func Example() {
+func Example_publish() {
 	queueName := "job_queue"
 	addr := "amqp://guest:guest@localhost:5672/"
 	queue := New(queueName, addr)
@@ -39,12 +40,14 @@ loop:
 		// Attempt to push a message every 2 seconds
 		case <-time.After(time.Second * 2):
 			if err := queue.Push(message); err != nil {
-				fmt.Printf("Push failed: %s\n", err)
+				log.Printf("Push failed: %s\n", err)
 			} else {
-				fmt.Println("Push succeeded!")
+				log.Println("Push succeeded!")
 			}
 		case <-ctx.Done():
-			queue.Close()
+			if err := queue.Close(); err != nil {
+				log.Printf("Close failed: %s\n", err)
+			}
 			break loop
 		}
 	}
@@ -55,15 +58,15 @@ func Example_consume() {
 	addr := "amqp://guest:guest@localhost:5672/"
 	queue := New(queueName, addr)
 
-	// Give the connection sometime to setup
+	// Give the connection sometime to set up
 	<-time.After(time.Second)
 
 	ctx, cancel := context.WithTimeout(context.Background(), time.Second*30)
 	defer cancel()
 
 	deliveries, err := queue.Consume()
 	if err != nil {
-		fmt.Printf("Could not start consuming: %s\n", err)
+		log.Printf("Could not start consuming: %s\n", err)
 		return
 	}
 
@@ -78,19 +81,22 @@ func Example_consume() {
 	for {
 		select {
 		case <-ctx.Done():
-			queue.Close()
+			err := queue.Close()
+			if err != nil {
+				log.Printf("Close failed: %s\n", err)
+			}
 			return
 
 		case amqErr := <-chClosedCh:
 			// This case handles the event of closed channel e.g. abnormal shutdown
-			fmt.Printf("AMQP Channel closed due to: %s\n", amqErr)
+			log.Printf("AMQP Channel closed due to: %s\n", amqErr)
 
 			deliveries, err = queue.Consume()
 			if err != nil {
 				// If the AMQP channel is not ready, it will continue the loop. Next
 				// iteration will enter this case because chClosedCh is closed by the
 				// library
-				fmt.Println("Error trying to consume, will try again")
+				log.Println("Error trying to consume, will try again")
 				continue
 			}
 
@@ -101,16 +107,21 @@ func Example_consume() {
 
 		case delivery := <-deliveries:
 			// Ack a message every 2 seconds
-			fmt.Printf("Received message: %s\n", delivery.Body)
+			log.Printf("Received message: %s\n", delivery.Body)
 			if err := delivery.Ack(false); err != nil {
-				fmt.Printf("Error acknowledging message: %s\n", err)
+				log.Printf("Error acknowledging message: %s\n", err)
 			}
 			<-time.After(time.Second * 2)
 		}
 	}
 }
 
+// Client is the base struct for handling connection recovery, consumption and
+// publishing. Note that this struct has an internal mutex to safeguard against
+// data races. As you develop and iterate over this example, you may need to add
+// further locks, or safeguards, to keep your application safe from data races
 type Client struct {
+	m               *sync.Mutex
 	queueName       string
 	logger          *log.Logger
 	connection      *amqp.Connection
@@ -143,6 +154,7 @@ var (
 // attempts to connect to the server.
 func New(queueName, addr string) *Client {
 	client := Client{
+		m:         &sync.Mutex{},
 		logger:    log.New(os.Stdout, "", log.LstdFlags),
 		queueName: queueName,
 		done:      make(chan bool),
@@ -155,7 +167,10 @@ func New(queueName, addr string) *Client {
 // notifyConnClose, and then continuously attempt to reconnect.
 func (client *Client) handleReconnect(addr string) {
 	for {
+		client.m.Lock()
 		client.isReady = false
+		client.m.Unlock()
+
 		client.logger.Println("Attempting to connect")
 
 		conn, err := client.connect(addr)
@@ -194,7 +209,9 @@ func (client *Client) connect(addr string) (*amqp.Connection, error) {
 // and then continuously attempt to re-initialize both channels
 func (client *Client) handleReInit(conn *amqp.Connection) bool {
 	for {
+		client.m.Lock()
 		client.isReady = false
+		client.m.Unlock()
 
 		err := client.init(conn)
 
@@ -251,7 +268,9 @@ func (client *Client) init(conn *amqp.Connection) error {
 	}
 
 	client.changeChannel(ch)
+	client.m.Lock()
 	client.isReady = true
+	client.m.Unlock()
 	client.logger.Println("Setup!")
 
 	return nil
@@ -275,13 +294,16 @@ func (client *Client) changeChannel(channel *amqp.Channel) {
 	client.channel.NotifyPublish(client.notifyConfirm)
 }
 
-// Push will push data onto the queue, and wait for a confirm.
-// This will block until the server sends a confirm. Errors are
+// Push will push data onto the queue, and wait for a confirmation.
+// This will block until the server sends a confirmation. Errors are
 // only returned if the push action itself fails, see UnsafePush.
 func (client *Client) Push(data []byte) error {
+	client.m.Lock()
 	if !client.isReady {
+		client.m.Unlock()
 		return errors.New("failed to push: not connected")
 	}
+	client.m.Unlock()
 	for {
 		err := client.UnsafePush(data)
 		if err != nil {
@@ -306,9 +328,12 @@ func (client *Client) Push(data []byte) error {
 // No guarantees are provided for whether the server will
 // receive the message.
 func (client *Client) UnsafePush(data []byte) error {
+	client.m.Lock()
 	if !client.isReady {
+		client.m.Unlock()
 		return errNotConnected
 	}
+	client.m.Unlock()
 
 	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
 	defer cancel()
@@ -331,13 +356,16 @@ func (client *Client) UnsafePush(data []byte) error {
 // successfully processed, or delivery.Nack when it fails.
 // Ignoring this will cause data to build up on the server.
 func (client *Client) Consume() (<-chan amqp.Delivery, error) {
+	client.m.Lock()
 	if !client.isReady {
+		client.m.Unlock()
 		return nil, errNotConnected
 	}
+	client.m.Unlock()
 
 	if err := client.channel.Qos(
 		1,     // prefetchCount
-		0,     // prefrechSize
+		0,     // prefetchSize
 		false, // global
 	); err != nil {
 		return nil, err
@@ -356,6 +384,11 @@ func (client *Client) Consume() (<-chan amqp.Delivery, error) {
 
 // Close will cleanly shut down the channel and connection.
 func (client *Client) Close() error {
+	client.m.Lock()
+	// we read and write isReady in two locations, so we grab the lock and hold onto
+	// it until we are finished
+	defer client.m.Unlock()
+
 	if !client.isReady {
 		return errAlreadyClosed
 	}