adding explicit context check before sends (#208)

Author: imiller31

Diff for: v2/sender.go

@@ -58,6 +58,12 @@ func NewSender(sender AzServiceBusSender, options *SenderOptions) *Sender {
 // SendMessage sends a payload on the bus.
 // the MessageBody is marshalled and set as the message body.
 func (d *Sender) SendMessage(ctx context.Context, mb MessageBody, options ...func(msg *azservicebus.Message) error) error {
+	// Check if there is a context error before doing anything since
+	// we rely on context failures to detect if the sender is dead.
+	if ctx.Err() != nil {
+		return fmt.Errorf("failed to send message: %w", ctx.Err())
+	}
+	
 	msg, err := d.ToServiceBusMessage(ctx, mb, options...)
 	if err != nil {
 		return err
@@ -124,6 +130,12 @@ func (d *Sender) ToServiceBusMessage(
 
 // SendMessageBatch sends the array of azservicebus messages as a batch.
 func (d *Sender) SendMessageBatch(ctx context.Context, messages []*azservicebus.Message) error {
+	// Check if there is a context error before doing anything since
+	// we rely on context failures to detect if the sender is dead.
+	if ctx.Err() != nil {
+		return fmt.Errorf("failed to send message: %w", ctx.Err())
+	}
+	
 	batch, err := d.sbSender.NewMessageBatch(ctx, &azservicebus.MessageBatchOptions{})
 	if err != nil {
 		return err

Diff for: v2/sender_test.go

@@ -178,6 +178,29 @@ func TestSender_WithContextCanceled(t *testing.T) {
 	g.Expect(err).To(MatchError(context.DeadlineExceeded))
 }
 
+func TestSender_SendWithCanceledContext(t *testing.T) {
+	g := NewWithT(t)
+	azSender := &fakeAzSender{
+		DoSendMessage: func(ctx context.Context, message *azservicebus.Message, options *azservicebus.SendMessageOptions) error {
+			return nil
+		},
+		DoSendMessageBatch: func(ctx context.Context, messages *azservicebus.MessageBatch, options *azservicebus.SendMessageBatchOptions) error {
+			return nil
+		},
+	}
+	sender := NewSender(azSender, &SenderOptions{
+		Marshaller: &DefaultJSONMarshaller{},
+	})
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	err := sender.SendMessage(ctx, "test")
+	g.Expect(err).To(MatchError(context.Canceled))
+	err = sender.SendMessageBatch(ctx, nil)
+	g.Expect(err).To(MatchError(context.Canceled))
+}
+
 func TestSender_DisabledSendTimeout(t *testing.T) {
 	g := NewWithT(t)
 	sendTimeout := -1 * time.Second