Fix settlement handling and add Message.SendSettled

* Message.SendSettled allows sending messages settled when
  LinkSenderSettle is ModeMixed.

client.go

@@ -390,8 +390,7 @@ func (s *Sender) send(ctx context.Context, msg *Message) (chan deliveryState, er
 	var (
 		maxPayloadSize = int64(s.link.session.conn.peerMaxFrameSize) - maxTransferFrameHeader
 		sndSettleMode  = s.link.senderSettleMode
-		rcvSettleMode  = s.link.receiverSettleMode
-		senderSettled  = sndSettleMode != nil && *sndSettleMode == ModeSettled
+		senderSettled  = sndSettleMode != nil && (*sndSettleMode == ModeSettled || (*sndSettleMode == ModeMixed && msg.SendSettled))
 		deliveryID     = atomic.AddUint32(&s.link.session.nextDeliveryID, 1)
 	)
 
@@ -416,16 +415,16 @@ func (s *Sender) send(ctx context.Context, msg *Message) (chan deliveryState, er
 		fr.Payload = append([]byte(nil), buf...)
 		fr.More = s.buf.len() > 0
 		if !fr.More {
+			// SSM=settled: overrides RSM; no acks.
+			// SSM=unsettled: sender should wait for receiver to ack
+			// RSM=first: receiver considers it settled immediately, but must still send ack (SSM=unsettled only)
+			// RSM=second: receiver sends ack and waits for return ack from sender (SSM=unsettled only)
+
 			// mark final transfer as settled when sender mode is settled
 			fr.Settled = senderSettled
 
-			// set done on last frame to be closed after network transmission
-			//
-			// If confirmSettlement is true (ReceiverSettleMode == "second"),
-			// Session.mux will intercept the done channel and close it when the
-			// receiver has confirmed settlement instead of on net transmit.
+			// set done on last frame
 			fr.done = make(chan deliveryState, 1)
-			fr.confirmSettlement = rcvSettleMode != nil && *rcvSettleMode == ModeSecond
 		}
 
 		select {
@@ -750,9 +749,9 @@ func (s *Session) mux(remoteBegin *performBegin) {
 				delete(handlesByDeliveryID, deliveryID)
 			}
 
-			// if confirmSettlement requested, add done chan to map
+			// if not settled, add done chan to map
 			// and clear from frame so conn doesn't close it.
-			if fr.confirmSettlement && fr.done != nil {
+			if !fr.Settled && fr.done != nil {
 				settlementByDeliveryID[deliveryID] = fr.done
 				fr.done = nil
 			}

integration_test.go

@@ -1131,6 +1131,8 @@ func TestIssue48_ReceiverModeSecond(t *testing.T) {
 		// Create a sender
 		sender, err := session.NewSender(
 			amqp.LinkTargetAddress(hubName),
+			amqp.LinkSenderSettle(amqp.ModeUnsettled),
+			amqp.LinkReceiverSettle(amqp.ModeFirst),
 		)
 		if err != nil {
 			t.Fatalf("%+v\n", err)
@@ -1144,17 +1146,6 @@ func TestIssue48_ReceiverModeSecond(t *testing.T) {
 				[]byte("there"),
 			},
 		})
-		time.Sleep(1 * time.Second) // Have to wait long enough for disposition to come through.
-		if err != nil {
-			t.Fatalf("Unexpected error response: %+v", err)
-		}
-
-		// Second send should get async error
-		err = sender.Send(context.Background(), &amqp.Message{
-			Data: [][]byte{
-				[]byte("hello"),
-			},
-		})
 		if err == nil {
 			t.Fatal("Expected error, got nil")
 		}

types.go

@@ -1262,10 +1262,10 @@ type performTransfer struct {
 	Payload []byte
 
 	// optional channel to indicate to sender that transfer has completed
+	//
+	// Settled=true: closed when the transferred on network.
+	// Settled=false: closed when the receiver has confirmed settlement.
 	done chan deliveryState
-	// complete when receiver has responded with disposition (ReceiverSettleMode = second)
-	// instead of when this message has been sent on network
-	confirmSettlement bool
 }
 
 func (t *performTransfer) frameBody() {}
@@ -1726,6 +1726,11 @@ type Message struct {
 	// encryption details).
 	Footer Annotations
 
+	// Mark the message as settled when LinkSenderSettle is ModeMixed.
+	//
+	// This field is ignored when LinkSenderSettle is not ModeMixed.
+	SendSettled bool
+
 	receiver   *Receiver // Receiver the message was received from
 	deliveryID uint32    // used when sending disposition
 	settled    bool      // whether transfer was settled by sender
@@ -1810,7 +1815,7 @@ func (m *Message) MarshalBinary() ([]byte, error) {
 }
 
 func (m *Message) shouldSendDisposition() bool {
-	return !m.settled || (m.receiver.link.receiverSettleMode != nil && *m.receiver.link.receiverSettleMode == ModeSecond)
+	return !m.settled
 }
 
 func (m *Message) marshal(wr *buffer) error {