Make it possible to get a notification when a message has been sent.

If a channel is passed in then true will be sent on the provided channel
after the message has successfully sent.

peer.go

@@ -89,6 +89,11 @@ func newNetAddress(addr net.Addr, services btcwire.ServiceFlag) (*btcwire.NetAdd
 	return na, nil
 }
 
+type outMsg struct {
+	msg      btcwire.Message
+	doneChan  chan bool
+}
+
 // peer provides a bitcoin peer for handling bitcoin communications.
 type peer struct {
 	server             *server
@@ -118,7 +123,7 @@ type peer struct {
 	requestQueue       *list.List
 	invSendQueue       *list.List
 	continueHash       *btcwire.ShaHash
-	outputQueue        chan btcwire.Message
+	outputQueue        chan outMsg
 	outputInvChan      chan *btcwire.InvVect
 	txProcessed        chan bool
 	blockProcessed     chan bool
@@ -1020,7 +1025,10 @@ out:
 	for {
 		select {
 		case msg := <-p.outputQueue:
-			p.writeMessage(msg)
+			p.writeMessage(msg.msg)
+			if msg.doneChan != nil {
+				msg.doneChan <- true
+			}
 
 		case iv := <-p.outputInvChan:
 			// No handshake? They'll find out soon enough.
@@ -1067,6 +1075,20 @@ out:
 			break out
 		}
 	}
+
+	// Drain any wait channels before we go away so we don't leave something
+	// waiting for us.
+cleanup:
+	for {
+		select {
+		case msg := <-p.outputQueue:
+			if msg.doneChan != nil {
+				msg.doneChan <- false
+			}
+		default:
+			break cleanup
+		}
+	}
 	log.Tracef("PEER: Peer output handler done for %s", p.addr)
 }
 
@@ -1074,7 +1096,7 @@ out:
 // uses a buffered channel to communicate with the output handler goroutine so
 // it is automatically rate limited and safe for concurrent access.
 func (p *peer) QueueMessage(msg btcwire.Message) {
-	p.outputQueue <- msg
+	p.outputQueue <- outMsg{msg: msg}
 }
 
 // QueueInventory adds the passed inventory to the inventory send queue which
@@ -1161,7 +1183,7 @@ func newPeerBase(s *server, inbound bool) *peer {
 		requestedBlocks: make(map[btcwire.ShaHash]bool),
 		requestQueue:    list.New(),
 		invSendQueue:    list.New(),
-		outputQueue:     make(chan btcwire.Message, outputBufferSize),
+		outputQueue:     make(chan outMsg, outputBufferSize),
 		outputInvChan:   make(chan *btcwire.InvVect, outputBufferSize),
 		txProcessed:     make(chan bool, 1),
 		blockProcessed:  make(chan bool, 1),