htlcswitch: utilize memoryMailBox within link, no longer spawn goroutine to forward in switch

In this commit, we’ve modified the link and the switch to start to use
the new mailBox in place of the existing synchronous message send
directly into the link’s upstream/downstream channels. With his change,
we no longer need to spawn a new goroutine each time an HTLC needs to
be forwarded, or a user payment is initiated.

htlcswitch/link.go

@@ -182,11 +182,11 @@ type channelLink struct {
 	// been processed because of the commitment transaction overflow.
 	overflowQueue *packetQueue
 
-	// availableBandwidth is an integer with units of millisatoshi which
-	// indicates the total available bandwidth of a link, taking into
-	// account any pending (uncommitted) HLTC's, and any HTLC's that are
-	// within the overflow queue.
-	availableBandwidth uint64
+	// mailBox is the main interface between the outside world and the
+	// link. All incoming messages will be sent over this mailBox. Messages
+	// include new updates from our connected peer, and new packets to be
+	// forwarded sent by the switch.
+	mailBox *memoryMailBox
 
 	// upstream is a channel that new messages sent from the remote peer to
 	// the local peer will be sent across.
@@ -219,19 +219,23 @@ type channelLink struct {
 func NewChannelLink(cfg ChannelLinkConfig, channel *lnwallet.LightningChannel,
 	currentHeight uint32) ChannelLink {
 
-	return &channelLink{
+	link := &channelLink{
 		cfg:         cfg,
 		channel:     channel,
-		upstream:    make(chan lnwire.Message),
-		downstream:  make(chan *htlcPacket),
+		mailBox:     newMemoryMailBox(),
 		linkControl: make(chan interface{}),
 		// TODO(roasbeef): just do reserve here?
 		availableBandwidth: uint64(channel.StateSnapshot().LocalBalance),
-		logCommitTimer:     time.NewTimer(300 * time.Millisecond),
-		overflowQueue:      newPacketQueue(lnwallet.MaxHTLCNumber / 2),
-		bestHeight:         currentHeight,
-		quit:               make(chan struct{}),
+		logCommitTimer: time.NewTimer(300 * time.Millisecond),
+		overflowQueue:  newPacketQueue(lnwallet.MaxHTLCNumber / 2),
+		bestHeight:     currentHeight,
+		quit:           make(chan struct{}),
 	}
+
+	link.upstream = link.mailBox.MessageOutBox()
+	link.downstream = link.mailBox.PacketOutBox()
+
+	return link
 }
 
 // A compile time check to ensure channelLink implements the ChannelLink
@@ -1020,10 +1024,7 @@ func (l *channelLink) String() string {
 //
 // NOTE: Part of the ChannelLink interface.
 func (l *channelLink) HandleSwitchPacket(packet *htlcPacket) {
-	select {
-	case l.downstream <- packet:
-	case <-l.quit:
-	}
+	l.mailBox.AddPacket(packet)
 }
 
 // HandleChannelUpdate handles the htlc requests as settle/add/fail which sent
@@ -1031,10 +1032,7 @@ func (l *channelLink) HandleSwitchPacket(packet *htlcPacket) {
 //
 // NOTE: Part of the ChannelLink interface.
 func (l *channelLink) HandleChannelUpdate(message lnwire.Message) {
-	select {
-	case l.upstream <- message:
-	case <-l.quit:
-	}
+	l.mailBox.AddMessage(message)
 }
 
 // updateChannelFee updates the commitment fee-per-kw on this channel by