htlcswitch: add usage of queue in channel link

In this commit usage of the pending packet queue have been added.
This queue will consume the downstream packets if state machine return
the error that we do not have enough capacity for htlc in commitment
transaction. Upon receiving settle/fail payment descriptors - add htlc
have been removed, we release the slot, and process pending add htlc
requests.

htlcswitch/link.go

@@ -9,6 +9,8 @@ import (
 
 	"io"
 
+	"encoding/hex"
+
 	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/roasbeef/btcd/btcec"
@@ -82,6 +84,10 @@ type channelLink struct {
 	// which may affect behaviour of the service.
 	cfg *ChannelLinkConfig
 
+	// queue is used to store the htlc add updates which haven't been
+	// processed because of the commitment trancation overflow.
+	queue *packetQueue
+
 	// upstream is a channel which responsible for propagating the
 	// received from remote peer messages, with which we have an opened
 	// channel, to handler function.
@@ -115,6 +121,7 @@ func NewChannelLink(cfg *ChannelLinkConfig,
 		downstream:    make(chan *htlcPacket),
 		control:       make(chan interface{}),
 		cancelReasons: make(map[uint64]lnwire.OpaqueReason),
+		queue:         newWaitingQueue(),
 		quit:          make(chan struct{}),
 	}
 }
@@ -248,7 +255,31 @@ out:
 				break out
 			}
 
+		// Previously add update have been added to the reprocessing
+		// queue because of the overflooding threat, and now we are
+		// trying to process it again.
+		case packet := <-l.queue.pending:
+			msg := packet.htlc.(*lnwire.UpdateAddHTLC)
+			log.Infof("Reprocess downstream add update "+
+				"with payment hash(%v)",
+				hex.EncodeToString(msg.PaymentHash[:]))
+			l.handleDownStreamPkt(packet)
+
 		case pkt := <-l.downstream:
+			// If we have non empty processing queue than in
+			// order to preserve the order of add updates
+			// consume it, and process it later.
+			htlc, ok := pkt.htlc.(*lnwire.UpdateAddHTLC)
+			if ok && l.queue.length() != 0 {
+				log.Infof("Downstream htlc add update with "+
+					"payment hash(%v) have been added to "+
+					"reprocessing queue, batch: %v",
+					hex.EncodeToString(htlc.PaymentHash[:]),
+					l.batchCounter)
+
+				l.queue.consume(pkt)
+				continue
+			}
 			l.handleDownStreamPkt(pkt)
 
 		case msg := <-l.upstream:
@@ -282,7 +313,15 @@ func (l *channelLink) handleDownStreamPkt(pkt *htlcPacket) {
 		// chains.
 		htlc.ChanID = l.ChanID()
 		index, err := l.channel.AddHTLC(htlc)
-		if err != nil {
+		if err == lnwallet.ErrMaxHTLCNumber {
+			log.Infof("Downstream htlc add update with "+
+				"payment hash(%v) have been added to "+
+				"reprocessing queue, batch: %v",
+				hex.EncodeToString(htlc.PaymentHash[:]),
+				l.batchCounter)
+			l.queue.consume(pkt)
+			return
+		} else if err != nil {
 			// TODO: possibly perform fallback/retry logic
 			// depending on type of error
 
@@ -298,6 +337,10 @@ func (l *channelLink) handleDownStreamPkt(pkt *htlcPacket) {
 				err)
 			return
 		}
+		log.Tracef("Receive downstream htlc with payment hash"+
+			"(%v), assign the index: %v, batch: %v",
+			hex.EncodeToString(htlc.PaymentHash[:]),
+			index, l.batchCounter+1)
 		htlc.ID = index
 
 		l.cfg.Peer.SendMessage(htlc)
@@ -381,6 +424,9 @@ func (l *channelLink) handleUpstreamMsg(msg lnwire.Message) {
 			l.cfg.Peer.Disconnect()
 			return
 		}
+		log.Tracef("Receive upstream htlc with payment hash(%v), "+
+			"assign the index: %v",
+			hex.EncodeToString(msg.PaymentHash[:]), index)
 
 		// TODO(roasbeef): perform sanity checks on per-hop payload
 		//  * time-lock is sane, fee, chain, etc
@@ -611,6 +657,7 @@ func (l *channelLink) processLockedInHtlcs(
 					&lnwire.UpdateFufillHTLC{
 						PaymentPreimage: pd.RPreimage,
 					}, pd.RHash, pd.Amount))
+			l.queue.release()
 
 		case lnwallet.Fail:
 			opaqueReason := l.cancelReasons[pd.ParentIndex]
@@ -625,6 +672,7 @@ func (l *channelLink) processLockedInHtlcs(
 						Reason: opaqueReason,
 						ChanID: l.ChanID(),
 					}, pd.RHash, pd.Amount))
+			l.queue.release()
 
 		case lnwallet.Add:
 			blob := l.blobs[pd.Index]