peer: eliminate possibility of concurrent map writes in htlcManager

This commit fixed an issue in the htlcManager goroutine which manages
channel state updates. Due to lack of a mutex protecting the two maps
written in the goroutine launched to forward HTLC’s to the switch.

This issue was detected by golang’s runtime which is able to detect
invalid concurrent map writes.

peer.go

@@ -1203,6 +1203,8 @@ type commitmentState struct {
 	channel   *lnwallet.LightningChannel
 	chanPoint *wire.OutPoint
 	chanID    lnwire.ChannelID
+
+	sync.RWMutex
 }
 
 // htlcManager is the primary goroutine which drives a channel's commitment
@@ -1564,7 +1566,9 @@ func (p *peer) handleUpstreamMsg(state *commitmentState, msg lnwire.Message) {
 		// switch, we'll attach the routing information so the switch
 		// can finalize the circuit.
 		case sphinx.MoreHops:
+			state.Lock()
 			state.pendingCircuits[index] = sphinxPacket
+			state.Unlock()
 		default:
 			peerLog.Errorf("mal formed onion packet")
 			state.htlcsToCancel[index] = lnwire.SphinxParseError
@@ -1590,7 +1594,9 @@ func (p *peer) handleUpstreamMsg(state *commitmentState, msg lnwire.Message) {
 			return
 		}
 
+		state.Lock()
 		state.cancelReasons[idx] = lnwire.FailCode(htlcPkt.Reason[0])
+		state.Unlock()
 
 	case *lnwire.CommitSig:
 		// We just received a new update to our local commitment chain,
@@ -1671,7 +1677,10 @@ func (p *peer) handleUpstreamMsg(state *commitmentState, msg lnwire.Message) {
 				// Otherwise, the HTLC failed, so we propagate
 				// the error back to the potential caller.
 				case lnwallet.Fail:
+					state.Lock()
 					errMsg := state.cancelReasons[parentIndex]
+					state.Unlock()
+
 					p.preImage <- [32]byte{}
 					p.err <- errors.New(errMsg.String())
 				}
@@ -1746,18 +1755,24 @@ func (p *peer) handleUpstreamMsg(state *commitmentState, msg lnwire.Message) {
 				// We don't need to forward any HTLCs that we
 				// just settled or cancelled above.
 				// TODO(roasbeef): key by index instead?
+				state.RLock()
 				if _, ok := settledPayments[htlc.RHash]; ok {
+					state.RUnlock()
 					continue
 				}
 				if _, ok := cancelledHtlcs[htlc.Index]; ok {
+					state.RUnlock()
 					continue
 				}
+				state.RUnlock()
 
+				state.Lock()
 				onionPkt := state.pendingCircuits[htlc.Index]
 				delete(state.pendingCircuits, htlc.Index)
 
 				reason := state.cancelReasons[htlc.ParentIndex]
 				delete(state.cancelReasons, htlc.ParentIndex)
+				state.Unlock()
 
 				// Send this fully activated HTLC to the htlc
 				// switch to continue the chained clear/settle.