htlcswitch: add new method to the ChannelLink interface, EligibleToForward

In this commit, we add a new method to the ChanneLink interface:
EligibleToForward. This method allows a link to be added to the switch,
but in an intermediate state which indicates that it isn’t yet ready to
forward any incoming HTLC’s.

htlcswitch/interfaces.go

@@ -85,6 +85,13 @@ type ChannelLink interface {
 	// the channel link opened.
 	Peer() Peer
 
+	// EligibleToForward returns a bool indicating if the channel is able
+	// to actively accept requests to forward HTLC's. A channel may be
+	// active, but not able to forward HTLC's if it hasn't yet finalized
+	// the pre-channel operation protocol with the remote peer. The switch
+	// will use this function in forwarding decisions accordingly.
+	EligibleToForward() bool
+
 	// Start/Stop are used to initiate the start/stop of the channel link
 	// functioning.
 	Start() error

htlcswitch/link.go

@@ -292,6 +292,14 @@ func (l *channelLink) Stop() {
 	l.cfg.BlockEpochs.Cancel()
 }
 
+// EligibleToForward returns a bool indicating if the channel is able to
+// actively accept requests to forward HTLC's. We're able to forward HTLC's if
+// we know the remote party's next revocation point. Otherwise, we can't
+// initiate new channel state.
+func (l *channelLink) EligibleToForward() bool {
+	return l.channel.RemoteNextRevocation() != nil
+}
+
 // sampleNetworkFee samples the current fee rate on the network to get into the
 // chain in a timely manner. The returned value is expressed in fee-per-kw, as
 // this is the native rate used when computing the fee for commitment

htlcswitch/mock.go

@@ -430,6 +430,7 @@ func (f *mockChannelLink) Bandwidth() lnwire.MilliSatoshi     { return 99999999
 func (f *mockChannelLink) Peer() Peer                         { return f.peer }
 func (f *mockChannelLink) Start() error                       { return nil }
 func (f *mockChannelLink) Stop()                              {}
+func (f *mockChannelLink) EligibleToForward() bool            { return true }
 
 var _ ChannelLink = (*mockChannelLink)(nil)