htlcswitch: add a new UpdateFeePolicy to ChannelLink interface

This commit adds a new method to the ChannelLink interface which is
meant to allow outside sub-system to update the forwarding policy of a
channel. This can be triggered either by a new RPC method, or
automatically by some sort of control system which seeks to optimize
fee revenue, or block off channels, etc.

htlcswitch/interfaces.go

@@ -57,8 +57,17 @@ type ChannelLink interface {
 	// short channel ID encodes the exact location in the main chain that
 	// the original funding output can be found.
 	ShortChanID() lnwire.ShortChannelID
+
+	// UpdateForwardingPolicy updates the forwarding policy for the target
+	// ChannelLink. Once updated, the link will use the new forwarding
+	// policy to govern if it an incoming HTLC should be forwarded or not.
+	UpdateForwardingPolicy(ForwardingPolicy)
+
 	// Bandwidth returns the amount of satoshis which current link might
-	// pass through channel link.
+	// pass through channel link. The value returned from this method
+	// represents the up to date available flow through the channel. This
+	// takes into account any forwarded but un-cleared HTLC's, and any
+	// HTLC's which have been set to the over flow queue.
 	Bandwidth() btcutil.Amount
 
 	// Stats return the statistics of channel link. Number of updates,

htlcswitch/link.go

@@ -740,6 +740,36 @@ func (l *channelLink) getBandwidth() btcutil.Amount {
 	return l.channel.LocalAvailableBalance() - l.overflowQueue.pendingAmount()
 }
 
+// policyUpdate is a message sent to a channel link when an outside sub-system
+// wishes to update the current forwarding policy.
+type policyUpdate struct {
+	policy ForwardingPolicy
+
+	done chan struct{}
+}
+
+// UpdateForwardingPolicy updates the forwarding policy for the target
+// ChannelLink. Once updated, the link will use the new forwarding policy to
+// govern if it an incoming HTLC should be forwarded or not.
+//
+// NOTE: Part of the ChannelLink interface.
+func (l *channelLink) UpdateForwardingPolicy(newPolicy ForwardingPolicy) {
+	cmd := &policyUpdate{
+		policy: newPolicy,
+		done:   make(chan struct{}),
+	}
+
+	select {
+	case l.linkControl <- cmd:
+	case <-l.quit:
+	}
+
+	select {
+	case <-cmd.done:
+	case <-l.quit:
+	}
+}
+
 // Stats returns the statistics of channel link.
 //
 // NOTE: Part of the ChannelLink interface.