lnwallet: add new ActiveHtlcs to channel state machine

In this commit, we’ve added a new method to the channel state machine:
ActiveHtlcs. This method will allow callers to poll the state of the
channel to retrieve the set of HTLC’s active on *both* commitment
transactions.

lnwallet/channel.go

@@ -5284,3 +5284,32 @@ func (lc *LightningChannel) State() *channeldb.OpenChannel {
 func (lc *LightningChannel) ObserverQuit() chan struct{} {
 	return lc.observerQuit
 }
+
+// ActiveHtlcs returns a slice of HTLC's which are currently active on *both*
+// commitment transactions.
+func (lc *LightningChannel) ActiveHtlcs() []channeldb.HTLC {
+	lc.RLock()
+	defer lc.RUnlock()
+
+	// We'll only return HTLC's that are locked into *both* commitment
+	// transactions. So we'll iterate through their set of HTLC's to note
+	// which ones are present on thir commitment.
+	remoteHtlcs := make(map[[32]byte]struct{})
+	for _, htlc := range lc.channelState.RemoteCommitment.Htlcs {
+		onionHash := sha256.Sum256(htlc.OnionBlob[:])
+		remoteHtlcs[onionHash] = struct{}{}
+	}
+
+	// Now tht we know which HTLC's they have, we'll only mark the HTLC's
+	// as active if *we* know them as well.
+	activeHtlcs := make([]channeldb.HTLC, 0, len(remoteHtlcs))
+	for _, htlc := range lc.channelState.LocalCommitment.Htlcs {
+		if _, ok := remoteHtlcs[sha256.Sum256(htlc.OnionBlob[:])]; !ok {
+			continue
+		}
+
+		activeHtlcs = append(activeHtlcs, htlc)
+	}
+
+	return activeHtlcs
+}