htlcswitch: Assign each pending payment a unique ID.

This simplifies the pending payment handling code because it allows it
be handled in nearly the same way as forwarded HTLCs by treating an
empty channel ID as local dispatch.

htlcswitch/link.go

@@ -721,19 +721,16 @@ func (l *channelLink) handleDownStreamPkt(pkt *htlcPacket, isReProcess bool) {
 			"local_log_index=%v, batch_size=%v",
 			htlc.PaymentHash[:], index, l.batchCounter+1)
 
-		// If packet was forwarded from another channel link then we should
-		// create circuit (remember the path) in order to forward settle/fail
+		// Create circuit (remember the path) in order to forward settle/fail
 		// packet back.
-		if pkt.incomingChanID != (lnwire.ShortChannelID{}) {
-			l.cfg.Switch.addCircuit(&PaymentCircuit{
-				PaymentHash:    htlc.PaymentHash,
-				IncomingChanID: pkt.incomingChanID,
-				IncomingHTLCID: pkt.incomingHTLCID,
-				OutgoingChanID: l.ShortChanID(),
-				OutgoingHTLCID: index,
-				ErrorEncrypter: pkt.obfuscator,
-			})
-		}
+		l.cfg.Switch.addCircuit(&PaymentCircuit{
+			PaymentHash:    htlc.PaymentHash,
+			IncomingChanID: pkt.incomingChanID,
+			IncomingHTLCID: pkt.incomingHTLCID,
+			OutgoingChanID: l.ShortChanID(),
+			OutgoingHTLCID: index,
+			ErrorEncrypter: pkt.obfuscator,
+		})
 
 		htlc.ID = index
 		l.cfg.Peer.SendMessage(htlc)