From 848801267a58b55a99af52e5b83513182a1c19d3 Mon Sep 17 00:00:00 2001
From: Yaacov Akiba Slama <ya@slamail.org>
Date: Tue, 17 Jan 2023 16:25:40 +0200
Subject: [PATCH] Open a channel only if the nextHop is unknown or the
 destination

---
 cln_interceptor.go | 19 +++++++++++++++++--
 intercept.go       |  4 ++--
 lnd_interceptor.go | 16 ++++++++++++++--
 3 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/cln_interceptor.go b/cln_interceptor.go
index 7ef5e66..b29fcbc 100644
--- a/cln_interceptor.go
+++ b/cln_interceptor.go
@@ -133,11 +133,26 @@ func (i *ClnHtlcInterceptor) intercept() error {
 				log.Printf("unexpected error in interceptor.Recv() %v", err)
 				break
 			}
+			nextHop := "<unknown>"
+			channels, err := i.client.client.GetChannel(request.Onion.ShortChannelId)
+			if err != nil {
+				for _, c := range channels {
+					if c.Source == i.config.NodePubkey {
+						nextHop = c.Destination
+						break
+					}
+					if c.Destination == i.config.NodePubkey {
+						nextHop = c.Source
+						break
+					}
+				}
+			}
 
-			log.Printf("correlationid: %v\nhtlc: %v\nchanID: %v\nincoming amount: %v\noutgoing amount: %v\nincoming expiry: %v\noutgoing expiry: %v\npaymentHash: %v\nonionBlob: %v\n\n",
+			log.Printf("correlationid: %v\nhtlc: %v\nchanID: %v\nnextHop: %v\nincoming amount: %v\noutgoing amount: %v\nincoming expiry: %v\noutgoing expiry: %v\npaymentHash: %v\nonionBlob: %v\n\n",
 				request.Correlationid,
 				request.Htlc,
 				request.Onion.ShortChannelId,
+				nextHop,
 				request.Htlc.AmountMsat, //with fees
 				request.Onion.ForwardMsat,
 				request.Htlc.CltvExpiryRelative,
@@ -153,7 +168,7 @@ func (i *ClnHtlcInterceptor) intercept() error {
 					interceptorClient.Send(i.defaultResolution(request))
 					i.doneWg.Done()
 				}
-				interceptResult := intercept(i.client, i.config, paymentHash, request.Onion.ForwardMsat, request.Htlc.CltvExpiry)
+				interceptResult := intercept(i.client, i.config, nextHop, paymentHash, request.Onion.ForwardMsat, request.Htlc.CltvExpiry)
 				switch interceptResult.action {
 				case INTERCEPT_RESUME_WITH_ONION:
 					interceptorClient.Send(i.resumeWithOnion(request, interceptResult))
diff --git a/intercept.go b/intercept.go
index d15c861..14a9305 100644
--- a/intercept.go
+++ b/intercept.go
@@ -45,7 +45,7 @@ type interceptResult struct {
 	onionBlob    []byte
 }
 
-func intercept(client LightningClient, config *NodeConfig, reqPaymentHash []byte, reqOutgoingAmountMsat uint64, reqOutgoingExpiry uint32) interceptResult {
+func intercept(client LightningClient, config *NodeConfig, nextHop string, reqPaymentHash []byte, reqOutgoingAmountMsat uint64, reqOutgoingExpiry uint32) interceptResult {
 	reqPaymentHashStr := hex.EncodeToString(reqPaymentHash)
 	resp, _, _ := payHashGroup.Do(reqPaymentHashStr, func() (interface{}, error) {
 		paymentHash, paymentSecret, destination, incomingAmountMsat, outgoingAmountMsat, channelPoint, err := paymentInfo(reqPaymentHash)
@@ -58,7 +58,7 @@ func intercept(client LightningClient, config *NodeConfig, reqPaymentHash []byte
 		}
 		log.Printf("paymentHash:%x\npaymentSecret:%x\ndestination:%x\nincomingAmountMsat:%v\noutgoingAmountMsat:%v",
 			paymentHash, paymentSecret, destination, incomingAmountMsat, outgoingAmountMsat)
-		if paymentSecret == nil {
+		if paymentSecret == nil || (nextHop != "<unknown>" && nextHop != hex.EncodeToString(destination)) {
 			return interceptResult{
 				action: INTERCEPT_RESUME,
 			}, nil
diff --git a/lnd_interceptor.go b/lnd_interceptor.go
index e89ca4c..5e0f812 100644
--- a/lnd_interceptor.go
+++ b/lnd_interceptor.go
@@ -123,9 +123,21 @@ func (i *LndHtlcInterceptor) intercept() error {
 				break
 			}
 
-			fmt.Printf("htlc: %v\nchanID: %v\nincoming amount: %v\noutgoing amount: %v\nincomin expiry: %v\noutgoing expiry: %v\npaymentHash: %x\nonionBlob: %x\n\n",
+			nextHop := "<unknown>"
+			chanInfo, err := i.client.client.GetChanInfo(context.Background(), &lnrpc.ChanInfoRequest{ChanId: request.OutgoingRequestedChanId})
+			if err == nil && chanInfo != nil {
+				if chanInfo.Node1Pub == i.config.NodePubkey {
+					nextHop = chanInfo.Node2Pub
+				}
+				if chanInfo.Node2Pub == i.config.NodePubkey {
+					nextHop = chanInfo.Node1Pub
+				}
+			}
+
+			fmt.Printf("htlc: %v\nchanID: %v\nnextHop: %v\nincoming amount: %v\noutgoing amount: %v\nincomin expiry: %v\noutgoing expiry: %v\npaymentHash: %x\nonionBlob: %x\n\n",
 				request.IncomingCircuitKey.HtlcId,
 				request.IncomingCircuitKey.ChanId,
+				nextHop,
 				request.IncomingAmountMsat,
 				request.OutgoingAmountMsat,
 				request.IncomingExpiry,
@@ -136,7 +148,7 @@ func (i *LndHtlcInterceptor) intercept() error {
 
 			i.doneWg.Add(1)
 			go func() {
-				interceptResult := intercept(i.client, i.config, request.PaymentHash, request.OutgoingAmountMsat, request.OutgoingExpiry)
+				interceptResult := intercept(i.client, i.config, nextHop, request.PaymentHash, request.OutgoingAmountMsat, request.OutgoingExpiry)
 				switch interceptResult.action {
 				case INTERCEPT_RESUME_WITH_ONION:
 					interceptorClient.Send(&routerrpc.ForwardHtlcInterceptResponse{