lnd+lnrpc: update fwdinghistory message

In this commit we update the returned message for fwdinghistory
to include the htlcindex for all forwarded htlcs.

itest/lnd_multi-hop-payments_test.go

@@ -213,6 +213,31 @@ func testMultiHopPayments(ht *lntest.HarnessTest) {
 		require.Equal(ht, aliceAlias, event.PeerAliasOut)
 	}
 
+	// Verify HTLC IDs are not nil and unique across all forwarding events.
+	seenIDs := make(map[uint64]bool)
+	for _, event := range fwdingHistory.ForwardingEvents {
+		// We check that the incoming and outgoing htlc indices are not
+		// set to nil. The indices are required for any forwarding event
+		// recorded after v0.20.
+		require.NotNil(ht, event.IncomingHtlcId)
+		require.NotNil(ht, event.OutgoingHtlcId)
+
+		require.False(ht, seenIDs[*event.IncomingHtlcId])
+		require.False(ht, seenIDs[*event.OutgoingHtlcId])
+		seenIDs[*event.IncomingHtlcId] = true
+		seenIDs[*event.OutgoingHtlcId] = true
+	}
+
+	// The HTLC IDs should be exactly 0, 1, 2, 3, 4.
+	expectedIDs := map[uint64]bool{
+		0: true,
+		1: true,
+		2: true,
+		3: true,
+		4: true,
+	}
+	require.Equal(ht, expectedIDs, seenIDs)
+
 	// We expect Carol to have successful forwards and settles for
 	// her sends.
 	ht.AssertHtlcEvents(

lnrpc/lightning.proto

@@ -4716,6 +4716,14 @@ message ForwardingEvent {
     // The peer alias of the outgoing channel.
     string peer_alias_out = 13;
 
+    // The ID of the incoming HTLC in the payment circuit. This field is
+    // optional and is unset for forwarding events happened before v0.20.
+    optional uint64 incoming_htlc_id = 14;
+
+    // The ID of the outgoing HTLC in the payment circuit. This field is
+    // optional and may be unset for legacy forwarding events.
+    optional uint64 outgoing_htlc_id = 15;
+
     // TODO(roasbeef): add settlement latency?
     //  * use FPE on the chan id?
     //  * also list failures?

lnrpc/lightning.swagger.json

@@ -5075,6 +5075,16 @@
         "peer_alias_out": {
           "type": "string",
           "description": "The peer alias of the outgoing channel."
+        },
+        "incoming_htlc_id": {
+          "type": "string",
+          "format": "uint64",
+          "description": "The ID of the incoming HTLC in the payment circuit. This field is\noptional and is unset for forwarding events happened before v0.20."
+        },
+        "outgoing_htlc_id": {
+          "type": "string",
+          "format": "uint64",
+          "description": "The ID of the outgoing HTLC in the payment circuit. This field is\noptional and may be unset for legacy forwarding events."
         }
       }
     },

rpcserver.go

@@ -8100,6 +8100,16 @@ func (r *rpcServer) ForwardingHistory(ctx context.Context,
 			AmtOutMsat:  uint64(amtOutMsat),
 		}
 
+		// If the incoming htlc id is present, add it to the response.
+		event.IncomingHtlcID.WhenSome(func(id uint64) {
+			resp.ForwardingEvents[i].IncomingHtlcId = &id
+		})
+
+		// If the outgoing htlc id is present, add it to the response.
+		event.OutgoingHtlcID.WhenSome(func(id uint64) {
+			resp.ForwardingEvents[i].OutgoingHtlcId = &id
+		})
+
 		if req.PeerAliasLookup {
 			aliasIn, err := getRemoteAlias(event.IncomingChanID)
 			if err != nil {