injectpayment_onion: fix fees for blinded paths

Changelog-Fixed: injectpayment_onion: fix fees for blinded paths, treat amount_msat as the incoming amount and not the forward amount.

Signed-off-by: Lagrang3 <lagrang3@protonmail.com>

Author: Lagrang3

contrib/msggen/msggen/schema.json

@@ -16124,7 +16124,7 @@
           "amount_msat": {
             "type": "msat",
             "description": [
-              "The amount for the first HTLC in millisatoshis.  This is also the amount which will be forwarded to the first peer (if any) as we do not charge fees on our own payments.  Note: this is shown in listsendpays as `amount_sent_msat`."
+              "The amount in millisatoshis this node would receive from a peer before forwarding the payment to the next."
             ]
           },
           "cltv_expiry": {

doc/schemas/injectpaymentonion.json

@@ -34,7 +34,7 @@
       "amount_msat": {
         "type": "msat",
         "description": [
-          "The amount for the first HTLC in millisatoshis.  This is also the amount which will be forwarded to the first peer (if any) as we do not charge fees on our own payments.  Note: this is shown in listsendpays as `amount_sent_msat`."
+          "The amount in millisatoshis this node would receive from a peer before forwarding the payment to the next."
         ]
       },
       "cltv_expiry": {

lightningd/pay.c

@@ -1973,15 +1973,15 @@ static struct command_result *json_injectpaymentonion(struct command *cmd,
 		register_payment_and_waiter(cmd,
 					    payment_hash,
 					    *partid, *groupid,
-					    *destination_msat, *msat, AMOUNT_MSAT(0),
+					    *destination_msat, payload->amt_to_forward, AMOUNT_MSAT(0),
 					    label, invstring, local_invreq_id,
 					    &shared_secret,
 					    destination);
 
 		/* Mark it pending now, though htlc_set_add might
 		 * not resolve immediately */
 		fixme_ignore(command_still_pending(cmd));
-		htlc_set_add(cmd->ld, cmd->ld->log, *msat, *payload->total_msat,
+		htlc_set_add(cmd->ld, cmd->ld->log, payload->amt_to_forward, *payload->total_msat,
 			     NULL, payment_hash, payload->payment_secret,
 			     selfpay_mpp_fail, selfpay_mpp_succeeded,
 			     selfpay);
@@ -2017,22 +2017,22 @@ static struct command_result *json_injectpaymentonion(struct command *cmd,
 					    "Unknown peer %s",
 					    fmt_node_id(tmpctx, &nid));
 
-		next = best_channel(cmd->ld, next_peer, *msat, NULL);
+		next = best_channel(cmd->ld, next_peer, payload->amt_to_forward, NULL);
 		if (!next)
 			return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
 					    "No available channel with peer %s",
 					    fmt_node_id(tmpctx, &nid));
 	}
 
-	if (amount_msat_greater(*msat, next->htlc_maximum_msat)
-	    || amount_msat_less(*msat, next->htlc_minimum_msat)) {
+	if (amount_msat_greater(payload->amt_to_forward, next->htlc_maximum_msat)
+	    || amount_msat_less(payload->amt_to_forward, next->htlc_minimum_msat)) {
 		/* Are we in old-range grace-period? */
 		if (!timemono_before(time_mono(), next->old_feerate_timeout)
-		    || amount_msat_less(*msat, next->old_htlc_minimum_msat)
-		    || amount_msat_greater(*msat, next->old_htlc_maximum_msat)) {
+		    || amount_msat_less(payload->amt_to_forward, next->old_htlc_minimum_msat)
+		    || amount_msat_greater(payload->amt_to_forward, next->old_htlc_maximum_msat)) {
 			return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
 					    "Amount %s not in htlc min/max range %s-%s",
-					    fmt_amount_msat(tmpctx, *msat),
+					    fmt_amount_msat(tmpctx, payload->amt_to_forward),
 					    fmt_amount_msat(tmpctx, next->htlc_minimum_msat),
 					    fmt_amount_msat(tmpctx, next->htlc_maximum_msat));
 		}
@@ -2085,11 +2085,11 @@ static struct command_result *json_injectpaymentonion(struct command *cmd,
 	if (command_check_only(cmd))
 		return command_check_done(cmd);
 
-	failmsg = send_htlc_out(tmpctx, next, *msat,
+	failmsg = send_htlc_out(tmpctx, next, payload->amt_to_forward,
 				*cltv,
 				/* If unknown, we set this equal (so accounting logs 0 fees) */
 				amount_msat_eq(*destination_msat, AMOUNT_MSAT(0))
-				? *msat : *destination_msat,
+				? payload->amt_to_forward : *destination_msat,
 				payment_hash,
 				next_path_key, NULL, *partid, *groupid,
 				serialize_onionpacket(tmpctx, rs->next),
@@ -2104,7 +2104,7 @@ static struct command_result *json_injectpaymentonion(struct command *cmd,
 	register_payment_and_waiter(cmd,
 				    payment_hash,
 				    *partid, *groupid,
-				    *destination_msat, *msat, AMOUNT_MSAT(0),
+				    *destination_msat, payload->amt_to_forward, AMOUNT_MSAT(0),
 				    label, invstring, local_invreq_id,
 				    &shared_secret,
 				    destination);

tests/test_pay.py

@@ -6360,7 +6360,7 @@ def test_injectpaymentonion_selfpay(node_factory, executor):
              'payload': serialize_payload_final_tlv(333, 18, 1000, blockheight, inv5['payment_secret']).hex()}]
     onion1 = l1.rpc.createonion(hops=hops1, assocdata=inv5['payment_hash'])
     hops2 = [{'pubkey': l1.info['id'],
-             'payload': serialize_payload_final_tlv(666, 18, 1000, blockheight, inv5['payment_secret']).hex()}]
+             'payload': serialize_payload_final_tlv(667, 18, 1000, blockheight, inv5['payment_secret']).hex()}]
     onion2 = l1.rpc.createonion(hops=hops2, assocdata=inv5['payment_hash'])
 
     fut1 = executor.submit(l1.rpc.injectpaymentonion,

tests/test_xpay.py

@@ -1074,7 +1074,6 @@ def mock_getblockhash(req):
     fut.result(TIMEOUT)
 
 
-@pytest.mark.xfail(strict=True)
 def test_blinded_path_fees(node_factory):
     """Test that we don't send the amount+fees to our direct peer (we should
     only send the required amount) when the sending node is the entry point in