pay: Add strict_first option to send_payment_core

send_payment_core uses non-strict channel selection on the first
hop. The newly introduced option strict_first forces the payment
core to use the channel with the scid given in the route for the
first hop.
If the channel can not add an htlc, the sendpay core returns an
error PAY_TRY_OTHER_ROUTE.

lightningd/pay.c

@@ -852,6 +852,7 @@ send_payment_core(struct lightningd *ld,
 		  u64 partid,
 		  u64 group,
 		  const struct route_hop *first_hop,
+		  const bool strict_first,
 		  struct amount_msat msat,
 		  struct amount_msat total_msat,
 		  const char *label TAKES,
@@ -1032,7 +1033,20 @@ send_payment_core(struct lightningd *ld,
 	if (offer_err)
 		return offer_err;
 
-	channel = find_channel_for_htlc_add(ld, &first_hop->node_id);
+	/* We want to use the channel with the given scid in the routes first hop
+	 * if strict_first is set. Use no-strict channel selection otherwise.
+	 */
+	if (strict_first) {
+		struct peer *peer = peer_by_id(ld, &first_hop->node_id);
+		if (!peer)
+			channel = NULL;
+
+		channel = find_channel_by_scid(peer, &first_hop->scid);
+		if (!channel_can_add_htlc(channel))
+			channel = NULL;
+	} else
+		channel = find_channel_for_htlc_add(ld, &first_hop->node_id);
+
 	if (!channel) {
 		struct json_stream *data
 			= json_stream_fail(cmd, PAY_TRY_OTHER_ROUTE,
@@ -1128,7 +1142,8 @@ send_payment(struct lightningd *ld,
 	     const char *description TAKES,
 	     const struct sha256 *local_offer_id,
 	     const struct secret *payment_secret,
-	     const u8 *payment_metadata)
+	     const u8 *payment_metadata,
+		 const bool strict_first)
 {
 	unsigned int base_expiry;
 	struct onionpacket *packet;
@@ -1198,6 +1213,7 @@ send_payment(struct lightningd *ld,
 		 n_hops, type_to_string(tmpctx, struct amount_msat, &msat));
 	packet = create_onionpacket(tmpctx, path, ROUTING_INFO_SIZE, &path_secrets);
 	return send_payment_core(ld, cmd, rhash, partid, group, &route[0],
+				 strict_first,
 				 msat, total_msat,
 				 label, invstring, description,
 				 packet, &ids[n_hops - 1], ids,
@@ -1276,6 +1292,7 @@ static struct command_result *json_sendonion(struct command *cmd,
 	struct amount_msat *msat;
 	u64 *partid, *group;
 	struct sha256 *local_offer_id = NULL;
+	bool *strict_first;
 
 	if (!param(cmd, buffer, params,
 		   p_req("onion", param_bin_from_hex, &onion),
@@ -1291,6 +1308,7 @@ static struct command_result *json_sendonion(struct command *cmd,
 		   p_opt("localofferid", param_sha256, &local_offer_id),
 		   p_opt("groupid", param_u64, &group),
 		   p_opt("description", param_string, &description),
+		   p_opt_def("strict_first", param_bool, &strict_first, false),
 		   NULL))
 		return command_param_failed();
 
@@ -1311,7 +1329,8 @@ static struct command_result *json_sendonion(struct command *cmd,
 				    failcode);
 
 	return send_payment_core(ld, cmd, payment_hash, *partid, *group,
-				 first_hop, *msat, AMOUNT_MSAT(0),
+				 first_hop, strict_first,
+				 *msat, AMOUNT_MSAT(0),
 				 label, invstring, description,
 				 packet, destination, NULL, NULL,
 				 path_secrets, local_offer_id);
@@ -1409,6 +1428,7 @@ static struct command_result *json_sendpay(struct command *cmd,
 	struct secret *payment_secret;
 	struct sha256 *local_offer_id;
 	u8 *payment_metadata;
+	bool *strict_first;
 
 	/* For generating help, give new-style. */
 	if (!param(cmd, buffer, params,
@@ -1424,6 +1444,7 @@ static struct command_result *json_sendpay(struct command *cmd,
 		   p_opt("groupid", param_u64, &group),
 		   p_opt("payment_metadata", param_bin_from_hex, &payment_metadata),
 		   p_opt("description", param_string, &description),
+		   p_opt_def("strict_first", param_bool, &strict_first, false),
 		   NULL))
 		return command_param_failed();
 
@@ -1474,7 +1495,7 @@ static struct command_result *json_sendpay(struct command *cmd,
 			    final_amount,
 			    msat ? *msat : final_amount,
 			    label, invstring, description, local_offer_id,
-			    payment_secret, payment_metadata);
+			    payment_secret, payment_metadata, strict_first);
 }
 
 static const struct json_command sendpay_command = {