"createonion" to accept an optional custom onion_size.

Changelog-Added: `createonion` RPC command now accepts an optional `onion_size`.

doc/lightning-createonion.7.md

@@ -4,7 +4,7 @@ lightning-createonion -- Low-level command to create a custom onion
 SYNOPSIS
 --------
 
-**createonion** *hops* *assocdata* \[*session_key*\]
+**createonion** *hops* *assocdata* \[*session_key*\] \[*onion_size*\]
 
 DESCRIPTION
 -----------
@@ -68,10 +68,10 @@ which the above *hops* parameter was generated:
 
  - Notice that the payload in the *hops* parameter is the hex-encoded version
    of the parameters in the `getroute` response.
- - The payloads are shifted left by one, i.e., payload 0 in `createonion`
-   corresponds to payload 1 from `getroute`.
+ - Except for the pubkey, the values are shifted left by one, i.e., the 1st
+   payload in `createonion` corresponds to the 2nd set of values from `getroute`.
  - The final payload is a copy of the last payload sans `channel`
- 
+
 These rules are directly derived from the onion construction. Please refer
 [BOLT 04][bolt04] for details and rationale.
 
@@ -85,6 +85,10 @@ should only be used for testing or if a specific shared secret is
 important. If not specified it will be securely generated internally, and the
 shared secrets will be returned.
 
+The optional *onion_size* parameter specifies a size different from the default
+payment onion (1300 bytes). May be used for custom protocols like trampoline
+routing.
+
 RETURN VALUE
 ------------
 
@@ -122,4 +126,3 @@ RESOURCES
 Main web site: <https://github.com/ElementsProject/lightning>
 
 [bolt04]: https://github.com/lightningnetwork/lightning-rfc/blob/master/04-onion-routing.md
-

lightningd/pay.c

@@ -1670,13 +1670,15 @@ static struct command_result *json_createonion(struct command *cmd,
 	struct secret *session_key, *shared_secrets;
 	struct sphinx_path *sp;
 	u8 *assocdata, *serialized;
+	u32 *packet_size;
 	struct onionpacket *packet;
 	struct sphinx_hop *hops;
 
 	if (!param(cmd, buffer, params,
 		   p_req("hops", param_hops_array, &hops),
 		   p_req("assocdata", param_bin_from_hex, &assocdata),
 		   p_opt("session_key", param_secret, &session_key),
+		   p_opt_def("onion_size", param_number, &packet_size, ROUTING_INFO_SIZE),
 		   NULL)) {
 		return command_param_failed();
 	}
@@ -1689,12 +1691,12 @@ static struct command_result *json_createonion(struct command *cmd,
 	for (size_t i=0; i<tal_count(hops); i++)
 		sphinx_add_hop(sp, &hops[i].pubkey, hops[i].raw_payload);
 
-	if (sphinx_path_payloads_size(sp) > ROUTING_INFO_SIZE)
+	if (sphinx_path_payloads_size(sp) > *packet_size)
 		return command_fail(
 		    cmd, JSONRPC2_INVALID_PARAMS,
 		    "Payloads exceed maximum onion packet size.");
 
-	packet = create_onionpacket(cmd, sp, ROUTING_INFO_SIZE, &shared_secrets);
+	packet = create_onionpacket(cmd, sp, *packet_size, &shared_secrets);
 	if (!packet)
 		return command_fail(cmd, LIGHTNINGD,
 				    "Could not create onion packet");