json-rpc: Add createonion command similar to the devtools/onion tool

This allows us to create an onion in the JSON-RPC that we can then later inject with the `sendonion` command that we're about to implement.

lightningd/pay.c

@@ -3,6 +3,7 @@
 #include <ccan/tal/str/str.h>
 #include <common/bolt11.h>
 #include <common/json_command.h>
+#include <common/json_helpers.h>
 #include <common/jsonrpc_errors.h>
 #include <common/param.h>
 #include <common/timeout.h>
@@ -1103,3 +1104,99 @@ static const struct json_command listsendpays_command = {
 	"Show sendpay, old and current, optionally limiting to {bolt11} or {payment_hash}."
 };
 AUTODATA(json_command, &listsendpays_command);
+
+static struct command_result *json_createonion(struct command *cmd,
+						const char *buffer,
+						const jsmntok_t *obj UNNEEDED,
+						const jsmntok_t *params)
+{
+	const jsmntok_t *hopstok, *hop, *payloadtok, *typetok, *pubkeytok;
+	enum sphinx_payload_type type;
+	size_t i, hop_count = 0;
+	struct json_stream *response;
+	struct pubkey pubkey;
+	struct secret *session_key, *shared_secrets;
+	struct sphinx_path *sp;
+	u8 *assocdata, *payload, *serialized;
+	struct onionpacket *packet;
+
+	if (!param(cmd, buffer, params,
+		   p_req("hops", param_array, &hopstok),
+		   p_req("assocdata", param_bin_from_hex, &assocdata),
+		   p_opt("session_key", param_secret, &session_key),
+		   NULL)) {
+		return command_param_failed();
+	}
+
+	if (session_key == NULL)
+		sp = sphinx_path_new(cmd, assocdata);
+	else
+		sp = sphinx_path_new_with_key(cmd, assocdata, session_key);
+
+	json_for_each_arr(i, hop, hopstok) {
+		payloadtok = json_get_member(buffer, hop, "payload");
+		typetok = json_get_member(buffer, hop, "type");
+		pubkeytok = json_get_member(buffer, hop, "pubkey");
+
+		if (!pubkeytok)
+			return command_fail(cmd, JSONRPC2_INVALID_REQUEST,
+					    "Hop %zu does not have a pubkey", i);
+
+		if (!payloadtok)
+			return command_fail(cmd, JSONRPC2_INVALID_REQUEST,
+					    "Hop %zu does not have a payload", i);
+
+		payload = json_tok_bin_from_hex(cmd, buffer, payloadtok);
+		if (!json_to_pubkey(buffer, pubkeytok, &pubkey))
+			return command_fail(
+			    cmd, JSONRPC2_INVALID_PARAMS,
+			    "'pubkey' should be a pubkey, not '%.*s'",
+			    pubkeytok->end - pubkeytok->start,
+			    buffer + pubkeytok->start);
+
+		if (!payload)
+			return command_fail(
+			    cmd, JSONRPC2_INVALID_PARAMS,
+			    "'payload' should be a hex encoded binary, not '%.*s'",
+			    pubkeytok->end - pubkeytok->start,
+			    buffer + pubkeytok->start);
+
+		if (!typetok || !json_tok_streq(buffer, typetok, "legacy")) {
+			type = SPHINX_RAW_PAYLOAD;
+		} else {
+			type = SPHINX_V0_PAYLOAD;
+		}
+
+		sphinx_add_raw_hop(sp, &pubkey, type, payload);
+		hop_count++;
+	}
+
+	if (hop_count == 0)
+		return command_fail(cmd, JSONRPC2_INVALID_REQUEST,
+				    "Cannot create an onion without hops.");
+
+
+	packet = create_onionpacket(cmd, sp, &shared_secrets);
+	if (!packet)
+		return command_fail(cmd, LIGHTNINGD,
+				    "Could not create onion packet");
+
+	serialized = serialize_onionpacket(cmd, packet);
+
+	response = json_stream_success(cmd);
+	json_add_hex(response, "onion", serialized, tal_bytelen(serialized));
+	json_array_start(response, "shared_secrets");
+	for (size_t i=0; i<hop_count; i++) {
+		json_add_secret(response, NULL, &shared_secrets[i]);
+	}
+	json_array_end(response);
+	return command_success(cmd, response);
+}
+
+static const struct json_command createonion_command = {
+	"createonion",
+	"payment",
+	json_createonion,
+	"Create an onion going through the provided nodes, each with its own payload"
+};
+AUTODATA(json_command, &createonion_command);