routing: Simplify code to read the route back in

This was overly complex since it was off-by-one and we were storing
some information elsewhere. Now this just loads the route as is into
structs, extracts some information for our outgoing HTLC, and then
shifts by the array of structs by one, and finally fills in the last
instruction, which is the terminal.

lightningd/pay.c

@@ -151,7 +151,7 @@ static void json_sendpay(struct command *cmd,
 	struct pubkey *ids;
 	jsmntok_t *routetok, *rhashtok;
 	const jsmntok_t *t, *end;
-	unsigned int delay, first_delay, base_expiry;
+	unsigned int delay, base_expiry;
 	size_t n_hops;
 	struct sha256 rhash;
 	struct peer *peer;
@@ -159,6 +159,7 @@ static void json_sendpay(struct command *cmd,
 	const u8 *onion;
 	u8 sessionkey[32];
 	struct hop_data *hop_data;
+	struct hop_data first_hop_data;
 	u64 amount, lastamount;
 	struct onionpacket *packet;
 	u8 *msg;
@@ -194,12 +195,9 @@ static void json_sendpay(struct command *cmd,
 	n_hops = 0;
 	ids = tal_arr(cmd, struct pubkey, n_hops);
 
-	/* Switching to hop_data in the next commit, and it causes a
-	 * double free in peer_control otherwise */
-	hop_data = tal_arr(NULL, struct hop_data, 0);
+	hop_data = tal_arr(cmd, struct hop_data, n_hops);
 	for (t = routetok + 1; t < end; t = json_next(t)) {
 		const jsmntok_t *amttok, *idtok, *delaytok, *chantok;
-		struct short_channel_id scid;
 
 		if (t->type != JSMN_OBJECT) {
 			command_fail(cmd, "route %zu '%.*s' is not an object",
@@ -218,29 +216,20 @@ static void json_sendpay(struct command *cmd,
 			return;
 		}
 
-		if (n_hops == 0) {
-			/* What we will send */
-			if (!json_tok_u64(buffer, amttok, &amount)) {
-				command_fail(cmd, "route %zu invalid msatoshi",
-					     n_hops);
-				return;
-			}
-			lastamount = amount;
-		} else {
+		tal_resize(&hop_data, n_hops + 1);
+		tal_resize(&ids, n_hops+1);
+		hop_data[n_hops].realm = 0;
 		/* What that hop will forward */
-			tal_resize(&hop_data, n_hops);
-			if (!json_tok_u64(buffer, amttok, &lastamount)) {
+		if (!json_tok_u64(buffer, amttok, &amount)) {
 			command_fail(cmd, "route %zu invalid msatoshi",
 				     n_hops);
 			return;
 		}
-			hop_data[n_hops - 1].amt_forward = lastamount;
-		}
+		hop_data[n_hops].amt_forward = amount;
 
-		tal_resize(&ids, n_hops+1);
 		if (!short_channel_id_from_str(buffer + chantok->start,
 					       chantok->end - chantok->start,
-					&scid)) {
+					       &hop_data[n_hops].channel_id)) {
 			command_fail(cmd, "route %zu invalid id", n_hops);
 			return;
 		}
@@ -254,13 +243,7 @@ static void json_sendpay(struct command *cmd,
 			command_fail(cmd, "route %zu invalid delay", n_hops);
 			return;
 		}
-		if (n_hops == 0)
-			first_delay = delay;
-		else {
-			hop_data[n_hops-1].outgoing_cltv
-				= base_expiry + delay;
-			hop_data[n_hops-1].channel_id = scid;
-		}
+		hop_data[n_hops].outgoing_cltv = base_expiry + delay;
 		n_hops++;
 	}
 
@@ -269,12 +252,20 @@ static void json_sendpay(struct command *cmd,
 		return;
 	}
 
-	/* Add payload for final hop */
-	tal_resize(&hop_data, n_hops);
-	/* Memset here since we need it in the onion creation but will
-	 * shave it off immediately again */
-	memset(&hop_data[n_hops-1], 0, sizeof(struct hop_data));
+	/* Store some info we'll need for our own HTLC */
+	amount = hop_data[0].amt_forward;
+	lastamount = hop_data[n_hops-1].amt_forward;
+	first_hop_data = hop_data[0];
+
+	/* Shift the hop_data down by one, so each hop gets its
+	 * instructions, not how we got there */
+	for (size_t i=0; i < n_hops - 1; i++) {
+		hop_data[i] = hop_data[i+1];
+	}
+	/* And finally set the final hop to the special values in
+	 * BOLT04 */
 	hop_data[n_hops-1].outgoing_cltv = base_expiry + delay;
+	memset(&hop_data[n_hops-1].channel_id, 0, sizeof(struct short_channel_id));
 
 	pc = find_pay_command(ld, &rhash);
 	if (pc) {
@@ -357,7 +348,7 @@ static void json_sendpay(struct command *cmd,
 	log_info(ld->log, "Sending %"PRIu64" over %zu hops to deliver %"PRIu64,
 		 amount, n_hops, lastamount);
 	msg = towire_channel_offer_htlc(pc, amount,
-					base_expiry + first_delay,
+					first_hop_data.outgoing_cltv,
 					&pc->rhash, onion);
 	subd_req(pc, peer->owner, take(msg), -1, 0, rcvd_htlc_reply, pc);