lightningd: prefer to forward on hinted channel

When receiving a forward request lightningd selects the best channel with
the next peer based on its ability to add an HTLC and its spendable amount.
If two forwarding request arrive at the same time pointing to the same
peer we can have a race condition in which both HTLCs compete
concurrently for the same spendable amount.
There shouldn't be a race condition if both requests are supposed to be
allocated on different parallel channels, hence instead of selecting the
"best channel" we select the hinted channel unless it cannot forward,
in that case we fallback to the "best channel".

Changelog-Fixed: racy forwarding requests in the presence of parallel channels.

Signed-off-by: Lagrang3 <[email protected]>

Author: Lagrang3

lightningd/peer_htlcs.c

@@ -668,6 +668,16 @@ static struct channel *best_channel(struct lightningd *ld,
 				    struct amount_msat amt_to_forward,
 				    struct channel *hint)
 {
+	/* Prefer using hint if it can forward the request. */
+	if (hint && channel_state_can_add_htlc(hint->state)) {
+		struct amount_msat spendable;
+		spendable = channel_amount_spendable(hint);
+		if (amount_msat_greater_eq(
+			amt_to_forward,
+			hint->channel_info.their_config.htlc_minimum) &&
+		    amount_msat_less_eq(amt_to_forward, spendable))
+			return hint;
+	}
 	struct amount_msat best_spendable = AMOUNT_MSAT(0);
 	struct channel *channel, *best = hint;