lease-rates: calculate the fee owed to peer for the funds lease

common/lease_rates.c

@@ -45,6 +45,41 @@ void lease_rates_get_commitment(struct pubkey *pubkey,
 	sha256_done(&sctx, sha);
 }
 
+bool lease_rates_calc_fee(struct lease_rates *rates,
+			  struct amount_sat accept_funding_sats,
+			  struct amount_sat requested_sats,
+			  u32 onchain_feerate,
+			  struct amount_sat *fee)
+{
+	struct amount_sat lease_fee, basis_sat, tx_fee;
+	/* BOLT- #2:
+	 * The lease fee is calculated as:
+	 * `lease_fee_base_sat` +
+	 * min(`accept_channel2`.`funding_satoshis`, `open_channel2`.`requested_sats`) * `lease_fee_basis` / 10_000 +
+	 * `funding_weight` * `funding_feerate_perkw` / 1000
+	 */
+
+	lease_fee = amount_sat(rates->lease_fee_base_sat);
+	basis_sat = amount_sat_less(accept_funding_sats, requested_sats)
+		? accept_funding_sats : requested_sats;
+
+	if (!amount_sat_scale(&basis_sat, basis_sat,
+			      rates->lease_fee_basis))
+		return false;
+
+	basis_sat = amount_sat_div(basis_sat, 10000);
+
+	if (!amount_sat_add(&lease_fee, lease_fee, basis_sat))
+		return false;
+
+	tx_fee = amount_tx_fee(onchain_feerate, rates->funding_weight);
+	if (!amount_sat_add(&lease_fee, lease_fee, tx_fee))
+		return false;
+
+	*fee = lease_fee;
+	return true;
+}
+
 bool lease_rates_set_chan_fee_base_msat(struct lease_rates *rates,
 					struct amount_msat amt)
 {

common/lease_rates.h

@@ -19,6 +19,12 @@ void lease_rates_get_commitment(struct pubkey *pubkey,
 				u16 chan_fee_ppt,
 				struct sha256 *sha);
 
+bool lease_rates_calc_fee(struct lease_rates *rates,
+			  struct amount_sat accept_funding_sats,
+			  struct amount_sat requested_sats,
+			  u32 onchain_feerate,
+			  struct amount_sat *fee);
+
 WARN_UNUSED_RESULT bool lease_rates_set_chan_fee_base_msat(struct lease_rates *rates, struct amount_msat amt);
 
 WARN_UNUSED_RESULT bool lease_rates_set_lease_fee_sat(struct lease_rates *rates, struct amount_sat amt);

common/test/run-lease_rates.c

@@ -95,11 +95,42 @@ static void check_lease_rate_commitment_hash(void)
 
 }
 
+static void check_lease_rate_fees(void)
+{
+	/* BOLT- #2:
+	 * E.g.
+	 * An node requests 1_000_000sats at a feerate of 2500perkw. They
+	 * are contributing 500_000sats. Their weight contribution to the
+	 * funding transaction will be 720.
+	 *
+	 * The accepter adds 1,100,000sats and charges a base funding fee of
+	 * 233sats with a lease fee basis of 22. Their funding weight is 444.
+	 * The lease fee is as follows:
+	 * 233 + min(1_000_000,1_100_000) * 22 / 10_000 + 444 * 2500 / 1000
+	 * The total lease fee for this open is 3543sats.
+	 */
+	struct amount_sat request_sats = amount_sat(1000000);
+	struct amount_sat accepter_contrib = amount_sat(1100000);
+	struct amount_sat fee;
+	struct lease_rates rates;
+	u32 feerate = 2500;
+
+	rates.lease_fee_basis = 22;
+	rates.lease_fee_base_sat = 233;
+	rates.funding_weight = 444;
+
+	assert(lease_rates_calc_fee(&rates, accepter_contrib,
+				    request_sats, feerate,
+				    &fee));
+	assert(amount_sat_eq(fee, amount_sat(3543)));
+}
+
 int main(int argc, char *argv[])
 {
 	common_setup(argv[0]);
 
 	check_lease_rate_commitment_hash();
+	check_lease_rate_fees();
 
 	common_shutdown();
 }

openingd/Makefile

@@ -57,6 +57,7 @@ OPENINGD_COMMON_OBJS :=				\
 	common/initial_commit_tx.o		\
 	common/key_derive.o			\
 	common/keyset.o				\
+	common/lease_rates.o			\
 	common/memleak.o			\
 	common/msg_queue.o			\
 	common/node_id.o			\

openingd/dualopend.c

@@ -192,6 +192,9 @@ struct state {
 
 	/* State of inflight funding transaction attempt */
 	struct tx_state *tx_state;
+
+	/* If delay til the channel funds lease expires */
+	u32 lease_expiry;
 };
 
 /* psbt_changeset_get_next - Get next message to send
@@ -2491,7 +2494,7 @@ static void opener_start(struct state *state, u8 *msg)
 	struct channel_id cid;
 	char *err_reason;
 	struct amount_sat total, requested_sats;
-	u32 current_blockheight, lease_expiry;
+	u32 current_blockheight;
 	bool dry_run;
 	struct tx_state *tx_state = state->tx_state;
 
@@ -2642,16 +2645,21 @@ static void opener_start(struct state *state, u8 *msg)
 		open_err_warn(state, "%s", "Abort requested");
 	}
 
-	/* FIXME: BOLT QUOTE */
+	/* BOLT- #2:
+	 * The accepting node:  ...
+	 *  - if they decide to accept the offer:
+	 *    - MUST include a `will_fund` tlv
+	 */
 	if (open_tlv->request_funds && a_tlv->will_fund) {
 		char *err_msg;
 		struct lease_rates *rates = &a_tlv->will_fund->lease_rates;
+		struct amount_sat lease_fee;
 
-		lease_expiry = current_blockheight + LEASE_RATE_DURATION;
+		state->lease_expiry = current_blockheight + LEASE_RATE_DURATION;
 
 		msg = towire_dualopend_validate_lease(NULL,
 						      &a_tlv->will_fund->signature,
-						      lease_expiry,
+						      state->lease_expiry,
 						      rates->channel_fee_max_base_msat,
 						      rates->channel_fee_max_proportional_thousandths,
 						      &state->their_funding_pubkey);
@@ -2666,8 +2674,39 @@ static void opener_start(struct state *state, u8 *msg)
 
 		if (err_msg)
 			open_err_warn(state, "%s", err_msg);
+
+		/* BOLT- #2:
+		 * The lease fee is added to the accepter's balance
+		 * in a channel, in addition to the `funding_satoshi`
+		 * that they are contributing. The channel initiator
+		 * must contribute enough funds to cover
+		 * `open_channel2`.`funding_satoshis`, the lease fee,
+		 * and their tx weight * `funding_feerate_perkw` / 1000.
+		 */
+		if (!lease_rates_calc_fee(rates, tx_state->accepter_funding,
+					  requested_sats,
+					  tx_state->feerate_per_kw_funding,
+					  &lease_fee))
+			negotiation_failed(state,
+					   "Unable to calculate lease fee");
+
+		/* Add it to the accepter's total */
+		if (!amount_sat_add(&tx_state->accepter_funding,
+				    tx_state->accepter_funding, lease_fee)) {
+
+			negotiation_failed(state,
+					   "Unable to add accepter's funding"
+					   " and channel lease fee (%s + %s)",
+					   type_to_string(tmpctx,
+							  struct amount_sat,
+							  &tx_state->accepter_funding),
+					   type_to_string(tmpctx,
+							  struct amount_sat,
+							  &lease_fee));
+			return;
+		}
 	} else
-		lease_expiry = 0;
+		state->lease_expiry = 0;
 
 	/* Check that total funding doesn't overflow */
 	if (!amount_sat_add(&total, tx_state->opener_funding,