closingd: allow higher closing fee if anchor_outputs.

This follows lightning/bolts#847.

For anchor_outputs, we pass down a max_feerate to closingd, and set the
fee ceiling to MAX.  It uses that to estimate the desired closing fee.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-EXPERIMENTAL: Anchor output mutual close allow a fee higher than the final commitment transaction (as per lightning-rfc ElementsProject#847)

Author: rustyrussell

closingd/closingd.c

@@ -584,16 +584,47 @@ static size_t closing_tx_weight_estimate(u8 *scriptpubkey[NUM_SIDES],
 static void calc_fee_bounds(size_t expected_weight,
 			    u32 min_feerate,
 			    u32 desired_feerate,
-			    struct amount_sat maxfee,
+			    u32 *max_feerate,
+			    struct amount_sat commitment_fee,
 			    struct amount_sat *minfee,
-			    struct amount_sat *desiredfee)
+			    struct amount_sat *desiredfee,
+			    struct amount_sat *maxfee)
 {
 	*minfee = amount_tx_fee(min_feerate, expected_weight);
 	*desiredfee = amount_tx_fee(desired_feerate, expected_weight);
 
+	/* BOLT-closing-fee_range #2:
+	 * - If the channel does not use `option_anchor_outputs`:
+	 *   - MUST set `fee_satoshis` less than or equal to the base fee of
+	 *     the final commitment transaction, as calculated in
+	 *     [BOLT #3](03-transactions.md#fee-calculation).
+	 */
+	if (max_feerate) {
+		*maxfee = amount_tx_fee(*max_feerate, expected_weight);
+
+		status_debug("deriving max fee from rate %u -> %s (not %s)",
+			     *max_feerate,
+			     type_to_string(tmpctx, struct amount_sat, maxfee),
+			     type_to_string(tmpctx, struct amount_sat, &commitment_fee));
+
+		/* option_anchor_outputs sets commitment_fee to max, so this
+		 * doesn't do anything */
+		if (amount_sat_greater(*maxfee, commitment_fee)) {
+			status_unusual("Maximum feerate %u would give fee %s:"
+				       " we must limit it to the final commitment fee %s",
+				       *max_feerate,
+				       type_to_string(tmpctx, struct amount_sat,
+						      maxfee),
+				       type_to_string(tmpctx, struct amount_sat,
+						      &commitment_fee));
+			*maxfee = commitment_fee;
+		}
+	} else
+		*maxfee = commitment_fee;
+
 	/* Can't exceed maxfee. */
-	if (amount_sat_greater(*minfee, maxfee))
-		*minfee = maxfee;
+	if (amount_sat_greater(*minfee, *maxfee))
+		*minfee = *maxfee;
 
 	if (amount_sat_less(*desiredfee, *minfee)) {
 		status_unusual("Our ideal fee is %s (%u sats/perkw),"
@@ -603,20 +634,20 @@ static void calc_fee_bounds(size_t expected_weight,
 			       type_to_string(tmpctx, struct amount_sat, minfee));
 		*desiredfee = *minfee;
 	}
-	if (amount_sat_greater(*desiredfee, maxfee)) {
+	if (amount_sat_greater(*desiredfee, *maxfee)) {
 		status_unusual("Our ideal fee is %s (%u sats/perkw),"
 			       " but our maximum is %s: using that",
 			       type_to_string(tmpctx, struct amount_sat, desiredfee),
 			       desired_feerate,
-			       type_to_string(tmpctx, struct amount_sat, &maxfee));
-		*desiredfee = maxfee;
+			       type_to_string(tmpctx, struct amount_sat, maxfee));
+		*desiredfee = *maxfee;
 	}
 
 	status_debug("Expected closing weight = %zu, fee %s (min %s, max %s)",
 		     expected_weight,
 		     type_to_string(tmpctx, struct amount_sat, desiredfee),
 		     type_to_string(tmpctx, struct amount_sat, minfee),
-		     type_to_string(tmpctx, struct amount_sat, &maxfee));
+		     type_to_string(tmpctx, struct amount_sat, maxfee));
 }
 
 /* We've received one offer; if we're opener, that means we've already sent one
@@ -799,8 +830,9 @@ int main(int argc, char *argv[])
 	u16 funding_txout;
 	struct amount_sat funding, out[NUM_SIDES];
 	struct amount_sat our_dust_limit;
-	struct amount_sat min_fee_to_accept, commitment_fee, offer[NUM_SIDES];
-	u32 min_feerate, initial_feerate;
+	struct amount_sat min_fee_to_accept, commitment_fee, offer[NUM_SIDES],
+		max_fee_to_accept;
+	u32 min_feerate, initial_feerate, *max_feerate;
 	struct feerange feerange;
 	enum side opener;
 	u8 *scriptpubkey[NUM_SIDES], *funding_wscript;
@@ -830,7 +862,7 @@ int main(int argc, char *argv[])
 				    &out[LOCAL],
 				    &out[REMOTE],
 				    &our_dust_limit,
-				    &min_feerate, &initial_feerate,
+				    &min_feerate, &initial_feerate, &max_feerate,
 				    &commitment_fee,
 				    &scriptpubkey[LOCAL],
 				    &scriptpubkey[REMOTE],
@@ -852,8 +884,9 @@ int main(int argc, char *argv[])
 	calc_fee_bounds(closing_tx_weight_estimate(scriptpubkey,
 						   funding_wscript,
 						   out, funding, our_dust_limit),
-			min_feerate, initial_feerate, commitment_fee,
-			&min_fee_to_accept, &offer[LOCAL]);
+			min_feerate, initial_feerate, max_feerate,
+			commitment_fee,
+			&min_fee_to_accept, &offer[LOCAL], &max_fee_to_accept);
 
 	/* Write values into tlv for updated closing fee neg */
 	their_feerange = tal(ctx, struct tlv_closing_signed_tlvs_fee_range *);
@@ -862,7 +895,7 @@ int main(int argc, char *argv[])
 	if (use_quickclose) {
 		our_feerange = tal(ctx, struct tlv_closing_signed_tlvs_fee_range);
 		our_feerange->min_fee_satoshis = min_fee_to_accept;
-		our_feerange->max_fee_satoshis = commitment_fee;
+		our_feerange->max_fee_satoshis = max_fee_to_accept;
 	} else
 		our_feerange = NULL;
 
@@ -892,7 +925,7 @@ int main(int argc, char *argv[])
 	    "Negotiating closing fee between %s and %s satoshi (ideal %s) "
 	    "using step %s",
 	    type_to_string(tmpctx, struct amount_sat, &min_fee_to_accept),
-	    type_to_string(tmpctx, struct amount_sat, &commitment_fee),
+	    type_to_string(tmpctx, struct amount_sat, &max_fee_to_accept),
 	    type_to_string(tmpctx, struct amount_sat, &offer[LOCAL]),
 	    fee_negotiation_step_str);
 
@@ -955,7 +988,7 @@ int main(int argc, char *argv[])
 	}
 
 	/* Now we have first two points, we can init fee range. */
-	init_feerange(&feerange, commitment_fee, offer);
+	init_feerange(&feerange, max_fee_to_accept, offer);
 
 	/* Apply (and check) opener offer now. */
 	adjust_feerange(&feerange, offer[opener], opener);
@@ -1014,6 +1047,7 @@ int main(int argc, char *argv[])
 	tal_free(wrong_funding);
 	tal_free(our_feerange);
 	tal_free(their_feerange);
+	tal_free(max_feerate);
 	closing_dev_memleak(ctx, scriptpubkey, funding_wscript);
 #endif
 

closingd/closingd_wire.csv

@@ -19,6 +19,7 @@ msgdata,closingd_init,remote_sat,amount_sat,
 msgdata,closingd_init,our_dust_limit,amount_sat,
 msgdata,closingd_init,min_feerate_perksipa,u32,
 msgdata,closingd_init,preferred_feerate_perksipa,u32,
+msgdata,closingd_init,max_feerate_perksipa,?u32,
 msgdata,closingd_init,fee_limit_satoshi,amount_sat,
 msgdata,closingd_init,local_scriptpubkey_len,u16,
 msgdata,closingd_init,local_scriptpubkey,u8,local_scriptpubkey_len

closingd/closingd_wiregen.c

@@ -197,7 +197,7 @@ void peer_start_closingd(struct channel *channel,
 			 struct per_peer_state *pps)
 {
 	u8 *initmsg;
-	u32 feerate;
+	u32 feerate, *max_feerate;
 	struct amount_msat their_msat;
 	struct amount_sat feelimit;
 	int hsmfd;
@@ -256,6 +256,19 @@ void peer_start_closingd(struct channel *channel,
 			feerate = feerate_floor();
 	}
 
+	/* We use a feerate if anchor_outputs, otherwise max fee is set by
+	 * the final unilateral. */
+	if (channel->option_anchor_outputs) {
+		max_feerate = tal(tmpctx, u32);
+		/* Aim for reasonable max, but use final if we don't know. */
+		*max_feerate = unilateral_feerate(ld->topology);
+		if (!*max_feerate)
+			*max_feerate = final_commit_feerate;
+		/* No other limit on fees */
+		feelimit = AMOUNT_SAT(-1ULL);
+	} else
+		max_feerate = NULL;
+
 	/* BOLT #3:
 	 *
 	 * Each node offering a signature:
@@ -288,7 +301,9 @@ void peer_start_closingd(struct channel *channel,
 				       amount_msat_to_sat_round_down(channel->our_msat),
 				       amount_msat_to_sat_round_down(their_msat),
 				       channel->our_config.dust_limit,
-				       feerate_min(ld, NULL), feerate, feelimit,
+				       feerate_min(ld, NULL), feerate,
+				       max_feerate,
+				       feelimit,
 				       channel->shutdown_scriptpubkey[LOCAL],
 				       channel->shutdown_scriptpubkey[REMOTE],
 				       channel->closing_fee_negotiation_step,