rfc-proposal: relax closing fee requirements

Implementation of lightning/bolts#847

In particular see comment:

	OK, perhaps we should split this further, to make it explicit that there's no back-and-forth if both sides support this?

	If you both include the TLV, there's no back-and-forth, it's either two messages or three.
	If nodes receive the tlv, it picks from that range for its reply: if it doesn't care, uses the same rate.
	If reply has different rate (and it's acceptable), reply with using that rate. Otherwise it's complete.
	That will be a nice simplification in future...

Needs tests, and interop check.

Waiting for finalized wording before importing more spec notes.

Author: niftynei

closingd/closingd.c

@@ -295,10 +295,12 @@ static void send_offer(struct per_peer_state *pps,
 		       enum side opener,
 		       struct amount_sat our_dust_limit,
 		       struct amount_sat fee_to_offer,
-		       const struct bitcoin_outpoint *wrong_funding)
+		       const struct bitcoin_outpoint *wrong_funding,
+		       struct tlv_closing_signed_tlvs_fee_range *tlv_fees)
 {
 	struct bitcoin_tx *tx;
 	struct bitcoin_signature our_sig;
+	struct tlv_closing_signed_tlvs *close_tlvs;
 	u8 *msg;
 
 	/* BOLT #2:
@@ -338,8 +340,14 @@ static void send_offer(struct per_peer_state *pps,
 	status_debug("sending fee offer %s",
 		     type_to_string(tmpctx, struct amount_sat, &fee_to_offer));
 
+	/* Add the new close_tlvs with our fee range */
+	close_tlvs = tlv_closing_signed_tlvs_new(msg);
+	close_tlvs->fee_range = tlv_fees;
+
 	assert(our_sig.sighash_type == SIGHASH_ALL);
-	msg = towire_closing_signed(NULL, channel_id, fee_to_offer, &our_sig.s);
+	msg = towire_closing_signed(NULL, channel_id, fee_to_offer, &our_sig.s,
+				    close_tlvs);
+
 	sync_crypto_write(pps, take(msg));
 }
 
@@ -376,13 +384,15 @@ receive_offer(struct per_peer_state *pps,
 	      struct amount_sat our_dust_limit,
 	      struct amount_sat min_fee_to_accept,
 	      const struct bitcoin_outpoint *wrong_funding,
-	      struct bitcoin_txid *closing_txid)
+	      struct bitcoin_txid *closing_txid,
+	      struct tlv_closing_signed_tlvs_fee_range **tlv_fees)
 {
 	u8 *msg;
 	struct channel_id their_channel_id;
 	struct amount_sat received_fee;
 	struct bitcoin_signature their_sig;
 	struct bitcoin_tx *tx;
+	struct tlv_closing_signed_tlvs *close_tlvs;
 
 	/* Wait for them to say something interesting */
 	do {
@@ -406,8 +416,10 @@ receive_offer(struct per_peer_state *pps,
 	} while (!msg);
 
 	their_sig.sighash_type = SIGHASH_ALL;
+	close_tlvs = tlv_closing_signed_tlvs_new(msg);
 	if (!fromwire_closing_signed(msg, &their_channel_id,
-				     &received_fee, &their_sig.s))
+				     &received_fee, &their_sig.s,
+				     close_tlvs))
 		peer_failed_warn(pps, channel_id,
 				 "Expected closing_signed: %s",
 				 tal_hex(tmpctx, msg));
@@ -481,6 +493,11 @@ receive_offer(struct per_peer_state *pps,
 	status_debug("Received fee offer %s",
 		     type_to_string(tmpctx, struct amount_sat, &received_fee));
 
+	if (close_tlvs)
+		*tlv_fees = close_tlvs->fee_range;
+	else
+		*tlv_fees = NULL;
+
 	/* Master sorts out what is best offer, we just tell it any above min */
 	if (amount_sat_greater_eq(received_fee, min_fee_to_accept)) {
 		status_debug("...offer is reasonable");
@@ -546,6 +563,23 @@ static void adjust_feerange(struct feerange *feerange,
 			      "Overflow in updating fee range");
 }
 
+static bool
+evaluate_their_offer(struct amount_sat their_offer,
+		     const struct tlv_closing_signed_tlvs_fee_range *our_range,
+		     const struct tlv_closing_signed_tlvs_fee_range *their_range)
+{
+	/* They'd better be within their own range */
+	if (amount_sat_greater(their_offer, their_range->max_fee_satoshis))
+		return false;
+	if (amount_sat_less(their_offer, their_range->min_fee_satoshis))
+		return false;
+	if (amount_sat_greater(their_offer, our_range->max_fee_satoshis))
+		return false;
+	if (amount_sat_less(their_offer, our_range->min_fee_satoshis))
+		return false;
+	return true;
+}
+
 /* Figure out what we should offer now. */
 static struct amount_sat
 adjust_offer(struct per_peer_state *pps, const struct channel_id *channel_id,
@@ -669,6 +703,7 @@ int main(int argc, char *argv[])
 	u8 *channel_reestablish;
 	struct secret last_remote_per_commit_secret;
 	struct bitcoin_outpoint *wrong_funding;
+	struct tlv_closing_signed_tlvs_fee_range *our_feerange, **their_feerange;
 
 	subdaemon_setup(argc, argv);
 
@@ -687,7 +722,8 @@ int main(int argc, char *argv[])
 				    &out[LOCAL],
 				    &out[REMOTE],
 				    &our_dust_limit,
-				    &min_fee_to_accept, &commitment_fee,
+				    &min_fee_to_accept,
+				    &commitment_fee,
 				    &offer[LOCAL],
 				    &scriptpubkey[LOCAL],
 				    &scriptpubkey[REMOTE],
@@ -706,6 +742,13 @@ int main(int argc, char *argv[])
 	/* stdin == requests, 3 == peer, 4 = gossip, 5 = gossip_store, 6 = hsmd */
 	per_peer_state_set_fds(notleak(pps), 3, 4, 5);
 
+	/* Write values into tlv for updated closing fee neg */
+	their_feerange = tal(ctx, struct tlv_closing_signed_tlvs_fee_range *);
+	*their_feerange = NULL;
+	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;
+
 	snprintf(fee_negotiation_step_str, sizeof(fee_negotiation_step_str),
 		 "%" PRIu64 "%s", fee_negotiation_step,
 		 fee_negotiation_step_unit ==
@@ -763,7 +806,8 @@ int main(int argc, char *argv[])
 				   funding, out, opener,
 				   our_dust_limit,
 				   offer[LOCAL],
-				   wrong_funding);
+				   wrong_funding,
+				   our_feerange);
 		} else {
 			if (i == 0)
 				peer_billboard(false, "Waiting for their initial"
@@ -785,7 +829,69 @@ int main(int argc, char *argv[])
 						our_dust_limit,
 						min_fee_to_accept,
 						wrong_funding,
-						&closing_txid);
+						&closing_txid,
+						their_feerange);
+
+			if (*their_feerange) {
+				if (!evaluate_their_offer(offer[REMOTE],
+							  our_feerange,
+							  *their_feerange)) {
+					peer_billboard(true,
+						       "Unable to agree on"
+						       " a feerate."
+						       " Our range %s-%s,"
+						       " their range %s-%s",
+						       type_to_string(tmpctx,
+							struct amount_sat,
+							&our_feerange->min_fee_satoshis),
+						       type_to_string(tmpctx,
+							struct amount_sat,
+							&our_feerange->max_fee_satoshis),
+						       type_to_string(tmpctx,
+							struct amount_sat,
+							&(*their_feerange)->min_fee_satoshis),
+						       type_to_string(tmpctx,
+							struct amount_sat,
+							&(*their_feerange)->max_fee_satoshis));
+					goto exit_thru_the_giftshop;
+				}
+
+				peer_billboard(true, "Using quick-close."
+					       " Updated our offer to match"
+					       " theirs (was %s, now %s",
+					       type_to_string(tmpctx,
+							      struct amount_sat,
+							      &offer[LOCAL]),
+					       type_to_string(tmpctx,
+							      struct amount_sat,
+							      &offer[REMOTE]));
+
+				/* We mirror it back */
+				if (i == 1 && !amount_sat_eq(offer[LOCAL],
+							     offer[REMOTE])) {
+					send_offer(pps, chainparams,
+						   &channel_id,
+						   funding_pubkey,
+						   funding_wscript,
+						   scriptpubkey,
+						   &funding_txid,
+						   funding_txout,
+						   funding, out, opener,
+						   our_dust_limit,
+						   offer[REMOTE],
+						   wrong_funding,
+						   our_feerange);
+				}
+
+				/* BOLT-9a0b1f68d6c6feefe2c459e28adee3475d02a62e #2:
+				 * The receiving node:
+				 * - if `fee_satoshis` matches its previously sent
+				 *   `fee_range`:
+				 *   - SHOULD use `fee_satoshis` to sign and
+				 *     broadcast the final closing transaction
+				 */
+				offer[LOCAL] = offer[REMOTE];
+			}
 		}
 	}
 
@@ -814,7 +920,8 @@ int main(int argc, char *argv[])
 				   funding, out, opener,
 				   our_dust_limit,
 				   offer[LOCAL],
-				   wrong_funding);
+				   wrong_funding,
+				   our_feerange);
 		} else {
 			peer_billboard(false, "Waiting for another"
 				       " closing fee offer:"
@@ -831,7 +938,8 @@ int main(int argc, char *argv[])
 						our_dust_limit,
 						min_fee_to_accept,
 						wrong_funding,
-						&closing_txid);
+						&closing_txid,
+						their_feerange);
 		}
 
 		whose_turn = !whose_turn;
@@ -841,6 +949,7 @@ int main(int argc, char *argv[])
 		       offer[LOCAL],
 		       type_to_string(tmpctx, struct bitcoin_txid, &closing_txid));
 
+exit_thru_the_giftshop:
 #if DEVELOPER
 	/* We don't listen for master commands, so always check memleak here */
 	tal_free(wrong_funding);

wire/extracted_peer_04_closing_fee_nego.patch

@@ -0,0 +1,13 @@
+--- wire/peer_wire.csv	2021-05-18 11:15:05.539134131 -0500
++++ -	2021-05-18 11:15:15.690789516 -0500
+@@ -90,6 +90,10 @@
+ msgdata,closing_signed,channel_id,channel_id,
+ msgdata,closing_signed,fee_satoshis,u64,
+ msgdata,closing_signed,signature,signature,
++msgdata,closing_signed,tlvs,closing_signed_tlvs,
++tlvtype,closing_signed_tlvs,fee_range,1
++tlvdata,closing_signed_tlvs,fee_range,min_fee_satoshis,u64,
++tlvdata,closing_signed_tlvs,fee_range,max_fee_satoshis,u64,
+ msgtype,update_add_htlc,128
+ msgdata,update_add_htlc,channel_id,channel_id,
+ msgdata,update_add_htlc,id,u64,

wire/test/run-peer-wire.c

@@ -611,20 +611,27 @@ static struct msg_funding_signed *fromwire_struct_funding_signed(const tal_t *ct
 static void *towire_struct_closing_signed(const tal_t *ctx,
 					  const struct msg_closing_signed *s)
 {
+	struct tlv_closing_signed_tlvs *close_tlvs;
+
+	close_tlvs = tlv_closing_signed_tlvs_new(ctx);
 	return towire_closing_signed(ctx,
 				     &s->channel_id,
 				     s->fee_satoshis,
-				     &s->signature);
+				     &s->signature,
+				     close_tlvs);
 }
 
 static struct msg_closing_signed *fromwire_struct_closing_signed(const tal_t *ctx, const void *p)
 {
 	struct msg_closing_signed *s = tal(ctx, struct msg_closing_signed);
+	struct tlv_closing_signed_tlvs *close_tlvs;
 
+	close_tlvs = tlv_closing_signed_tlvs_new(ctx);
 	if (fromwire_closing_signed(p,
 				    &s->channel_id,
 				    &s->fee_satoshis,
-				    &s->signature))
+				    &s->signature,
+				    close_tlvs))
 		return s;
 	return tal_free(s);
 }