simpleclosed.c: add heuristic to delay our tx broadcast if our amount is less AND our fee is less than our peer's amount and fee

in case of a reboot the tx will be broadcast as usual.

simple_close_control.c: add delay logic to `drop_to_chain` after 1 hour.
test_closing.py: add test to verify the heuristic is being applied.

Author: nGoline

closingd/simpleclosed.c

@@ -309,7 +309,8 @@ static struct bitcoin_tx *handle_closing_complete(
 					struct amount_sat local_sat,
 					struct amount_sat dust_limit,
 					const u8 *our_last_script,
-					const u8 *msg)
+					const u8 *msg,
+					struct amount_sat *their_fee_out)
 {
 	struct channel_id their_cid = {};
 	u8 *closer_script, *closee_script;
@@ -331,6 +332,8 @@ static struct bitcoin_tx *handle_closing_complete(
 				 "Bad closing_complete: %s",
 				 tal_hex(tmpctx, msg));
 
+	*their_fee_out = fee_sat;
+
 	/* BOLT #2:
 	 * The receiver of `closing_complete` (aka. "the closee"):
 	 * ...
@@ -611,7 +614,7 @@ int main(int argc, char *argv[])
 	bool got_peer_complete, got_our_sig;
 	struct tlv_closing_tlvs *sent_tlvs;
 	u8 *sent_closer_script, *sent_closee_script;
-	struct amount_sat sent_fee;
+	struct amount_sat sent_fee, their_fee;
 	u32 sent_locktime = 0;
 
 	subdaemon_setup(argc, argv);
@@ -667,7 +670,8 @@ int main(int argc, char *argv[])
 			}
 			handle_closing_complete(&funding, funding_sats,
 				&local_fundingkey, &remote_fundingkey, local_wallet_index,
-				local_wallet_ext_key, local_sat, dust_limit, local_script, msg);
+				local_wallet_ext_key, local_sat, dust_limit, local_script,
+				msg, &their_fee);
 			got_peer_complete = true;
 			break;
 
@@ -761,7 +765,27 @@ int main(int argc, char *argv[])
 		}
 	}
 
-	wire_sync_write(REQ_FD, take(towire_simpleclosed_complete(NULL)));
+	/* Decide whether to ask master to delay broadcasting our closer tx.
+	 * If our output (closer pays the full fee) is less than the peer's
+	 * output AND our proposed fee is lower than theirs, their tx has a
+	 * better chance of being mined first — give it an hour head-start.
+	 * Skip when closer_amount is zero: our output is dust and will be
+	 * omitted regardless, so there is nothing to protect by waiting. */
+	struct amount_sat closer_amount;
+	if (!amount_sat_sub(&closer_amount, local_sat, sent_fee))
+		closer_amount = AMOUNT_SAT(0);
+	bool delay_broadcast = amount_sat_greater(closer_amount, AMOUNT_SAT(0))
+		&& amount_sat_less(closer_amount, remote_sat)
+		&& amount_sat_less(sent_fee, their_fee);
+	if (delay_broadcast)
+		status_debug("Simple close: requesting delayed broadcast"
+			" (our output %s < peer %s, our fee %s < their fee %s)",
+			fmt_amount_sat(tmpctx, closer_amount),
+			fmt_amount_sat(tmpctx, remote_sat),
+			fmt_amount_sat(tmpctx, sent_fee),
+			fmt_amount_sat(tmpctx, their_fee));
+
+	wire_sync_write(REQ_FD, take(towire_simpleclosed_complete(NULL, delay_broadcast)));
 	tal_free(ctx);
 	daemon_shutdown();
 }

closingd/simpleclosed_wire.csv

@@ -47,3 +47,4 @@ msgdata,simpleclosed_closee_broadcast,sig,bitcoin_signature,
 
 # Negotiations complete, exiting.
 msgtype,simpleclosed_complete,3004
+msgdata,simpleclosed_complete,delay_broadcast,bool,

lightningd/simple_close_control.c

@@ -5,7 +5,9 @@
 #include <ccan/tal/str/str.h>
 #include <closingd/simpleclosed_wiregen.h>
 #include <common/fee_states.h>
+#include <common/memleak.h>
 #include <common/shutdown_scriptpubkey.h>
+#include <common/timeout.h>
 #include <errno.h>
 #include <hsmd/permissions.h>
 #include <inttypes.h>
@@ -153,9 +155,17 @@ static void handle_simpleclosed_closee_broadcast(struct channel *channel,
 		fmt_bitcoin_txid(tmpctx, &txid));
 }
 
+static void delayed_drop_to_chain(struct channel *channel)
+{
+	log_info(channel->log, "Simple close: broadcast delay elapsed, broadcasting closing tx");
+	drop_to_chain(channel->peer->ld, channel, true, NULL);
+}
+
 static void handle_simpleclosed_complete(struct channel *channel, const u8 *msg)
 {
-	if (!fromwire_simpleclosed_complete(msg)) {
+	struct lightningd *ld = channel->peer->ld;
+	bool delay_broadcast;
+	if (!fromwire_simpleclosed_complete(msg, &delay_broadcast)) {
 		channel_internal_error(channel,
 			"bad simpleclosed_complete: %s",
 			tal_hex(msg, msg));
@@ -176,7 +186,24 @@ static void handle_simpleclosed_complete(struct channel *channel, const u8 *msg)
 		REASON_UNKNOWN,
 		"Simple close complete");
 
-	drop_to_chain(channel->peer->ld, channel, true, NULL);
+	if (delay_broadcast) {
+		log_info(channel->log,
+			"Simple close: delaying broadcast by 1 hour"
+			" (peer has higher-fee tx)");
+		/* Watch the funding outpoint now so onchaind starts when the
+		 * peer's higher-fee tx confirms.  Also resolves any pending
+		 * `close` RPC immediately rather than blocking for an hour. */
+		channel_watch_funding_out(ld, channel);
+		const struct bitcoin_tx **txs
+			= tal_arr(tmpctx, const struct bitcoin_tx *, 1);
+		txs[0] = channel->last_tx;
+		resolve_close_command(ld, channel, true, txs);
+		notleak(new_reltimer(ld->timers, channel,
+			time_from_sec(3600),
+			delayed_drop_to_chain, channel));
+	} else {
+		drop_to_chain(ld, channel, true, NULL);
+	}
 }
 
 static unsigned int simpleclosed_msg(struct subd *sd, const u8 *msg,

tests/test_closing.py

@@ -4320,6 +4320,54 @@ def test_simple_close_closee_path(node_factory, bitcoind):
     wait_for(lambda: confirmed_txid in {o['txid'] for o in l2.rpc.listfunds()['outputs']})
 
 
+def test_simple_close_delay_broadcast(node_factory, bitcoind, executor):
+    """When the closer has less output AND proposes a lower fee than the peer,
+    it must log a 1-hour delay and let the peer's higher-fee tx get mined first.
+    The peer (l2) has the reversed conditions and must NOT delay.
+
+    We verify the delay via the log message only — we cannot wait an hour."""
+    # feerates[3] (100-block ECONOMICAL target) drives BOTH mutual_close_feerate
+    # AND the anchor commitment feerate used during channel open.  The anchor
+    # path clamps to a floor of 1250 sat/kw, and l2 enforces a minimum of
+    # feerates[3]//2 = 7500//2 = 3750 sat/kw on the proposed commitment rate.
+    # So l1 needs feerates[3] >= 3750 to pass l2's open-channel check, yet
+    # still be strictly lower than l2's 7500 to trigger the delay heuristic.
+    # Setting per-node feerates at startup avoids smoothing: the first poll
+    # copies raw values directly with no exponential smoothing applied.
+    l1_opts = {'experimental-simple-close': None,
+               'feerates': (7500, 7500, 7500, 3750)}
+    l2_opts = {'experimental-simple-close': None,
+               'feerates': (7500, 7500, 7500, 7500)}
+    l1, l2 = node_factory.line_graph(2, opts=[l1_opts, l2_opts])
+
+    # Pay 600 000 sat l1 → l2: afterwards l1 ≈ 400 000 sat, l2 ≈ 600 000 sat.
+    l1.pay(l2, 600_000_000)
+    wait_for(lambda: only_one(l2.rpc.listpeerchannels()['channels'])['htlcs'] == [])
+
+    # l1's close RPC returns as soon as the mutual-close tx is stored, even
+    # though the broadcast itself is delayed 1 hour.  Run it in a thread so
+    # the test can proceed without blocking.
+    fut = executor.submit(l1.rpc.close, l2.info['id'])
+
+    # l1 as closer: closer_amount < remote_sat (l2 has more)
+    # AND sent_fee (≈3750*weight/1000) < their_fee (≈7500*weight/1000) → delay.
+    l1.daemon.wait_for_log('Simple close: delaying broadcast by 1 hour')
+
+    # l2 as closee: no delay expected.
+    wait_for(lambda: only_one(l2.rpc.listpeerchannels()['channels'])['state']
+             == 'CLOSINGD_COMPLETE')
+    assert not l2.daemon.is_in_log('Simple close: delaying broadcast')
+
+    # l2 broadcasts immediately; wait until its tx is confirmed.
+    # We wait for the broadcast log rather than polling getrawmempool(),
+    # which can miss a just-submitted tx under the rpcproxy timing.
+    l2.daemon.wait_for_log('Broadcasting txid')
+    bitcoind.generate_block(1)
+    l1.daemon.wait_for_log('Resolved FUNDING_TRANSACTION/FUNDING_OUTPUT by MUTUAL_CLOSE')
+    l2.daemon.wait_for_log('Resolved FUNDING_TRANSACTION/FUNDING_OUTPUT by MUTUAL_CLOSE')
+    fut.result(timeout=10)
+
+
 def test_simple_close_no_feature_fallback(node_factory, bitcoind, chainparams):
     """Without option_simple_close the nodes must fall back to legacy closingd
     (iterative closing_signed fee negotiation) and produce a single mutually-