Allow a single P2A output to be below dust in check_spends!

Author: tankyleo

lightning/src/ln/functional_test_utils.rs

@@ -1941,23 +1941,41 @@ pub fn update_nodes_with_chan_announce<'a, 'b, 'c, 'd>(
 pub fn do_check_spends<F: Fn(&bitcoin::transaction::OutPoint) -> Option<TxOut>>(
 	tx: &Transaction, get_output: F,
 ) {
+	let mut p2a_output_below_dust = false;
+	let mut has_p2a_output = false;
 	for outp in tx.output.iter() {
-		assert!(
-			outp.value >= outp.script_pubkey.minimal_non_dust(),
-			"Spending tx output didn't meet dust limit"
-		);
+		let is_p2a = outp.script_pubkey == crate::ln::chan_utils::shared_anchor_script_pubkey();
+		has_p2a_output |= is_p2a;
+		if outp.value < outp.script_pubkey.minimal_non_dust() {
+			if p2a_output_below_dust || !is_p2a {
+				panic!("Spending tx output didn't meet dust limit");
+			}
+			p2a_output_below_dust = true;
+		};
 	}
 	let mut total_value_in = 0;
 	for input in tx.input.iter() {
-		total_value_in += get_output(&input.previous_output).unwrap().value.to_sat();
+		let output = get_output(&input.previous_output).unwrap();
+		if output.script_pubkey == crate::ln::chan_utils::shared_anchor_script_pubkey() {
+			assert!(input.witness.is_empty());
+		}
+		total_value_in += output.value.to_sat();
 	}
 	let mut total_value_out = 0;
 	for output in tx.output.iter() {
 		total_value_out += output.value.to_sat();
 	}
-	let min_fee = (tx.weight().to_wu() as u64 + 3) / 4; // One sat per vbyte (ie per weight/4, rounded up)
-													// Input amount - output amount = fee, so check that out + min_fee is smaller than input
-	assert!(total_value_out + min_fee <= total_value_in);
+	if p2a_output_below_dust {
+		assert_eq!(
+			total_value_in, total_value_out,
+			"Spending tx has one output below dust, while not zero fee"
+		);
+	// 0FC commitment transactions will have their fee bumped by a child, so don't require that
+	// they meet the 1sat/vB minimum.
+	} else if !has_p2a_output {
+		let min_fee = (tx.weight().to_wu() as u64 + 3) / 4; // One sat per vbyte (ie per weight/4, rounded up)
+		assert!(total_value_out + min_fee <= total_value_in);
+	}
 	tx.verify(get_output).unwrap();
 }
 
@@ -1966,8 +1984,15 @@ macro_rules! check_spends {
 	($tx: expr, $($spends_txn: expr),*) => {
 		{
 			$(
+			let mut single_output_below_dust = false;
 			for outp in $spends_txn.output.iter() {
-				assert!(outp.value >= outp.script_pubkey.minimal_non_dust(), "Input tx output didn't meet dust limit");
+				if outp.value < outp.script_pubkey.minimal_non_dust() {
+					if single_output_below_dust || outp.script_pubkey != $crate::ln::chan_utils::shared_anchor_script_pubkey() {
+						panic!("Input tx output didn't meet dust limit");
+					} else {
+						single_output_below_dust = true;
+					}
+				}
 			}
 			)*
 			let get_output = |out_point: &bitcoin::transaction::OutPoint| {