wip: witness weight estimates

Author: dunxen

lightning/src/ln/interactivetxs.rs

@@ -23,7 +23,7 @@ use crate::events::bump_transaction::{BASE_INPUT_WEIGHT, EMPTY_SCRIPT_SIG_WEIGHT
 use crate::ln::channel::TOTAL_BITCOIN_SUPPLY_SATOSHIS;
 use crate::ln::msgs::SerialId;
 use crate::ln::{msgs, ChannelId};
-use crate::sign::EntropySource;
+use crate::sign::{EntropySource, P2WPKH_WITNESS_WEIGHT};
 use crate::util::ser::TransactionU16LenLimited;
 
 /// The number of received `tx_add_input` messages during a negotiation at which point the
@@ -364,9 +364,6 @@ impl NegotiationContext {
 			return Err(AbortReason::ExceededNumberOfInputsOrOutputs);
 		}
 
-		// TODO: How do we enforce their fees cover the witness without knowing its expected length?
-		const INPUT_WEIGHT: u64 = BASE_INPUT_WEIGHT + EMPTY_SCRIPT_SIG_WEIGHT;
-
 		// - the peer's paid feerate does not meet or exceed the agreed feerate (based on the minimum fee).
 		let mut counterparty_weight_contributed: u64 = self
 			.counterparty_outputs_contributed()
@@ -375,8 +372,32 @@ impl NegotiationContext {
 					* WITNESS_SCALE_FACTOR as u64
 			})
 			.sum();
-		counterparty_weight_contributed +=
-			self.counterparty_inputs_contributed().count() as u64 * INPUT_WEIGHT;
+		// We don't know the counterparty's witnesses ahead of time obviously, so we need to have some
+		// estimates for their weights based on script type. We make some reasonably assumptions here.
+		let mut total_inputs_weight: u64 = 0;
+		for TxInputWithPrevOutput { prev_output, .. } in self.counterparty_inputs_contributed() {
+			total_inputs_weight = total_inputs_weight
+				.saturating_add(BASE_INPUT_WEIGHT)
+				.saturating_add(if prev_output.script_pubkey.is_v0_p2wpkh() {
+					// The weight for a P2WPKH spend is fixed.
+					EMPTY_SCRIPT_SIG_WEIGHT + P2WPKH_WITNESS_WEIGHT
+				} else if prev_output.script_pubkey.is_v0_p2wsh() {
+					// We cover a 2-of-2 multisig spend case as a minimum estimated weight. We assume signature
+					// grinding (71 byte signatures) here as we do add fee margin of error anyway.
+					EMPTY_SCRIPT_SIG_WEIGHT + 1 /* witness items */ + 1 /* multisig dummy element */ + 1 /* 1st sig len */
+						+ 71 /* 1st sig */ + 1 /* 2nd sig len */ + 71 /* 2nd sig */ + 1 /* witness script len */
+						+ 1 /* signature count */ + 1 /* 1st pubkey len */ + 33 /* 1st pubkey */ + 1 /* 2nd pubkey len */
+						+ 33 /* 2nd pubkey */ + 1 /* pubkey count */ + 1 /* OP_CHECKMULTISIG */
+				} else if prev_output.script_pubkey.is_v1_p2tr() {
+					// We cover the key path spend case as a minimum estimated weight.
+					EMPTY_SCRIPT_SIG_WEIGHT + 1 /* witness items */ + 1 /* schnorr sig len */ + 64 /* schnorr sig */
+				} else {
+					// The case for witness programs with versions greater than V1. At this stage they're anyone-can-spend.
+					EMPTY_SCRIPT_SIG_WEIGHT
+				});
+		}
+		counterparty_weight_contributed =
+			counterparty_weight_contributed.saturating_add(total_inputs_weight);
 		let counterparty_fees_contributed =
 			counterparty_inputs_value.saturating_sub(counterparty_outputs_value);
 		let mut required_counterparty_contribution_fee =