Add a note that the anchor transaction in 0FC channels has a max size of 1000vB

Author: tankyleo

lightning/src/events/bump_transaction/mod.rs

@@ -364,6 +364,8 @@ pub trait CoinSelectionSource {
 	/// other claims, implementations must be willing to double spend their UTXOs. The choice of
 	/// which UTXOs to double spend is left to the implementation, but it must strive to keep the
 	/// set of other claims being double spent to a minimum.
+	///
+	/// Note that in 0FC channels, the anchor transaction has a max size of 1000vB.
 	fn select_confirmed_utxos<'a>(
 		&'a self, claim_id: ClaimId, must_spend: Vec<Input>, must_pay_to: &'a [TxOut],
 		target_feerate_sat_per_1000_weight: u32,
@@ -833,6 +835,12 @@ where
 				assert!(package_fee >= expected_package_fee);
 			}
 
+			#[cfg(debug_assertions)]
+			if channel_type.supports_anchor_zero_fee_commitments() {
+				assert!(commitment_tx.weight().to_wu() < chan_utils::MAX_TRUC_WEIGHT);
+				assert!(anchor_tx.weight().to_wu() < chan_utils::UNCONF_ANCESTOR_MAX_TRUC_WEIGHT);
+			}
+
 			log_info!(
 				self.logger,
 				"Broadcasting anchor transaction {} to bump channel close with txid {}",

lightning/src/ln/chan_utils.rs

@@ -96,9 +96,12 @@ pub const P2A_ANCHOR_INPUT_WITNESS_WEIGHT: u64 = 1;
 /// The maximum value of a P2A anchor.
 pub const P2A_MAX_VALUE: u64 = 240;
 
-/// The maximum weight of a TRUC transaction, see BIP431
+/// The maximum weight of a TRUC transaction, see BIP431.
 pub const MAX_TRUC_WEIGHT: u64 = 10_000 * WITNESS_SCALE_FACTOR as u64;
 
+/// The maximum weight of a TRUC transaction with an unconfirmed TRUC ancestor, see BIP431.
+pub const UNCONF_ANCESTOR_MAX_TRUC_WEIGHT: u64 = 1000 * WITNESS_SCALE_FACTOR as u64;
+
 /// The maximum weight of any standard transaction, see bitcoin/src/policy/policy.h
 pub const MAX_STANDARD_TX_WEIGHT: u64 = 400_000;