Remove input value fields from ConstructedTransaction

The local and remote input values are used to determine which node sends
tx_signatures first. Instead of persisting these values, compute them
only when needed from the input metadata. The spec states that the
entire shared input value is to be included for the node sending the
corresponding tx_add_input, so it isn't necessary to know the local and
remote balances which the metadata does not contain.

Author: jkczyz

lightning/src/ln/interactivetxs.rs

@@ -197,14 +197,9 @@ impl Display for AbortReason {
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub(crate) struct ConstructedTransaction {
 	holder_is_initiator: bool,
-
 	input_metadata: Vec<TxInMetadata>,
 	output_metadata: Vec<TxOutMetadata>,
 	tx: Transaction,
-
-	local_inputs_value_satoshis: u64,
-	remote_inputs_value_satoshis: u64,
-
 	shared_input_index: Option<u32>,
 }
 
@@ -249,9 +244,7 @@ impl_writeable_tlv_based!(ConstructedTransaction, {
 	(3, input_metadata, required),
 	(5, output_metadata, required),
 	(7, tx, required),
-	(9, local_inputs_value_satoshis, required),
-	(11, remote_inputs_value_satoshis, required),
-	(13, shared_input_index, option),
+	(9, shared_input_index, option),
 });
 
 impl ConstructedTransaction {
@@ -271,12 +264,6 @@ impl ConstructedTransaction {
 			return Err(AbortReason::MissingFundingOutput);
 		}
 
-		let local_inputs_value_satoshis = context
-			.inputs
-			.iter()
-			.fold(0u64, |value, (_, input)| value.saturating_add(input.local_value()));
-		let remote_inputs_value_satoshis = context.remote_inputs_value();
-
 		let satisfaction_weight =
 			Weight::from_wu(context.inputs.iter().fold(0u64, |value, (_, input)| {
 				value.saturating_add(input.satisfaction_weight().to_wu())
@@ -313,14 +300,9 @@ impl ConstructedTransaction {
 
 		Ok(Self {
 			holder_is_initiator: context.holder_is_initiator,
-
 			input_metadata,
 			output_metadata,
 			tx,
-
-			local_inputs_value_satoshis,
-			remote_inputs_value_satoshis,
-
 			shared_input_index,
 		})
 	}
@@ -337,6 +319,26 @@ impl ConstructedTransaction {
 		self.tx().compute_txid()
 	}
 
+	/// Returns the total input value from all local contributions, including the entire shared
+	/// input value if applicable.
+	fn local_contributed_input_value(&self) -> Amount {
+		self.input_metadata
+			.iter()
+			.filter(|input| input.is_local(self.holder_is_initiator))
+			.map(|input| input.prev_output.value)
+			.sum()
+	}
+
+	/// Returns the total input value from all remote contributions, including the entire shared
+	/// input value if applicable.
+	fn remote_contributed_input_value(&self) -> Amount {
+		self.input_metadata
+			.iter()
+			.filter(|input| !input.is_local(self.holder_is_initiator))
+			.map(|input| input.prev_output.value)
+			.sum()
+	}
+
 	/// Adds provided holder witnesses to holder inputs of unsigned transaction.
 	///
 	/// Note that it is assumed that the witness count equals the holder input count.
@@ -1379,11 +1381,13 @@ macro_rules! define_state_transitions {
 				let tx = context.validate_tx()?;
 
 				// Strict ordering prevents deadlocks during tx_signatures exchange
+				let local_contributed_input_value = tx.local_contributed_input_value();
+				let remote_contributed_input_value = tx.remote_contributed_input_value();
 				let holder_sends_tx_signatures_first =
-					if tx.local_inputs_value_satoshis == tx.remote_inputs_value_satoshis {
+					if local_contributed_input_value == remote_contributed_input_value {
 						holder_node_id.serialize() < counterparty_node_id.serialize()
 					} else {
-						tx.local_inputs_value_satoshis < tx.remote_inputs_value_satoshis
+						local_contributed_input_value < remote_contributed_input_value
 					};
 
 				let signing_session = InteractiveTxSigningSession {
@@ -3306,8 +3310,6 @@ mod tests {
 			input_metadata,
 			output_metadata: vec![], // N/A for test
 			tx: transaction.clone(),
-			local_inputs_value_satoshis: 0,  // N/A for test
-			remote_inputs_value_satoshis: 0, // N/A for test
 			shared_input_index: None,
 		};