Store shared output index in ConstructedTransaction

Currently, only the shared input index is stored in
ConstructedTransaction. This will be used later to filter out the shared
input when constructing an error during interactive tx negotiation.
Store the shared output index as well so that the shared output can be
filtered out as well.

Author: jkczyz

lightning/src/ln/interactivetxs.rs

@@ -200,6 +200,7 @@ pub(crate) struct ConstructedTransaction {
 	output_metadata: Vec<TxOutMetadata>,
 	tx: Transaction,
 	shared_input_index: Option<u32>,
+	shared_output_index: u16,
 }
 
 #[derive(Clone, Debug, Eq, PartialEq)]
@@ -244,6 +245,7 @@ impl_writeable_tlv_based!(ConstructedTransaction, {
 	(5, output_metadata, required),
 	(7, tx, required),
 	(9, shared_input_index, option),
+	(11, shared_output_index, required),
 });
 
 impl ConstructedTransaction {
@@ -280,12 +282,19 @@ impl ConstructedTransaction {
 					.map(|position| position as u32)
 			});
 
+		let shared_output_index = output
+			.iter()
+			.position(|txout| *txout == context.shared_funding_output.tx_out)
+			.map(|position| position as u16)
+			.unwrap_or(u16::MAX);
+
 		let tx = ConstructedTransaction {
 			holder_is_initiator: context.holder_is_initiator,
 			input_metadata,
 			output_metadata,
 			tx: Transaction { version: Version::TWO, lock_time, input, output },
 			shared_input_index,
+			shared_output_index,
 		};
 
 		// The receiving node:
@@ -315,7 +324,7 @@ impl ConstructedTransaction {
 			return Err(AbortReason::MissingFundingInput);
 		}
 
-		if !tx.tx.output.iter().any(|txout| *txout == context.shared_funding_output.tx_out) {
+		if tx.shared_output_index == u16::MAX {
 			return Err(AbortReason::MissingFundingOutput);
 		}
 
@@ -3329,6 +3338,7 @@ mod tests {
 			output_metadata: vec![], // N/A for test
 			tx: transaction.clone(),
 			shared_input_index: None,
+			shared_output_index: 0,
 		};
 
 		let secp_ctx = Secp256k1::new();