Extend begin_interactive_funding_tx_construction() with splicing-specific extra input

Author: optout21

lightning/src/ln/channel.rs

@@ -1684,12 +1684,17 @@ impl<SP: Deref> PendingV2Channel<SP> where SP::Target: SignerProvider {
 	#[allow(dead_code)] // TODO(dual_funding): Remove once contribution to V2 channels is enabled
 	fn begin_interactive_funding_tx_construction<ES: Deref>(
 		&mut self, signer_provider: &SP, entropy_source: &ES, holder_node_id: PublicKey,
+		prev_funding_input: Option<(TxIn, TransactionU16LenLimited)>,
 	) -> Result<Option<InteractiveTxMessageSend>, APIError>
 	where ES::Target: EntropySource
 	{
 		let mut funding_inputs = Vec::new();
 		mem::swap(&mut self.dual_funding_context.our_funding_inputs, &mut funding_inputs);
 
+		if let Some(prev_funding_input) = prev_funding_input {
+			funding_inputs.push(prev_funding_input);
+		}
+
 		let funding_inputs_prev_outputs = DualFundingChannelContext::txouts_from_input_prev_txs(&funding_inputs)
 			.map_err(|err| APIError::APIMisuseError { err: err.to_string() })?;
 

lightning/src/ln/interactivetxs.rs

@@ -2728,4 +2728,58 @@ mod tests {
 			assert_eq!(res.unwrap(), 154);
 		}
 	}
+
+	#[test]
+	fn test_calculate_change_output_value_splice() {
+		let input_prevouts_owned = vec![
+			TxOut { value: Amount::from_sat(70_000), script_pubkey: ScriptBuf::new() },
+			TxOut { value: Amount::from_sat(60_000), script_pubkey: ScriptBuf::new() },
+		];
+		let input_prevouts: Vec<&TxOut> = input_prevouts_owned.iter().collect();
+		let our_contributed = 110_000;
+		let txout = TxOut { value: Amount::from_sat(148_000), script_pubkey: ScriptBuf::new() };
+		let outputs = vec![OutputOwned::Shared(SharedOwnedOutput::new(txout, our_contributed))];
+		let funding_feerate_sat_per_1000_weight = 3000;
+
+		let total_inputs: u64 = input_prevouts.iter().map(|o| o.value.to_sat()).sum();
+		let gross_change = total_inputs - our_contributed;
+		let fees = 1746;
+		let common_fees = 126;
+		{
+			// There is leftover for change
+			let res = calculate_change_output_value(
+				true,
+				our_contributed,
+				&input_prevouts,
+				&outputs,
+				funding_feerate_sat_per_1000_weight,
+				300,
+			);
+			assert_eq!(res.unwrap(), gross_change - fees - common_fees);
+		}
+		{
+			// Very small leftover
+			let res = calculate_change_output_value(
+				false,
+				128_100,
+				&input_prevouts,
+				&outputs,
+				funding_feerate_sat_per_1000_weight,
+				300,
+			);
+			assert!(res.is_none());
+		}
+		{
+			// Small leftover, but not dust
+			let res = calculate_change_output_value(
+				false,
+				128_100,
+				&input_prevouts,
+				&outputs,
+				funding_feerate_sat_per_1000_weight,
+				100,
+			);
+			assert_eq!(res.unwrap(), 154);
+		}
+	}
 }