Refactor shared output support (interactivetxs)

This simplifies tracking separately the expected and actual shared output.
In the initiator case, we can just provide the shared output separately,
instead of including it within other outputs, and marking which one is the output.
We can use the same field for the intended shared output in the initiator
case, and the expected one in the acceptor case.

Author: optout21

lightning/src/ln/channel.rs

@@ -57,7 +57,7 @@ use crate::ln::channelmanager::{
 use crate::ln::interactivetxs::{
 	calculate_change_output_value, get_output_weight, AbortReason, HandleTxCompleteResult,
 	InteractiveTxConstructor, InteractiveTxConstructorArgs, InteractiveTxMessageSend,
-	InteractiveTxMessageSendResult, InteractiveTxSigningSession, OutputOwned, SharedOwnedOutput,
+	InteractiveTxMessageSendResult, InteractiveTxSigningSession, SharedOwnedOutput,
 	TX_COMMON_FIELDS_WEIGHT,
 };
 use crate::ln::msgs;
@@ -2756,24 +2756,12 @@ where
 		// Note: For the error case when the inputs are insufficient, it will be handled after
 		// the `calculate_change_output_value` call below
 		let mut funding_outputs = Vec::new();
-		let mut expected_remote_shared_funding_output = None;
 
 		let shared_funding_output = TxOut {
 			value: Amount::from_sat(self.funding.get_value_satoshis()),
 			script_pubkey: self.funding.get_funding_redeemscript().to_p2wsh(),
 		};
 
-		if self.funding.is_outbound() {
-			funding_outputs.push(
-				OutputOwned::Shared(SharedOwnedOutput::new(
-					shared_funding_output, self.dual_funding_context.our_funding_satoshis,
-				))
-			);
-		} else {
-			let TxOut { value, script_pubkey } = shared_funding_output;
-			expected_remote_shared_funding_output = Some((script_pubkey, value.to_sat()));
-		}
-
 		// Optionally add change output
 		let change_script = if let Some(script) = change_destination_opt {
 			script
@@ -2783,7 +2771,7 @@ where
 		};
 		let change_value_opt = calculate_change_output_value(
 			self.funding.is_outbound(), self.dual_funding_context.our_funding_satoshis,
-			&funding_inputs, &funding_outputs,
+			&shared_funding_output.script_pubkey, &funding_inputs, &funding_outputs,
 			self.dual_funding_context.funding_feerate_sat_per_1000_weight,
 			change_script.minimal_non_dust().to_sat(),
 		)?;
@@ -2798,7 +2786,7 @@ where
 			// Check dust limit again
 			if change_value_decreased_with_fee > self.context.holder_dust_limit_satoshis {
 				change_output.value = Amount::from_sat(change_value_decreased_with_fee);
-				funding_outputs.push(OutputOwned::Single(change_output));
+				funding_outputs.push(change_output);
 			}
 		}
 
@@ -2811,8 +2799,8 @@ where
 			is_initiator: self.funding.is_outbound(),
 			funding_tx_locktime: self.dual_funding_context.funding_tx_locktime,
 			inputs_to_contribute: funding_inputs,
+			shared_funding_output: SharedOwnedOutput::new(shared_funding_output, self.dual_funding_context.our_funding_satoshis),
 			outputs_to_contribute: funding_outputs,
-			expected_remote_shared_funding_output,
 		};
 		let mut tx_constructor = InteractiveTxConstructor::new(constructor_args)?;
 		let msg = tx_constructor.take_initiator_first_message();
@@ -11809,6 +11797,10 @@ where
 			funding_feerate_sat_per_1000_weight: msg.funding_feerate_sat_per_1000_weight,
 			our_funding_inputs: our_funding_inputs.clone(),
 		};
+		let shared_funding_output = TxOut {
+			value: Amount::from_sat(funding.get_value_satoshis()),
+			script_pubkey: funding.get_funding_redeemscript().to_p2wsh(),
+		};
 
 		let interactive_tx_constructor = Some(InteractiveTxConstructor::new(
 			InteractiveTxConstructorArgs {
@@ -11820,8 +11812,8 @@ where
 				funding_tx_locktime: dual_funding_context.funding_tx_locktime,
 				is_initiator: false,
 				inputs_to_contribute: our_funding_inputs,
+				shared_funding_output: SharedOwnedOutput::new(shared_funding_output, our_funding_satoshis),
 				outputs_to_contribute: Vec::new(),
-				expected_remote_shared_funding_output: Some((funding.get_funding_redeemscript().to_p2wsh(), funding.get_value_satoshis())),
 			}
 		).map_err(|_| ChannelError::Close((
 			"V2 channel rejected due to sender error".into(),