Expose UTXO spent for anchor and HTLC input descriptors

This may be required by some wallets that rely on PSBTs internally to
create/sign transactions.

Author: wpaulino

lightning/src/events/bump_transaction.rs

@@ -16,6 +16,7 @@ use core::ops::Deref;
 use crate::chain::chaininterface::BroadcasterInterface;
 use crate::chain::ClaimId;
 use crate::io_extras::sink;
+use crate::ln::channel::ANCHOR_OUTPUT_VALUE_SATOSHI;
 use crate::ln::chan_utils;
 use crate::ln::chan_utils::{
 	ANCHOR_INPUT_WITNESS_WEIGHT, HTLC_SUCCESS_INPUT_ANCHOR_WITNESS_WEIGHT,
@@ -74,6 +75,15 @@ pub struct AnchorDescriptor {
 }
 
 impl AnchorDescriptor {
+	/// Returns the UTXO to be spent by the anchor input, which can be obtained via
+	/// [`Self::unsigned_tx_input`].
+	pub fn spending_utxo(&self) -> TxOut {
+		TxOut {
+			script_pubkey: self.witness_script().to_v0_p2wsh(),
+			value: ANCHOR_OUTPUT_VALUE_SATOSHI,
+		}
+	}
+
 	/// Returns the unsigned transaction input spending the anchor output in the commitment
 	/// transaction.
 	pub fn unsigned_tx_input(&self) -> TxIn {
@@ -134,6 +144,15 @@ pub struct HTLCDescriptor {
 }
 
 impl HTLCDescriptor {
+	/// Returns the UTXO to be spent by the HTLC input, which can be obtained via
+	/// [`Self::unsigned_tx_input`].
+	pub fn spending_utxo<C: secp256k1::Signing + secp256k1::Verification>(&self, secp: &Secp256k1<C>) -> TxOut {
+		TxOut {
+			script_pubkey: self.witness_script(secp).to_v0_p2wsh(),
+			value: self.htlc.amount_msat / 1000,
+		}
+	}
+
 	/// Returns the unsigned transaction input spending the HTLC output in the commitment
 	/// transaction.
 	pub fn unsigned_tx_input(&self) -> TxIn {