Signer extended with method to sign prev funding transaction input

lightning/src/sign/ecdsa.rs

@@ -1,6 +1,6 @@
 //! Defines ECDSA-specific signer types.
 
-use bitcoin::transaction::Transaction;
+use bitcoin::{Script, Transaction};
 
 use bitcoin::secp256k1;
 use bitcoin::secp256k1::ecdsa::Signature;
@@ -209,4 +209,12 @@ pub trait EcdsaChannelSigner: ChannelSigner {
 	fn sign_channel_announcement_with_funding_key(
 		&self, msg: &UnsignedChannelAnnouncement, secp_ctx: &Secp256k1<secp256k1::All>,
 	) -> Result<Signature, ()>;
+	/// Sign an input of a transaction with our funding key.
+	/// Used for splicing, when signing the previous funding transaction.
+	/// The previous funding transaction becomes an input to the new funding transaction,
+	/// and it is a multisig, which we also need to sign.
+	fn sign_transaction_input(
+		&self, tx: &Transaction, input_index: u16, input_value: u64, input_redeem_script: &Script,
+		secp_ctx: &Secp256k1<secp256k1::All>,
+	) -> Result<Signature, ()>;
 }

lightning/src/sign/mod.rs

@@ -1695,6 +1695,22 @@ impl EcdsaChannelSigner for InMemorySigner {
 		let msghash = hash_to_message!(&Sha256dHash::hash(&msg.encode()[..])[..]);
 		Ok(secp_ctx.sign_ecdsa(&msghash, &self.funding_key))
 	}
+
+	fn sign_transaction_input(
+		&self, tx: &Transaction, input_index: u16, input_value: u64, input_redeem_script: &Script,
+		secp_ctx: &Secp256k1<secp256k1::All>,
+	) -> Result<Signature, ()> {
+		let sighash = &sighash::SighashCache::new(tx)
+			.p2wsh_signature_hash(
+				input_index as usize,
+				&input_redeem_script,
+				Amount::from_sat(input_value),
+				EcdsaSighashType::All,
+			)
+			.unwrap()[..];
+		let msg = hash_to_message!(sighash);
+		Ok(sign(secp_ctx, &msg, &self.funding_key))
+	}
 }
 
 #[cfg(taproot)]

lightning/src/util/test_channel_signer.rs

@@ -22,7 +22,7 @@ use core::cmp;
 use crate::sync::{Mutex, Arc};
 #[cfg(test)] use crate::sync::MutexGuard;
 
-use bitcoin::transaction::Transaction;
+use bitcoin::{Script, Transaction};
 use bitcoin::hashes::Hash;
 use bitcoin::sighash;
 use bitcoin::sighash::EcdsaSighashType;
@@ -353,6 +353,18 @@ impl EcdsaChannelSigner for TestChannelSigner {
 	) -> Result<Signature, ()> {
 		self.inner.sign_channel_announcement_with_funding_key(msg, secp_ctx)
 	}
+
+	fn sign_transaction_input(
+		&self, tx: &Transaction, input_index: u16, input_value: u64, input_redeem_script: &Script, secp_ctx: &Secp256k1<secp256k1::All>
+	) -> Result<Signature, ()> {
+		self.inner.sign_transaction_input(
+			tx,
+			input_index,
+			input_value,
+			input_redeem_script,
+			secp_ctx,
+		)
+	}
 }
 
 #[cfg(taproot)]