[Splicing] Signer extended with method to sign prev funding transaction input

lightning/src/sign/ecdsa.rs

@@ -209,4 +209,18 @@ pub trait EcdsaChannelSigner: ChannelSigner {
 	fn sign_channel_announcement_with_funding_key(
 		&self, msg: &UnsignedChannelAnnouncement, secp_ctx: &Secp256k1<secp256k1::All>,
 	) -> Result<Signature, ()>;
+
+	/// Signs the input of a splicing funding transaction with our funding key.
+	///
+	/// In splicing, the previous funding transaction output is spent as the input of
+	/// the new funding transaction, and is a 2-of-2 multisig.
+	///
+	/// `input_index`: The index of the input within the new funding transaction `tx`,
+	///    spending the previous funding transaction's output
+	///
+	/// `input_value`: The value of the previous funding transaction output.
+	fn sign_splicing_funding_input(
+		&self, tx: &Transaction, input_index: usize, input_value: u64,
+		secp_ctx: &Secp256k1<secp256k1::All>,
+	) -> Result<Signature, ()>;
 }

lightning/src/sign/mod.rs

@@ -1695,6 +1695,27 @@ impl EcdsaChannelSigner for InMemorySigner {
 		let msghash = hash_to_message!(&Sha256dHash::hash(&msg.encode()[..])[..]);
 		Ok(secp_ctx.sign_ecdsa(&msghash, &self.funding_key))
 	}
+
+	fn sign_splicing_funding_input(
+		&self, tx: &Transaction, input_index: usize, input_value: u64,
+		secp_ctx: &Secp256k1<secp256k1::All>,
+	) -> Result<Signature, ()> {
+		let funding_pubkey = PublicKey::from_secret_key(secp_ctx, &self.funding_key);
+		let counterparty_funding_key =
+			&self.counterparty_pubkeys().expect(MISSING_PARAMS_ERR).funding_pubkey;
+		let funding_redeemscript =
+			make_funding_redeemscript(&funding_pubkey, counterparty_funding_key);
+		let sighash = &sighash::SighashCache::new(tx)
+			.p2wsh_signature_hash(
+				input_index,
+				&funding_redeemscript,
+				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

@@ -353,6 +353,13 @@ impl EcdsaChannelSigner for TestChannelSigner {
 	) -> Result<Signature, ()> {
 		self.inner.sign_channel_announcement_with_funding_key(msg, secp_ctx)
 	}
+
+	fn sign_splicing_funding_input(
+		&self, tx: &Transaction, input_index: usize, input_value: u64,
+		secp_ctx: &Secp256k1<secp256k1::All>,
+	) -> Result<Signature, ()> {
+		self.inner.sign_splicing_funding_input(tx, input_index, input_value, secp_ctx)
+	}
 }
 
 #[cfg(taproot)]