Add a method to fetch all possible remote-closure script_pubkeys

In the previous commit we (finally) allowed users to opt into a
static `remote_key` derivation scheme, enabling them to scan the
chain for funds on counterparty commitment transactions without any
state at all.

This is only possible, however, of course, if they have the full
list of scripts to scan the chain for, which we expose here.

Author: TheBlueMatt

lightning/src/sign/mod.rs

@@ -2072,6 +2072,35 @@ impl KeysManager {
 		self.node_secret
 	}
 
+	/// Gets the set of possible `script_pubkey`s which can appear on chain for our
+	/// non-HTLC-encumbered balance if our counterparty force-closes the channel.
+	///
+	/// Only channels opened or spliced when using a [`KeysManager`] with the
+	/// `v2_remote_key_derivation` argument to [`KeysManager::new`] will close to such scripts,
+	/// other channels will close to a randomly-generated `script_pubkey`.
+	pub fn possible_v2_counterparty_closed_balance_spks<C: Signing>(
+		&self, secp_ctx: &Secp256k1<C>,
+	) -> Vec<ScriptBuf> {
+		let mut res = Vec::with_capacity(usize::from(STATIC_PAYMENT_KEY_COUNT) * 2);
+		let static_remote_key_features = ChannelTypeFeatures::only_static_remote_key();
+		let mut zero_fee_htlc_features = ChannelTypeFeatures::only_static_remote_key();
+		zero_fee_htlc_features.set_anchors_zero_fee_htlc_tx_required();
+		for idx in 0..STATIC_PAYMENT_KEY_COUNT {
+			let key = self
+				.static_payment_key
+				.derive_priv(
+					&self.secp_ctx,
+					&ChildNumber::from_hardened_idx(u32::from(idx)).expect("key space exhausted"),
+				)
+				.expect("Your RNG is busted")
+				.private_key;
+			let pubkey = PublicKey::from_secret_key(secp_ctx, &key);
+			res.push(get_counterparty_payment_script(&static_remote_key_features, &pubkey));
+			res.push(get_counterparty_payment_script(&zero_fee_htlc_features, &pubkey));
+		}
+		res
+	}
+
 	fn derive_payment_key_v2(&self, params: &[u8; 32]) -> SecretKey {
 		let mut eight_bytes = [0; 8];
 		eight_bytes.copy_from_slice(&params[0..8]);
@@ -2173,6 +2202,15 @@ impl KeysManager {
 						let signer = self.derive_channel_keys(&descriptor.channel_keys_id);
 						keys_cache = Some((signer, descriptor.channel_keys_id));
 					}
+					#[cfg(test)]
+					if self.v2_remote_key_derivation {
+						// In tests, we don't have to deal with upgrades from V1 sighers with
+						// `v2_remote_key_derivation` set, so use this opportunity to test
+						// `possible_v2_counterparty_closed_balance_spks`.
+						let possible_spks =
+							self.possible_v2_counterparty_closed_balance_spks(secp_ctx);
+						assert!(possible_spks.contains(&descriptor.output.script_pubkey));
+					}
 					let witness = keys_cache.as_ref().unwrap().0.sign_counterparty_payment_input(
 						&psbt.unsigned_tx,
 						input_idx,