Return optional Transaction from register_output

Electrum clients primarily operate by subscribing to notifications of
transactions by script pubkeys. Therefore, they will send filtered
transaction data without including dependent transactions. Outputs for
such transactions must be explicitly registered with these clients.

Therefore, upon block_connected, provide a mechanism for an Electrum-
backed chain::Filter to return new transaction data to scan.

Author: jkczyz

lightning/src/chain/mod.rs

@@ -11,7 +11,7 @@
 
 use bitcoin::blockdata::block::{Block, BlockHeader};
 use bitcoin::blockdata::script::Script;
-use bitcoin::blockdata::transaction::TxOut;
+use bitcoin::blockdata::transaction::{Transaction, TxOut};
 use bitcoin::hash_types::{BlockHash, Txid};
 
 use chain::channelmonitor::{ChannelMonitor, ChannelMonitorUpdate, ChannelMonitorUpdateErr, MonitorEvent};
@@ -131,7 +131,10 @@ pub trait Filter: Send + Sync {
 
 	/// Registers interest in spends of a transaction output identified by `outpoint` having
 	/// `script_pubkey` as the spending condition.
-	fn register_output(&self, outpoint: &OutPoint, script_pubkey: &Script);
+	///
+	/// Optionally, returns any transaction dependent on the output. This is useful for Electrum
+	/// clients to facilitate registering in-block descendants.
+	fn register_output(&self, outpoint: &OutPoint, script_pubkey: &Script) -> Option<(usize, Transaction)>;
 }
 
 impl<T: Listen> Listen for std::ops::Deref<Target = T> {

lightning/src/util/test_utils.rs

@@ -546,7 +546,8 @@ impl chain::Filter for TestChainSource {
 		self.watched_txn.lock().unwrap().insert((*txid, script_pubkey.clone()));
 	}
 
-	fn register_output(&self, outpoint: &OutPoint, script_pubkey: &Script) {
+	fn register_output(&self, outpoint: &OutPoint, script_pubkey: &Script) -> Option<(usize, Transaction)> {
 		self.watched_outputs.lock().unwrap().insert((*outpoint, script_pubkey.clone()));
+		None
 	}
 }