Return TransactionData 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

@@ -16,7 +16,7 @@ use bitcoin::hash_types::{BlockHash, Txid};
 
 use chain::channelmonitor::{ChannelMonitor, ChannelMonitorUpdate, ChannelMonitorUpdateErr, MonitorEvent};
 use chain::keysinterface::Sign;
-use chain::transaction::OutPoint;
+use chain::transaction::{OutPoint, TransactionData};
 
 pub mod chaininterface;
 pub mod chainmonitor;
@@ -134,7 +134,10 @@ 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 transactions dependent on the output. This is useful for Electrum
+	/// clients to facilitate registering any in-block descendants.
+	fn register_output(&self, outpoint: &OutPoint, script_pubkey: &Script) -> Option<TransactionData>;
 }
 
 impl<T: Listen> Listen for std::ops::Deref<Target = T> {

lightning/src/chain/transaction.rs

@@ -13,6 +13,37 @@ use bitcoin::hash_types::Txid;
 use bitcoin::blockdata::transaction::OutPoint as BitcoinOutPoint;
 use bitcoin::blockdata::transaction::Transaction;
 
+/// Transaction data where each item consists of a transaction paired with the index of the
+/// transaction within a block.
+///
+/// Useful for returning enumerated transactions from a block, possibly filtered, in order to retain
+/// the transaction index.
+///
+/// ```
+/// extern crate bitcoin;
+/// extern crate lightning;
+///
+/// use bitcoin::blockdata::block::Block;
+/// use bitcoin::blockdata::constants::genesis_block;
+/// use bitcoin::network::constants::Network;
+/// use lightning::chain::transaction::TransactionData;
+///
+/// let block = genesis_block(Network::Bitcoin);
+/// let coinbase = block.txdata[0].clone();
+///
+/// let mut txdata = enumerate_txdata(block);
+/// assert_eq!(txdata.len(), 1);
+///
+/// let (index, tx) = txdata.pop().unwrap();
+/// assert_eq!(index, 0);
+/// assert_eq!(tx, coinbase);
+///
+/// fn enumerate_txdata(mut block: Block) -> TransactionData {
+/// 	block.txdata.drain(..).enumerate().collect()
+/// }
+/// ```
+pub type TransactionData = Vec<(usize, Transaction)>;
+
 /// Transaction data where each item consists of a transaction reference paired with the index of
 /// the transaction within a block.
 ///

lightning/src/util/test_utils.rs

@@ -13,7 +13,7 @@ use chain::chaininterface::ConfirmationTarget;
 use chain::chainmonitor;
 use chain::channelmonitor;
 use chain::channelmonitor::MonitorEvent;
-use chain::transaction::OutPoint;
+use chain::transaction::{OutPoint, TransactionData};
 use chain::keysinterface;
 use ln::features::{ChannelFeatures, InitFeatures};
 use ln::msgs;
@@ -545,7 +545,8 @@ 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<TransactionData> {
 		self.watched_outputs.lock().unwrap().insert((*outpoint, script_pubkey.clone()));
+		None
 	}
 }