Drop the redundant/broken ChannelMonitor::get_monitored_outpoints

In review of the final doc changes in #649, I noticed there
appeared to be redundant monitored-outpoints function in
`ChannelMonitor` - `get_monitored_outpoints()` and
`get_outputs_to_watch()`.

In 6f08779,
get_monitored_outpoints() was added, with its behavior largely the
same as today's - only returning the set of remote commitment txn
outputs that we've learned about on-chain. This is clearly not
sufficient, and in 73dce20,
`get_outputs_to_watch` was added which was overly cautious to
ensure nothing was missed. Still, the author of 73dce20
(me) seemed entirely unaware of the work in 6f08779
(also me), despite the function being the literal next function in
the same file. This is presumably because it was assumed that
`get_monitored_outpoints` referred to oupoints for which we should
monitor for spends of (which is true), while `get_outputs_to_watch`
referred to outpouts which we should monitor for the transaction
containing said output (which is not true), or something of that
nature. Specifically, it is the expected behavior that the only
time we care about `Filter::register_tx` is for the funding
transaction (which we aren't aware of the inputs of), but for all
other transactions we register interest on the basis of an outpoint
in the previous transaction (ie via `Filter::register_output`).

Here we drop the broken-on-day-one `get_monitored_outpoints()`
version, but assert in testing that the values which it would return
are all present in `get_outputs_to_watch()`.

Author: TheBlueMatt

lightning/src/chain/channelmonitor.rs

@@ -1214,23 +1214,17 @@ impl<ChanSigner: ChannelKeys> ChannelMonitor<ChanSigner> {
 	///
 	/// (C-not exported) because we have no HashMap bindings
 	pub fn get_outputs_to_watch(&self) -> &HashMap<Txid, Vec<Script>> {
-		&self.outputs_to_watch
-	}
-
-	/// Gets the sets of all outpoints which this ChannelMonitor expects to hear about spends of.
-	/// Generally useful when deserializing as during normal operation the return values of
-	/// block_connected are sufficient to ensure all relevant outpoints are being monitored (note
-	/// that the get_funding_txo outpoint and transaction must also be monitored for!).
-	///
-	/// (C-not exported) as there is no practical way to track lifetimes of returned values.
-	pub fn get_monitored_outpoints(&self) -> Vec<(Txid, u32, &Script)> {
-		let mut res = Vec::with_capacity(self.counterparty_commitment_txn_on_chain.len() * 2);
-		for (ref txid, &(_, ref outputs)) in self.counterparty_commitment_txn_on_chain.iter() {
-			for (idx, output) in outputs.iter().enumerate() {
-				res.push(((*txid).clone(), idx as u32, output));
+		// If we've detected a counterparty commitment tx on chain, we must include it in the set
+		// of outputs to watch for spends of, otherwise we're likely to lose user funds. Because
+		// its trivial to do, double-check that here.
+		for (txid, &(_, ref outputs)) in self.counterparty_commitment_txn_on_chain.iter() {
+			let watched_outputs = self.outputs_to_watch.get(txid).expect("Counterparty commitment txn which have been broadcast should have outputs registered");
+			assert_eq!(watched_outputs.len(), outputs.len());
+			for (watched, output) in watched_outputs.iter().zip(outputs.iter()) {
+				assert_eq!(watched, output);
 			}
 		}
-		res
+		&self.outputs_to_watch
 	}
 
 	/// Get the list of HTLCs who's status has been updated on chain. This should be called by

lightning/src/ln/channelmanager.rs

@@ -3730,7 +3730,7 @@ impl<ChanSigner: ChannelKeys + Writeable, M: Deref, T: Deref, K: Deref, F: Deref
 ///    This may result in closing some Channels if the ChannelMonitor is newer than the stored
 ///    ChannelManager state to ensure no loss of funds. Thus, transactions may be broadcasted.
 /// 3) Register all relevant ChannelMonitor outpoints with your chain watch mechanism using
-///    ChannelMonitor::get_monitored_outpoints and ChannelMonitor::get_funding_txo().
+///    ChannelMonitor::get_outputs_to_watch() and ChannelMonitor::get_funding_txo().
 /// 4) Reconnect blocks on your ChannelMonitors.
 /// 5) Move the ChannelMonitors into your local chain::Watch.
 /// 6) Disconnect/connect blocks on the ChannelManager.