Don't create chan-closed mon update for outbound never-signed chans

Like the previous commit for channel-closed monitor updates for
inbound channels during processing of a funding_created message,
this resolves a more general issue for closing outbound channels
which have sent a funding_created but not yet received a
funding_signed.

This issue was also detected by full_stack_target.

To make similar issues easier to detect in testing and fuzzing, an
additional assertion is added to panic on updates to a channel
monitor before registering it.

Author: TheBlueMatt

lightning/src/chain/chainmonitor.rs

@@ -194,6 +194,13 @@ where C::Target: chain::Filter,
 		match monitors.get_mut(&funding_txo) {
 			None => {
 				log_error!(self.logger, "Failed to update channel monitor: no such monitor registered");
+
+				// We should never ever trigger this from within ChannelManager. Technically a
+				// user could use this object with some proxying in between which makes this
+				// possible, but in tests and fuzzing, this should be a panic.
+				#[cfg(any(test, feature = "fuzztarget"))]
+				panic!();
+				#[cfg(not(any(test, feature = "fuzztarget")))]
 				Err(ChannelMonitorUpdateErr::PermanentFailure)
 			},
 			Some(orig_monitor) => {

lightning/src/ln/channel.rs

@@ -4016,11 +4016,23 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
 				_ => {}
 			}
 		}
+		let funding_txo = if let Some(funding_txo) = self.funding_txo {
+			// If we haven't yet exchanged funding signatures (ie channel_state < FundingSent),
+			// returning a channel monitor update here would imply a channel monitor update before
+			// we even registered the channel monitor to begin with, which is invalid.
+			// Thus, if we aren't actually at a point where we could conceivably broadcast the
+			// funding transaction, don't return a funding txo (which prevents providing the
+			// monitor update to the user, even if we return one).
+			// See test_duplicate_chan_id and test_pre_lockin_no_chan_closed_update for more.
+			if self.channel_state & (ChannelState::FundingSent as u32 | ChannelState::ChannelFunded as u32 | ChannelState::ShutdownComplete as u32) != 0 {
+				Some(funding_txo.clone())
+			} else { None }
+		} else { None };
 
 		self.channel_state = ChannelState::ShutdownComplete as u32;
 		self.update_time_counter += 1;
 		self.latest_monitor_update_id += 1;
-		(self.funding_txo.clone(), ChannelMonitorUpdate {
+		(funding_txo, ChannelMonitorUpdate {
 			update_id: self.latest_monitor_update_id,
 			updates: vec![ChannelMonitorUpdateStep::ChannelForceClosed { should_broadcast }],
 		}, dropped_outbound_htlcs)

lightning/src/ln/general_unit_tests.rs

@@ -647,3 +647,40 @@ fn test_duplicate_chan_id() {
 	update_nodes_with_chan_announce(&nodes, 0, 1, &announcement, &as_update, &bs_update);
 	send_payment(&nodes[0], &[&nodes[1]], 8000000, 8_000_000);
 }
+
+#[test]
+fn test_pre_lockin_no_chan_closed_update() {
+	// Test that if a peer closes a channel in response to a funding_created message we don't
+	// generate a channel update (as the channel cannot appear on chain without a funding_signed
+	// message).
+	//
+	// Doing so would imply a channel monitor update before the initial channel monitor
+	// registration, violating our API guarantees.
+	//
+	// Previously, full_stack_target managed to hit this case by opening then closing a channel,
+	// then opening a second channel with the same funding output as the first (which is not
+	// rejected because the first channel does not exist in the ChannelManager) and closing it
+	// before receiving funding_signed.
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+
+	// Create an initial channel
+	nodes[0].node.create_channel(nodes[1].node.get_our_node_id(), 100000, 10001, 42, None).unwrap();
+	let mut open_chan_msg = get_event_msg!(nodes[0], MessageSendEvent::SendOpenChannel, nodes[1].node.get_our_node_id());
+	nodes[1].node.handle_open_channel(&nodes[0].node.get_our_node_id(), InitFeatures::known(), &open_chan_msg);
+	let accept_chan_msg = get_event_msg!(nodes[1], MessageSendEvent::SendAcceptChannel, nodes[0].node.get_our_node_id());
+	nodes[0].node.handle_accept_channel(&nodes[1].node.get_our_node_id(), InitFeatures::known(), &accept_chan_msg);
+
+	// Move the first channel through the funding flow...
+	let (temporary_channel_id, _tx, funding_output) = create_funding_transaction(&nodes[0], 100000, 42);
+
+	nodes[0].node.funding_transaction_generated(&temporary_channel_id, funding_output);
+	check_added_monitors!(nodes[0], 0);
+
+	let funding_created_msg = get_event_msg!(nodes[0], MessageSendEvent::SendFundingCreated, nodes[1].node.get_our_node_id());
+	let channel_id = ::chain::transaction::OutPoint { txid: funding_created_msg.funding_txid, index: funding_created_msg.funding_output_index }.to_channel_id();
+	nodes[0].node.handle_error(&nodes[1].node.get_our_node_id(), &msgs::ErrorMessage { channel_id, data: "Hi".to_owned() });
+	assert!(nodes[0].chain_monitor.added_monitors.lock().unwrap().is_empty());
+}