Fix spurious panic on receipt of a block while awaiting funding

When we receive a block we always test if we should send our
channel_ready via `check_get_channel_ready`. If the channel in
question requires confirmations, we quickly return if the funding
transaction has not yet confirmed (or even been defined), however
for 0conf channels the checks are necessarily more involved.

In any case, we wish to panic if the funding transaction has
confirmations prior to when it should have been broadcasted. This
is useful as it is easy for users to violate our broadcast-time
invariants without noticing and the panic gives us an opportunity
to catch it.

Sadly, in the case of 0conf channels, if we hadn't yet seen the
funding transaction at all but receive a block we would hit this
sanity check as we don't check whether there are actually funding
transaction confirmations prior to panicing.

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -4755,6 +4755,9 @@ impl<Signer: Sign> Channel<Signer> {
 	}
 
 	fn check_get_channel_ready(&mut self, height: u32) -> Option<msgs::ChannelReady> {
+		// Called:
+		//  * always when a new block/transactions are confirmed with the new height
+		//  * when funding is signed with a height of 0
 		if self.funding_tx_confirmation_height == 0 && self.minimum_depth != Some(0) {
 			return None;
 		}
@@ -4780,7 +4783,7 @@ impl<Signer: Sign> Channel<Signer> {
 			// We got a reorg but not enough to trigger a force close, just ignore.
 			false
 		} else {
-			if self.channel_state < ChannelState::ChannelFunded as u32 {
+			if self.funding_tx_confirmation_height != 0 && self.channel_state < ChannelState::ChannelFunded as u32 {
 				// We should never see a funding transaction on-chain until we've received
 				// funding_signed (if we're an outbound channel), or seen funding_generated (if we're
 				// an inbound channel - before that we have no known funding TXID). The fuzzer,

lightning/src/ln/priv_short_conf_tests.rs

@@ -1021,3 +1021,45 @@ fn test_zero_conf_accept_reject() {
 		_ => panic!(),
 	}
 }
+
+#[test]
+fn test_connect_before_funding() {
+	// Tests for a particularly dumb explicit panic that existed prior to 0.0.111 for 0conf
+	// channels. If we received a block while awaiting funding for 0-conf channels we'd hit an
+	// explicit panic when deciding if we should broadcast our channel_ready message.
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+
+	let mut manually_accept_conf = test_default_channel_config();
+	manually_accept_conf.manually_accept_inbound_channels = true;
+
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, Some(manually_accept_conf)]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+
+	nodes[0].node.create_channel(nodes[1].node.get_our_node_id(), 100_000, 10_001, 42, None).unwrap();
+	let open_channel = 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_channel);
+	let events = nodes[1].node.get_and_clear_pending_events();
+	assert_eq!(events.len(), 1);
+	match events[0] {
+		Event::OpenChannelRequest { temporary_channel_id, .. } => {
+			nodes[1].node.accept_inbound_channel_from_trusted_peer_0conf(&temporary_channel_id, &nodes[0].node.get_our_node_id(), 0).unwrap();
+		},
+		_ => panic!("Unexpected event"),
+	};
+
+	let mut accept_channel = get_event_msg!(nodes[1], MessageSendEvent::SendAcceptChannel, nodes[0].node.get_our_node_id());
+	assert_eq!(accept_channel.minimum_depth, 0);
+	nodes[0].node.handle_accept_channel(&nodes[1].node.get_our_node_id(), InitFeatures::known(), &accept_channel);
+
+	let events = nodes[0].node.get_and_clear_pending_events();
+	assert_eq!(events.len(), 1);
+	match events[0] {
+		Event::FundingGenerationReady { .. } => {},
+		_ => panic!("Unexpected event"),
+	}
+
+	connect_blocks(&nodes[0], 1);
+	connect_blocks(&nodes[1], 1);
+}