Avoid panic when 0conf channel's ann. sigs race on-chain confirmation

A channel's `short_channel_id` is currently only set when the funding
transaction is confirmed via `transactions_confirmed`, which might be
well after the channel initally becomes usable, e.g., in the 0conf case.

Previously we would panic due to a reachable `unwrap` when receiving a
counterparty's `announcement_signatures` message for a 0conf channel
pending confirmation on-chain.

Here we fix this bug by avoiding unsafe `unwrap`s and just erroring out
and ignoring the announcement_signatures message if the `short_channel_id`
hasn't been set yet.

lightning/src/ln/channel.rs

@@ -948,9 +948,9 @@ impl<Signer: ChannelSigner> ChannelContext<Signer> {
 		&self.channel_type
 	}
 
-	/// Guaranteed to be Some after both ChannelReady messages have been exchanged (and, thus,
+	/// Gets the channel's `short_channel_id`.
-	/// is_usable() returns true).
+	///
-	/// Allowed in any state (including after shutdown)
+	/// Will return `None` if the channel hasn't been confirmed yet.
 	pub fn get_short_channel_id(&self) -> Option<u64> {
 		self.short_channel_id
 	}
@@ -4832,6 +4832,8 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 			return Err(ChannelError::Ignore("Cannot get a ChannelAnnouncement if the channel is not currently usable".to_owned()));
 		}
 
+		let short_channel_id = self.context.get_short_channel_id()
+			.ok_or(ChannelError::Ignore("Cannot get a ChannelAnnouncement if the channel has not been confirmed yet".to_owned()))?;
 		let node_id = NodeId::from_pubkey(&node_signer.get_node_id(Recipient::Node)
 			.map_err(|_| ChannelError::Ignore("Failed to retrieve own public key".to_owned()))?);
 		let counterparty_node_id = NodeId::from_pubkey(&self.context.get_counterparty_node_id());
@@ -4840,7 +4842,7 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 		let msg = msgs::UnsignedChannelAnnouncement {
 			features: channelmanager::provided_channel_features(&user_config),
 			chain_hash,
-			short_channel_id: self.context.get_short_channel_id().unwrap(),
+			short_channel_id,
 			node_id_1: if were_node_one { node_id } else { counterparty_node_id },
 			node_id_2: if were_node_one { counterparty_node_id } else { node_id },
 			bitcoin_key_1: NodeId::from_pubkey(if were_node_one { &self.context.get_holder_pubkeys().funding_pubkey } else { self.context.counterparty_funding_pubkey() }),

lightning/src/ln/priv_short_conf_tests.rs

@@ -1007,3 +1007,38 @@ fn test_connect_before_funding() {
 	connect_blocks(&nodes[0], 1);
 	connect_blocks(&nodes[1], 1);
 }
+
+#[test]
+fn test_0conf_ann_sigs_racing_conf() {
+	// Previously we had a bug where we'd panic when receiving a counterparty's
+	// announcement_signatures message for a 0conf channel pending confirmation on-chain. Here we
+	// check that we just error out, ignore the announcement_signatures message, and proceed
+	// instead.
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let mut chan_config = test_default_channel_config();
+	chan_config.manually_accept_inbound_channels = true;
+
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, Some(chan_config)]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+
+	// This is the default but we force it on anyway
+	chan_config.channel_handshake_config.announced_channel = true;
+	let (tx, ..) = open_zero_conf_channel(&nodes[0], &nodes[1], Some(chan_config));
+
+	// We can use the channel immediately, but we can't announce it until we get 6+ confirmations
+	send_payment(&nodes[0], &[&nodes[1]], 100_000);
+
+	let scid = confirm_transaction(&nodes[0], &tx);
+	let as_announcement_sigs = get_event_msg!(nodes[0], MessageSendEvent::SendAnnouncementSignatures, nodes[1].node.get_our_node_id());
+
+	// Handling the announcement_signatures prior to the first confirmation would panic before.
+	nodes[1].node.handle_announcement_signatures(&nodes[0].node.get_our_node_id(), &as_announcement_sigs);
+
+	assert_eq!(confirm_transaction(&nodes[1], &tx), scid);
+	let bs_announcement_sigs = get_event_msg!(nodes[1], MessageSendEvent::SendAnnouncementSignatures, nodes[0].node.get_our_node_id());
+
+	nodes[0].node.handle_announcement_signatures(&nodes[1].node.get_our_node_id(), &bs_announcement_sigs);
+	let as_announcement = nodes[0].node.get_and_clear_pending_msg_events();
+	assert_eq!(as_announcement.len(), 1);
+}