Expose chan type in Event::OpenChannelRequest & ChannelDetails

As we add new supported channel types, inbound channels which use
new features may cause backwards-compatibility issues for clients.
If a new channel is opened using new features while a client still
wishes to ensure support for downgrading to a previous version of
LDK, that new channel may cause the `ChannelManager` to fail
deserialization due to unsupported feature flags.

By exposing the channel type flags to the user in channel requests,
users wishing to support downgrading to previous versions of LDK
can reject channels which use channel features which previous
versions of LDK do not understand.

fuzz/src/router.rs

@@ -215,6 +215,7 @@ pub fn do_test<Out: test_logger::Output>(data: &[u8], out: Out) {
 									forwarding_info: None,
 								},
 								funding_txo: Some(OutPoint { txid: bitcoin::Txid::from_slice(&[0; 32]).unwrap(), index: 0 }),
+								channel_type: None,
 								short_channel_id: Some(scid),
 								inbound_scid_alias: None,
 								channel_value_satoshis: slice_to_be64(get_slice!(8)),

lightning/src/ln/channel.rs

@@ -4232,6 +4232,11 @@ impl<Signer: Sign> Channel<Signer> {
 		self.user_id
 	}
 
+	/// Gets the channel's type
+	pub fn get_channel_type(&self) -> &ChannelTypeFeatures {
+		&self.channel_type
+	}
+
 	/// Guaranteed to be Some after both FundingLocked messages have been exchanged (and, thus,
 	/// is_usable() returns true).
 	/// Allowed in any state (including after shutdown)

lightning/src/ln/channelmanager.rs

@@ -42,7 +42,7 @@ use chain::transaction::{OutPoint, TransactionData};
 // construct one themselves.
 use ln::{PaymentHash, PaymentPreimage, PaymentSecret};
 use ln::channel::{Channel, ChannelError, ChannelUpdateStatus, UpdateFulfillCommitFetch};
-use ln::features::{InitFeatures, NodeFeatures};
+use ln::features::{ChannelTypeFeatures, InitFeatures, NodeFeatures};
 use routing::router::{PaymentParameters, Route, RouteHop, RoutePath, RouteParameters};
 use ln::msgs;
 use ln::msgs::NetAddress;
@@ -1200,6 +1200,10 @@ pub struct ChannelDetails {
 	/// Note that, if this has been set, `channel_id` will be equivalent to
 	/// `funding_txo.unwrap().to_channel_id()`.
 	pub funding_txo: Option<OutPoint>,
+	/// The features which this channel operates with. See individual features for more info.
+	///
+	/// `None` until negotiation completes and the channel type is finalized.
+	pub channel_type: Option<ChannelTypeFeatures>,
 	/// The position of the funding transaction in the chain. None if the funding transaction has
 	/// not yet been confirmed and the channel fully opened.
 	///
@@ -1922,6 +1926,9 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 						forwarding_info: channel.counterparty_forwarding_info(),
 					},
 					funding_txo: channel.get_funding_txo(),
+					// Note that accept_channel (or open_channel) is always the first message, so
+					// `have_received_message` indicates that type negotiation has completed.
+					channel_type: if channel.have_received_message() { Some(channel.get_channel_type().clone()) } else { None },
 					short_channel_id: channel.get_short_channel_id(),
 					inbound_scid_alias: channel.latest_inbound_scid_alias(),
 					channel_value_satoshis: channel.get_value_satoshis(),
@@ -4315,6 +4322,7 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 							counterparty_node_id: counterparty_node_id.clone(),
 							funding_satoshis: msg.funding_satoshis,
 							push_msat: msg.push_msat,
+							channel_type: channel.get_channel_type().clone(),
 						}
 					);
 				}
@@ -6054,6 +6062,7 @@ impl_writeable_tlv_based!(ChannelCounterparty, {
 impl_writeable_tlv_based!(ChannelDetails, {
 	(1, inbound_scid_alias, option),
 	(2, channel_id, required),
+	(3, channel_type, option),
 	(4, counterparty, required),
 	(6, funding_txo, option),
 	(8, short_channel_id, option),

lightning/src/routing/router.rs

@@ -1667,6 +1667,7 @@ mod tests {
 				forwarding_info: None,
 			},
 			funding_txo: Some(OutPoint { txid: bitcoin::Txid::from_slice(&[0; 32]).unwrap(), index: 0 }),
+			channel_type: None,
 			short_channel_id,
 			inbound_scid_alias: None,
 			channel_value_satoshis: 0,
@@ -5363,6 +5364,7 @@ mod benches {
 			funding_txo: Some(OutPoint {
 				txid: bitcoin::Txid::from_slice(&[0; 32]).unwrap(), index: 0
 			}),
+			channel_type: None,
 			short_channel_id: Some(1),
 			inbound_scid_alias: None,
 			channel_value_satoshis: 10_000_000,

lightning/src/util/events.rs

@@ -17,6 +17,7 @@
 use chain::keysinterface::SpendableOutputDescriptor;
 use ln::channelmanager::PaymentId;
 use ln::channel::FUNDING_CONF_DEADLINE_BLOCKS;
+use ln::features::ChannelTypeFeatures;
 use ln::msgs;
 use ln::msgs::DecodeError;
 use ln::{PaymentPreimage, PaymentHash, PaymentSecret};
@@ -429,6 +430,10 @@ pub enum Event {
 		funding_satoshis: u64,
 		/// Our starting balance in the channel if the request is accepted, in milli-satoshi.
 		push_msat: u64,
+		/// The features that this channel will operate with. If you reject the channel, a
+		/// well-behaved counterparty may automatically re-attempt the channel with a new set of
+		/// feature flags.
+		channel_type: ChannelTypeFeatures,
 	},
 }