OnionMessenger: support generating peer connection events.

Useful if we are in the mode of interception OMs for offline peers, so
users know when to re-inject intercepted OMs.

Author: valentinewallace

lightning/src/events/mod.rs

@@ -1053,6 +1053,18 @@ pub enum Event {
 		/// The onion message intended to be forwarded to `peer_node_id`.
 		message: msgs::OnionMessage,
 	},
+	/// Indicates that an onion message supporting peer has come online and it may
+	/// be time to forward any onion messages that were previously intercepted for
+	/// them. This event will only be generated if the `OnionMessenger` was
+	/// initialized with
+	/// [`OnionMessenger::new_with_offline_peer_interception`], see its docs.
+	///
+	/// [`OnionMessenger::new_with_offline_peer_interception`]: crate::onion_message::messenger::OnionMessenger::new_with_offline_peer_interception
+	OnionMessagePeerConnected {
+		/// The node id of the peer we just connected to, who advertises support for
+		/// onion messages.
+		peer_node_id: PublicKey,
+	}
 }
 
 impl Writeable for Event {
@@ -1304,6 +1316,12 @@ impl Writeable for Event {
 					(0, peer_node_id, required),
 					(2, message, required),
 				});
+			},
+			&Event::OnionMessagePeerConnected { ref peer_node_id } => {
+				39u8.write(writer)?;
+				write_tlv_fields!(writer, {
+					(0, peer_node_id, required),
+				});
 			}
 			// Note that, going forward, all new events must only write data inside of
 			// `write_tlv_fields`. Versions 0.0.101+ will ignore odd-numbered events that write
@@ -1728,6 +1746,17 @@ impl MaybeReadable for Event {
 				};
 				f()
 			},
+			39u8 => {
+				let mut f = || {
+					_init_and_read_len_prefixed_tlv_fields!(reader, {
+						(0, peer_node_id, required),
+					});
+					Ok(Some(Event::OnionMessagePeerConnected {
+						peer_node_id: peer_node_id.0.unwrap()
+					}))
+				};
+				f()
+			},
 			// Versions prior to 0.0.100 did not ignore odd types, instead returning InvalidValue.
 			// Version 0.0.100 failed to properly ignore odd types, possibly resulting in corrupt
 			// reads.

lightning/src/onion_message/messenger.rs

@@ -1140,6 +1140,11 @@ where
 				.entry(*their_node_id)
 				.or_insert_with(|| OnionMessageRecipient::ConnectedPeer(VecDeque::new()))
 				.mark_connected();
+			if self.intercept_messages_for_offline_peers {
+				self.pending_events.lock().unwrap().push(
+					Event::OnionMessagePeerConnected { peer_node_id: *their_node_id }
+				);
+			}
 		} else {
 			self.message_recipients.lock().unwrap().remove(their_node_id);
 		}