Add NodeIdMessageRouter and NullMessageRouter

Introduced two new default routers:

- `NodeIdMessageRouter` – for creating full-length blinded paths using
  peer's node id.
- `NullMessageRouter` – for intentionally creating no blinded paths

This makes the purpose of each router explicit and ensures their functions
remain unique.
It also builds on the flexibility introduced in the previous commit,
where `create_blinded_paths` was updated to support a MessageForwardNode.

Author: shaavan

lightning/src/onion_message/messenger.rs

@@ -704,6 +704,114 @@ where
 	}
 }
 
+/// This message router is similar to [`DefaultMessageRouter`], but it always creates
+/// full-length blinded paths, using the peer's [`NodeId`].
+///
+/// This message router can only route to a directly connected [`Destination`].
+///
+/// # Privacy
+///
+/// Creating [`BlindedMessagePath`]s may affect privacy since, if a suitable path cannot be found,
+/// it will create a one-hop path using the recipient as the introduction node if it is a announced
+/// node. Otherwise, there is no way to find a path to the introduction node in order to send a
+/// message, and thus an `Err` is returned.
+pub struct NodeIdMessageRouter<G: Deref<Target = NetworkGraph<L>>, L: Deref, ES: Deref>
+where
+	L::Target: Logger,
+	ES::Target: EntropySource,
+{
+	network_graph: G,
+	entropy_source: ES,
+}
+
+impl<G: Deref<Target = NetworkGraph<L>>, L: Deref, ES: Deref> NodeIdMessageRouter<G, L, ES>
+where
+	L::Target: Logger,
+	ES::Target: EntropySource,
+{
+	/// Creates a [`DefaultMessageRouter`] using the given [`NetworkGraph`].
+	pub fn new(network_graph: G, entropy_source: ES) -> Self {
+		Self { network_graph, entropy_source }
+	}
+}
+
+impl<G: Deref<Target = NetworkGraph<L>>, L: Deref, ES: Deref> MessageRouter
+	for NodeIdMessageRouter<G, L, ES>
+where
+	L::Target: Logger,
+	ES::Target: EntropySource,
+{
+	fn find_path(
+		&self, sender: PublicKey, peers: Vec<PublicKey>, destination: Destination,
+	) -> Result<OnionMessagePath, ()> {
+		DefaultMessageRouter::<G, L, ES>::find_path(&self.network_graph, sender, peers, destination)
+	}
+
+	fn create_blinded_paths<T: secp256k1::Signing + secp256k1::Verification>(
+		&self, recipient: PublicKey, context: MessageContext, peers: Vec<MessageForwardNode>,
+		secp_ctx: &Secp256k1<T>,
+	) -> Result<Vec<BlindedMessagePath>, ()> {
+		DefaultMessageRouter::create_blinded_paths_from_iter(
+			&self.network_graph,
+			recipient,
+			context,
+			peers.into_iter(),
+			&self.entropy_source,
+			secp_ctx,
+			false,
+		)
+	}
+
+	fn create_compact_blinded_paths<T: secp256k1::Signing + secp256k1::Verification>(
+		&self, recipient: PublicKey, context: MessageContext, peers: Vec<MessageForwardNode>,
+		secp_ctx: &Secp256k1<T>,
+	) -> Result<Vec<BlindedMessagePath>, ()> {
+		DefaultMessageRouter::create_blinded_paths_from_iter(
+			&self.network_graph,
+			recipient,
+			context,
+			peers.into_iter(),
+			&self.entropy_source,
+			secp_ctx,
+			false,
+		)
+	}
+}
+
+/// A special [`MessageRouter`] implementation that performs no routing.
+///
+/// # Note
+/// [`NullMessageRouter`] **must not** be used with [`ChannelManager`] as a parameter.
+///
+/// # Reason
+/// [`ChannelManager`] requires a functioning [`MessageRouter`] to create blinded paths,
+/// which are necessary for constructing reply paths in onion message communication.
+///
+/// [`ChannelManager`]: crate::ln::channelmanager::ChannelManager
+pub struct NullMessageRouter {}
+
+impl MessageRouter for NullMessageRouter {
+	fn find_path(
+		&self, _sender: PublicKey, _peers: Vec<PublicKey>, _destination: Destination,
+	) -> Result<OnionMessagePath, ()> {
+		unreachable!()
+	}
+
+	fn create_blinded_paths<T: secp256k1::Signing + secp256k1::Verification>(
+		&self, _recipient: PublicKey, _context: MessageContext, _peers: Vec<MessageForwardNode>,
+		_secp_ctx: &Secp256k1<T>,
+	) -> Result<Vec<BlindedMessagePath>, ()> {
+		Ok(vec![])
+	}
+
+	fn create_compact_blinded_paths<T: secp256k1::Signing + secp256k1::Verification>(
+		&self, _recipient: PublicKey, _context: MessageContext, _peers: Vec<MessageForwardNode>,
+		_secp_ctx: &Secp256k1<T>,
+	) -> Result<Vec<BlindedMessagePath>, ()> {
+		Ok(vec![])
+	}
+}
+
 /// A path for sending an [`OnionMessage`].
 #[derive(Clone)]
 pub struct OnionMessagePath {