Multi-hop blinded payment paths in ChannelManager

When constructing blinded payment paths for Bolt12Invoice, delegate to
Router::create_blinded_payment_paths which may produce multi-hop blinded
paths. Fallback to one-hop blinded paths if the Router fails or returns
no paths.

Author: jkczyz

lightning/src/ln/channelmanager.rs

@@ -7658,9 +7658,7 @@ where
 
 		match self.create_inbound_payment(Some(amount_msats), relative_expiry, None) {
 			Ok((payment_hash, payment_secret)) => {
-				let payment_paths = vec![
-					self.create_one_hop_blinded_payment_path(payment_secret),
-				];
+				let payment_paths = self.create_blinded_payment_paths(amount_msats, payment_secret);
 				#[cfg(not(feature = "no-std"))]
 				let builder = refund.respond_using_derived_keys(
 					payment_paths, payment_hash, expanded_key, entropy
@@ -7821,6 +7819,18 @@ where
 			.and_then(|paths| paths.into_iter().next().ok_or(()))
 	}
 
+	/// Creates multi-hop blinded payment paths for the given `amount_msats` by delegating to
+	/// [`Router::create_blinded_payment_paths`]. If the router returns an error or no paths,
+	/// creates a one-hop blinded payment path instead.
+	fn create_blinded_payment_paths(
+		&self, amount_msats: u64, payment_secret: PaymentSecret
+	) -> Vec<(BlindedPayInfo, BlindedPath)> {
+		self.create_multi_hop_blinded_payment_paths(amount_msats, payment_secret)
+			.ok()
+			.and_then(|paths| (!paths.is_empty()).then(|| paths))
+			.unwrap_or_else(|| vec![self.create_one_hop_blinded_payment_path(payment_secret)])
+	}
+
 	/// Creates a one-hop blinded payment path with [`ChannelManager::get_our_node_id`] as the
 	/// introduction node.
 	fn create_one_hop_blinded_payment_path(
@@ -7844,6 +7854,34 @@ where
 		).unwrap()
 	}
 
+	/// Creates multi-hop blinded payment paths for the given `amount_msats` by delegating to
+	/// [`Router::create_blinded_payment_paths`].
+	///
+	/// May return no paths if no peers [support route blinding] or whose channels don't have enough
+	/// inbound liquidity.
+	///
+	/// [support route blinding]: crate::ln::features::InitFeatures::supports_route_blinding
+	fn create_multi_hop_blinded_payment_paths(
+		&self, amount_msats: u64, payment_secret: PaymentSecret
+	) -> Result<Vec<(BlindedPayInfo, BlindedPath)>, ()> {
+		let entropy_source = self.entropy_source.deref();
+		let secp_ctx = &self.secp_ctx;
+
+		let first_hops = self.list_usable_channels();
+		let payee_node_id = self.get_our_node_id();
+		let max_cltv_expiry = self.best_block.read().unwrap().height() + LATENCY_GRACE_PERIOD_BLOCKS;
+		let payee_tlvs = ReceiveTlvs {
+			payment_secret,
+			payment_constraints: PaymentConstraints {
+				max_cltv_expiry,
+				htlc_minimum_msat: 1,
+			},
+		};
+		self.router.create_blinded_payment_paths(
+			payee_node_id, first_hops, payee_tlvs, amount_msats, entropy_source, secp_ctx
+		)
+	}
+
 	/// Gets a fake short channel id for use in receiving [phantom node payments]. These fake scids
 	/// are used when constructing the phantom invoice's route hints.
 	///
@@ -9081,7 +9119,7 @@ where
 				let amount_msats = match InvoiceBuilder::<DerivedSigningPubkey>::amount_msats(
 					&invoice_request
 				) {
-					Ok(amount_msats) => Some(amount_msats),
+					Ok(amount_msats) => amount_msats,
 					Err(error) => return Some(OffersMessage::InvoiceError(error.into())),
 				};
 				let invoice_request = match invoice_request.verify(expanded_key, secp_ctx) {
@@ -9093,11 +9131,11 @@ where
 				};
 				let relative_expiry = DEFAULT_RELATIVE_EXPIRY.as_secs() as u32;
 
-				match self.create_inbound_payment(amount_msats, relative_expiry, None) {
+				match self.create_inbound_payment(Some(amount_msats), relative_expiry, None) {
 					Ok((payment_hash, payment_secret)) if invoice_request.keys.is_some() => {
-						let payment_paths = vec![
-							self.create_one_hop_blinded_payment_path(payment_secret),
-						];
+						let payment_paths = self.create_blinded_payment_paths(
+							amount_msats, payment_secret
+						);
 						#[cfg(not(feature = "no-std"))]
 						let builder = invoice_request.respond_using_derived_keys(
 							payment_paths, payment_hash
@@ -9116,9 +9154,9 @@ where
 						}
 					},
 					Ok((payment_hash, payment_secret)) => {
-						let payment_paths = vec![
-							self.create_one_hop_blinded_payment_path(payment_secret),
-						];
+						let payment_paths = self.create_blinded_payment_paths(
+							amount_msats, payment_secret
+						);
 						#[cfg(not(feature = "no-std"))]
 						let builder = invoice_request.respond_with(payment_paths, payment_hash);
 						#[cfg(feature = "no-std")]