Introduce ResponseInstructions for OnionMessage Handling

fuzz/src/onion_message.rs

@@ -16,7 +16,7 @@ use lightning::sign::{Recipient, KeyMaterial, EntropySource, NodeSigner, SignerP
 use lightning::util::test_channel_signer::TestChannelSigner;
 use lightning::util::logger::Logger;
 use lightning::util::ser::{Readable, Writeable, Writer};
-use lightning::onion_message::messenger::{CustomOnionMessageHandler, Destination, MessageRouter, OnionMessagePath, OnionMessenger, PendingOnionMessage};
+use lightning::onion_message::messenger::{CustomOnionMessageHandler, Destination, MessageRouter, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction};
 use lightning::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use lightning::onion_message::packet::OnionMessageContents;
 
@@ -97,8 +97,8 @@ impl MessageRouter for TestMessageRouter {
 struct TestOffersMessageHandler {}
 
 impl OffersMessageHandler for TestOffersMessageHandler {
-	fn handle_message(&self, _message: OffersMessage) -> Option<OffersMessage> {
-None
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ResponseInstruction::NoResponse
 	}
 }
 
@@ -112,6 +112,9 @@ impl OnionMessageContents for TestCustomMessage {
 	fn tlv_type(&self) -> u64 {
  CUSTOM_MESSAGE_TYPE
 	}
+	fn msg_type(&self) -> &'static str {
+ "Custom Message"
+	}
 }
 
 impl Writeable for TestCustomMessage {
@@ -124,8 +127,11 @@ struct TestCustomMessageHandler {}
 
 impl CustomOnionMessageHandler for TestCustomMessageHandler {
 	type CustomMessage = TestCustomMessage;
-	fn handle_custom_message(&self, _msg: Self::CustomMessage) -> Option<Self::CustomMessage> {
- Some(TestCustomMessage {})
+	fn handle_custom_message(&self, message: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
+ match responder {
+ Some(responder) => responder.respond(message),
+ None => ResponseInstruction::NoResponse
+ }
 	}
 	fn read_custom_message<R: io::Read>(&self, _message_type: u64, buffer: &mut R) -> Result<Option<Self::CustomMessage>, msgs::DecodeError> {
  let mut buf = Vec::new();
@@ -280,9 +286,9 @@ mod tests {
  "Received an onion message with path_id None and a reply_path: Custom(TestCustomMessage)"
  .to_string())), Some(&1));
  assert_eq!(log_entries.get(&("lightning::onion_message::messenger".to_string(),
- "Constructing onion message when responding to Custom onion message with path_id None: TestCustomMessage".to_string())), Some(&1));
+ "Constructing onion message when responding with Custom Message to an onion message with path_id None: TestCustomMessage".to_string())), Some(&1));
  assert_eq!(log_entries.get(&("lightning::onion_message::messenger".to_string(),
- "Buffered onion message when responding to Custom onion message with path_id None".to_string())), Some(&1));
+ "Buffered onion message when responding with Custom Message to an onion message with path_id None".to_string())), Some(&1));
  }
 
  let two_unblinded_hops_om = "\

lightning/src/ln/channelmanager.rs

@@ -64,7 +64,7 @@ use crate::offers::invoice_request::{DerivedPayerId, InvoiceRequestBuilder};
 use crate::offers::offer::{Offer, OfferBuilder};
 use crate::offers::parse::Bolt12SemanticError;
 use crate::offers::refund::{Refund, RefundBuilder};
-use crate::onion_message::messenger::{Destination, MessageRouter, PendingOnionMessage, new_pending_onion_message};
+use crate::onion_message::messenger::{new_pending_onion_message, Destination, MessageRouter, PendingOnionMessage, Responder, ResponseInstruction};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use crate::sign::{EntropySource, NodeSigner, Recipient, SignerProvider};
 use crate::sign::ecdsa::WriteableEcdsaChannelSigner;
@@ -75,6 +75,7 @@ use crate::util::string::UntrustedString;
 use crate::util::ser::{BigSize, FixedLengthReader, Readable, ReadableArgs, MaybeReadable, Writeable, Writer, VecWriter};
 use crate::util::logger::{Level, Logger, WithContext};
 use crate::util::errors::APIError;
+
 #[cfg(not(c_bindings))]
 use {
 	crate::offers::offer::DerivedMetadata,
@@ -10332,23 +10333,27 @@ where
 	R::Target: Router,
 	L::Target: Logger,
 {
-	fn handle_message(&self, message: OffersMessage) -> Option<OffersMessage> {
+	fn handle_message(&self, message: OffersMessage, responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
  let secp_ctx = &self.secp_ctx;
  let expanded_key = &self.inbound_payment_key;
 
  match message {
  OffersMessage::InvoiceRequest(invoice_request) => {
+ let responder = match responder {
+ Some(responder) => responder,
+ None => return ResponseInstruction::NoResponse,
+ };
  let amount_msats = match InvoiceBuilder::<DerivedSigningPubkey>::amount_msats(
  &invoice_request
  ) {
  Ok(amount_msats) => amount_msats,
- Err(error) => return Some(OffersMessage::InvoiceError(error.into())),
+ Err(error) => return responder.respond(OffersMessage::InvoiceError(error.into())),
  };
  let invoice_request = match invoice_request.verify(expanded_key, secp_ctx) {
  Ok(invoice_request) => invoice_request,
  Err(()) => {
  let error = Bolt12SemanticError::InvalidMetadata;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10359,7 +10364,7 @@ where
  Ok((payment_hash, payment_secret)) => (payment_hash, payment_secret),
  Err(()) => {
  let error = Bolt12SemanticError::InvalidAmount;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10373,7 +10378,7 @@ where
  Ok(payment_paths) => payment_paths,
  Err(()) => {
  let error = Bolt12SemanticError::MissingPaths;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10418,8 +10423,8 @@ where
  };
 
  match response {
- Ok(invoice) => Some(OffersMessage::Invoice(invoice)),
- Err(error) => Some(OffersMessage::InvoiceError(error.into())),
+ Ok(invoice) => return responder.respond(OffersMessage::Invoice(invoice)),
+ Err(error) => return responder.respond(OffersMessage::InvoiceError(error.into())),
  }
  },
  OffersMessage::Invoice(invoice) => {
@@ -10439,14 +10444,21 @@ where
  }
  });
 
- match response {
- Ok(()) => None,
- Err(e) => Some(OffersMessage::InvoiceError(e)),
+ match (responder, response) {
+ (Some(responder), Err(e)) => responder.respond(OffersMessage::InvoiceError(e)),
+ (None, Err(_)) => {
+ log_trace!(
+ self.logger,
+ "A response was generated, but there is no reply_path specified for sending the response."
+ );
+ return ResponseInstruction::NoResponse;
+ }
+ _ => return ResponseInstruction::NoResponse,
  }
  },
  OffersMessage::InvoiceError(invoice_error) => {
  log_trace!(self.logger, "Received invoice_error: {}", invoice_error);
-None
+return ResponseInstruction::NoResponse;
  },
  }
 	}

lightning/src/ln/peer_handler.rs

@@ -28,7 +28,7 @@ use crate::util::ser::{VecWriter, Writeable, Writer};
 use crate::ln::peer_channel_encryptor::{PeerChannelEncryptor, NextNoiseStep, MessageBuf, MSG_BUF_ALLOC_SIZE};
 use crate::ln::wire;
 use crate::ln::wire::{Encode, Type};
-use crate::onion_message::messenger::{CustomOnionMessageHandler, PendingOnionMessage};
+use crate::onion_message::messenger::{CustomOnionMessageHandler, PendingOnionMessage, Responder, ResponseInstruction};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use crate::onion_message::packet::OnionMessageContents;
 use crate::routing::gossip::{NodeId, NodeAlias};
@@ -123,6 +123,7 @@ impl RoutingMessageHandler for IgnoringMessageHandler {
 	}
 	fn processing_queue_high(&self) -> bool { false }
 }
+
 impl OnionMessageHandler for IgnoringMessageHandler {
 	fn handle_onion_message(&self, _their_node_id: &PublicKey, _msg: &msgs::OnionMessage) {}
 	fn next_onion_message_for_peer(&self, _peer_node_id: PublicKey) -> Option<msgs::OnionMessage> { None }
@@ -134,12 +135,15 @@ impl OnionMessageHandler for IgnoringMessageHandler {
  InitFeatures::empty()
 	}
 }
+
 impl OffersMessageHandler for IgnoringMessageHandler {
-	fn handle_message(&self, _msg: OffersMessage) -> Option<OffersMessage> { None }
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ ResponseInstruction::NoResponse
+	}
 }
 impl CustomOnionMessageHandler for IgnoringMessageHandler {
 	type CustomMessage = Infallible;
-	fn handle_custom_message(&self, _msg: Infallible) -> Option<Infallible> {
+	fn handle_custom_message(&self, _message: Self::CustomMessage, _responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
  // Since we always return `None` in the read the handle method should never be called.
  unreachable!();
 	}
@@ -153,6 +157,7 @@ impl CustomOnionMessageHandler for IgnoringMessageHandler {
 
 impl OnionMessageContents for Infallible {
 	fn tlv_type(&self) -> u64 { unreachable!(); }
+	fn msg_type(&self) -> &'static str { unreachable!(); }
 }
 
 impl Deref for IgnoringMessageHandler {

lightning/src/onion_message/functional_tests.rs

@@ -18,7 +18,7 @@ use crate::routing::test_utils::{add_channel, add_or_update_node};
 use crate::sign::{NodeSigner, Recipient};
 use crate::util::ser::{FixedLengthReader, LengthReadable, Writeable, Writer};
 use crate::util::test_utils;
-use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, SendError};
+use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction, SendError};
 use super::offers::{OffersMessage, OffersMessageHandler};
 use super::packet::{OnionMessageContents, Packet};
 
@@ -62,8 +62,8 @@ struct MessengerNode {
 struct TestOffersMessageHandler {}
 
 impl OffersMessageHandler for TestOffersMessageHandler {
-	fn handle_message(&self, _message: OffersMessage) -> Option<OffersMessage> {
-None
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ResponseInstruction::NoResponse
 	}
 }
 
@@ -85,6 +85,9 @@ impl OnionMessageContents for TestCustomMessage {
  TestCustomMessage::Response => CUSTOM_RESPONSE_MESSAGE_TYPE,
  }
 	}
+	fn msg_type(&self) -> &'static str {
+ "Custom Message"
+	}
 }
 
 impl Writeable for TestCustomMessage {
@@ -123,15 +126,19 @@ impl Drop for TestCustomMessageHandler {
 
 impl CustomOnionMessageHandler for TestCustomMessageHandler {
 	type CustomMessage = TestCustomMessage;
-	fn handle_custom_message(&self, msg: Self::CustomMessage) -> Option<Self::CustomMessage> {
+	fn handle_custom_message(&self, msg: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
  match self.expected_messages.lock().unwrap().pop_front() {
  Some(expected_msg) => assert_eq!(expected_msg, msg),
  None => panic!("Unexpected message: {:?}", msg),
  }
-
- match msg {
+ let response_option = match msg {
  TestCustomMessage::Request => Some(TestCustomMessage::Response),
  TestCustomMessage::Response => None,
+ };
+ if let (Some(response), Some(responder)) = (response_option, responder) {
+ responder.respond(response)
+ } else {
+ ResponseInstruction::NoResponse
  }
 	}
 	fn read_custom_message<R: io::Read>(&self, message_type: u64, buffer: &mut R) -> Result<Option<Self::CustomMessage>, DecodeError> where Self: Sized {
@@ -422,6 +429,9 @@ fn invalid_custom_message_type() {
  // Onion message contents must have a TLV >= 64.
  63
  }
+ fn msg_type(&self) -> &'static str {
+ "Invalid Message"
+ }
 	}
 
 	impl Writeable for InvalidCustomMessage {

lightning/src/onion_message/messenger.rs

@@ -135,6 +135,7 @@ pub(super) const MAX_TIMER_TICKS: usize = 2;
 /// # let your_custom_message_type = 42;
 /// your_custom_message_type
 /// }
+/// fn msg_type(&self) -> &'static str { "YourCustomMessageType" }
 /// }
 /// // Send a custom onion message to a node id.
 /// let destination = Destination::Node(destination_node_id);
@@ -246,6 +247,50 @@ impl OnionMessageRecipient {
 	}
 }
 
+
+/// The `Responder` struct creates an appropriate [`ResponseInstruction`]
+/// for responding to a message.
+pub struct Responder {
+	/// The path along which a response can be sent.
+	reply_path: BlindedPath,
+	path_id: Option<[u8; 32]>
+}
+
+impl Responder {
+	/// Creates a new [`Responder`] instance with the provided reply path.
+	fn new(reply_path: BlindedPath, path_id: Option<[u8; 32]>) -> Self {
+ Responder {
+ reply_path,
+ path_id,
+ }
+	}
+
+	/// Creates the appropriate [`ResponseInstruction`] for a given response.
+	pub fn respond<T: OnionMessageContents>(self, response: T) -> ResponseInstruction<T> {
+ ResponseInstruction::WithoutReplyPath(OnionMessageResponse {
+ message: response,
+ reply_path: self.reply_path,
+ path_id: self.path_id,
+ })
+	}
+}
+
+/// This struct contains the information needed to reply to a received message.
+pub struct OnionMessageResponse<T: OnionMessageContents> {
+	message: T,
+	reply_path: BlindedPath,
+	path_id: Option<[u8; 32]>,
+}
+
+/// `ResponseInstruction` represents instructions for responding to received messages.
+pub enum ResponseInstruction<T: OnionMessageContents> {
+	/// Indicates that a response should be sent without including a reply path
+	/// for the recipient to respond back.
+	WithoutReplyPath(OnionMessageResponse<T>),
+	/// Indicates that there's no response to send back.
+	NoResponse,
+}
+
 /// An [`OnionMessage`] for [`OnionMessenger`] to send.
 ///
 /// These are obtained when released from [`OnionMessenger`]'s handlers after which they are
@@ -546,7 +591,7 @@ pub trait CustomOnionMessageHandler {
 	/// Called with the custom message that was received, returning a response to send, if any.
 	///
 	/// The returned [`Self::CustomMessage`], if any, is enqueued to be sent by [`OnionMessenger`].
-	fn handle_custom_message(&self, msg: Self::CustomMessage) -> Option<Self::CustomMessage>;
+	fn handle_custom_message(&self, message: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage>;
 
 	/// Read a custom message of type `message_type` from `buffer`, returning `Ok(None)` if the
 	/// message type is unknown.
@@ -933,19 +978,18 @@ where
 	}
 
 	fn handle_onion_message_response<T: OnionMessageContents>(
- &self, response: Option<T>, reply_path: Option<BlindedPath>, log_suffix: fmt::Arguments
+ &self, response: ResponseInstruction<T>
 	) {
- if let Some(response) = response {
- match reply_path {
- Some(reply_path) => {
- let _ = self.find_path_and_enqueue_onion_message(
- response, Destination::BlindedPath(reply_path), None, log_suffix
- );
- },
- None => {
- log_trace!(self.logger, "Missing reply path {}", log_suffix);
- },
- }
+ if let ResponseInstruction::WithoutReplyPath(response) = response {
+ let message_type = response.message.msg_type();
+ let _ = self.find_path_and_enqueue_onion_message(
+ response.message, Destination::BlindedPath(response.reply_path), None,
+ format_args!(
+ "when responding with {} to an onion message with path_id {:02x?}",
+ message_type,
+ response.path_id
+ )
+ );
  }
 	}
 
@@ -1029,22 +1073,18 @@ where
 
  match message {
  ParsedOnionMessageContents::Offers(msg) => {
- let response = self.offers_handler.handle_message(msg);
- self.handle_onion_message_response(
- response, reply_path, format_args!(
- "when responding to Offers onion message with path_id {:02x?}",
- path_id
- )
+ let responder = reply_path.map(
+ |reply_path| Responder::new(reply_path, path_id)
  );
+ let response_instructions = self.offers_handler.handle_message(msg, responder);
+ self.handle_onion_message_response(response_instructions);
  },
  ParsedOnionMessageContents::Custom(msg) => {
- let response = self.custom_handler.handle_custom_message(msg);
- self.handle_onion_message_response(
- response, reply_path, format_args!(
- "when responding to Custom onion message with path_id {:02x?}",
- path_id
- )
+ let responder = reply_path.map(
+ |reply_path| Responder::new(reply_path, path_id)
  );
+ let response_instructions = self.custom_handler.handle_custom_message(msg, responder);
+ self.handle_onion_message_response(response_instructions);
  },
  }
  },

fuzz/src/onion_message.rs

@@ -16,7 +16,7 @@ use lightning::sign::{Recipient, KeyMaterial, EntropySource, NodeSigner, SignerP
 use lightning::util::test_channel_signer::TestChannelSigner;
 use lightning::util::logger::Logger;
 use lightning::util::ser::{Readable, Writeable, Writer};
-use lightning::onion_message::messenger::{CustomOnionMessageHandler, Destination, MessageRouter, OnionMessagePath, OnionMessenger, PendingOnionMessage};
+use lightning::onion_message::messenger::{CustomOnionMessageHandler, Destination, MessageRouter, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction};
 use lightning::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use lightning::onion_message::packet::OnionMessageContents;
 
@@ -97,8 +97,8 @@ impl MessageRouter for TestMessageRouter {
 struct TestOffersMessageHandler {}
 
 impl OffersMessageHandler for TestOffersMessageHandler {
-	fn handle_message(&self, _message: OffersMessage) -> Option<OffersMessage> {
-None
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ResponseInstruction::NoResponse
 	}
 }
 
@@ -112,6 +112,9 @@ impl OnionMessageContents for TestCustomMessage {
 	fn tlv_type(&self) -> u64 {
  CUSTOM_MESSAGE_TYPE
 	}
+	fn msg_type(&self) -> &'static str {
+ "Custom Message"
+	}
 }
 
 impl Writeable for TestCustomMessage {
@@ -124,8 +127,11 @@ struct TestCustomMessageHandler {}
 
 impl CustomOnionMessageHandler for TestCustomMessageHandler {
 	type CustomMessage = TestCustomMessage;
-	fn handle_custom_message(&self, _msg: Self::CustomMessage) -> Option<Self::CustomMessage> {
- Some(TestCustomMessage {})
+	fn handle_custom_message(&self, message: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
+ match responder {
+ Some(responder) => responder.respond(message),
+ None => ResponseInstruction::NoResponse
+ }
 	}
 	fn read_custom_message<R: io::Read>(&self, _message_type: u64, buffer: &mut R) -> Result<Option<Self::CustomMessage>, msgs::DecodeError> {
  let mut buf = Vec::new();
@@ -280,9 +286,9 @@ mod tests {
  "Received an onion message with path_id None and a reply_path: Custom(TestCustomMessage)"
  .to_string())), Some(&1));
  assert_eq!(log_entries.get(&("lightning::onion_message::messenger".to_string(),
- "Constructing onion message when responding to Custom onion message with path_id None: TestCustomMessage".to_string())), Some(&1));
+ "Constructing onion message when responding with Custom Message to an onion message with path_id None: TestCustomMessage".to_string())), Some(&1));
  assert_eq!(log_entries.get(&("lightning::onion_message::messenger".to_string(),
- "Buffered onion message when responding to Custom onion message with path_id None".to_string())), Some(&1));
+ "Buffered onion message when responding with Custom Message to an onion message with path_id None".to_string())), Some(&1));
  }
 
  let two_unblinded_hops_om = "\

lightning/src/ln/channelmanager.rs

@@ -64,7 +64,7 @@ use crate::offers::invoice_request::{DerivedPayerId, InvoiceRequestBuilder};
 use crate::offers::offer::{Offer, OfferBuilder};
 use crate::offers::parse::Bolt12SemanticError;
 use crate::offers::refund::{Refund, RefundBuilder};
-use crate::onion_message::messenger::{Destination, MessageRouter, PendingOnionMessage, new_pending_onion_message};
+use crate::onion_message::messenger::{new_pending_onion_message, Destination, MessageRouter, PendingOnionMessage, Responder, ResponseInstruction};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use crate::sign::{EntropySource, NodeSigner, Recipient, SignerProvider};
 use crate::sign::ecdsa::WriteableEcdsaChannelSigner;
@@ -75,6 +75,7 @@ use crate::util::string::UntrustedString;
 use crate::util::ser::{BigSize, FixedLengthReader, Readable, ReadableArgs, MaybeReadable, Writeable, Writer, VecWriter};
 use crate::util::logger::{Level, Logger, WithContext};
 use crate::util::errors::APIError;
+
 #[cfg(not(c_bindings))]
 use {
 	crate::offers::offer::DerivedMetadata,
@@ -10332,23 +10333,27 @@ where
 	R::Target: Router,
 	L::Target: Logger,
 {
-	fn handle_message(&self, message: OffersMessage) -> Option<OffersMessage> {
+	fn handle_message(&self, message: OffersMessage, responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
  let secp_ctx = &self.secp_ctx;
  let expanded_key = &self.inbound_payment_key;
 
  match message {
  OffersMessage::InvoiceRequest(invoice_request) => {
+ let responder = match responder {
+ Some(responder) => responder,
+ None => return ResponseInstruction::NoResponse,
+ };
  let amount_msats = match InvoiceBuilder::<DerivedSigningPubkey>::amount_msats(
  &invoice_request
  ) {
  Ok(amount_msats) => amount_msats,
- Err(error) => return Some(OffersMessage::InvoiceError(error.into())),
+ Err(error) => return responder.respond(OffersMessage::InvoiceError(error.into())),
  };
  let invoice_request = match invoice_request.verify(expanded_key, secp_ctx) {
  Ok(invoice_request) => invoice_request,
  Err(()) => {
  let error = Bolt12SemanticError::InvalidMetadata;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10359,7 +10364,7 @@ where
  Ok((payment_hash, payment_secret)) => (payment_hash, payment_secret),
  Err(()) => {
  let error = Bolt12SemanticError::InvalidAmount;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10373,7 +10378,7 @@ where
  Ok(payment_paths) => payment_paths,
  Err(()) => {
  let error = Bolt12SemanticError::MissingPaths;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10418,8 +10423,8 @@ where
  };
 
  match response {
- Ok(invoice) => Some(OffersMessage::Invoice(invoice)),
- Err(error) => Some(OffersMessage::InvoiceError(error.into())),
+ Ok(invoice) => return responder.respond(OffersMessage::Invoice(invoice)),
+ Err(error) => return responder.respond(OffersMessage::InvoiceError(error.into())),
  }
  },
  OffersMessage::Invoice(invoice) => {
@@ -10439,14 +10444,21 @@ where
  }
  });
 
- match response {
- Ok(()) => None,
- Err(e) => Some(OffersMessage::InvoiceError(e)),
+ match (responder, response) {
+ (Some(responder), Err(e)) => responder.respond(OffersMessage::InvoiceError(e)),
+ (None, Err(_)) => {
+ log_trace!(
+ self.logger,
+ "A response was generated, but there is no reply_path specified for sending the response."
+ );
+ return ResponseInstruction::NoResponse;
+ }
+ _ => return ResponseInstruction::NoResponse,
  }
  },
  OffersMessage::InvoiceError(invoice_error) => {
  log_trace!(self.logger, "Received invoice_error: {}", invoice_error);
-None
+return ResponseInstruction::NoResponse;
  },
  }
 	}

lightning/src/ln/peer_handler.rs

@@ -28,7 +28,7 @@ use crate::util::ser::{VecWriter, Writeable, Writer};
 use crate::ln::peer_channel_encryptor::{PeerChannelEncryptor, NextNoiseStep, MessageBuf, MSG_BUF_ALLOC_SIZE};
 use crate::ln::wire;
 use crate::ln::wire::{Encode, Type};
-use crate::onion_message::messenger::{CustomOnionMessageHandler, PendingOnionMessage};
+use crate::onion_message::messenger::{CustomOnionMessageHandler, PendingOnionMessage, Responder, ResponseInstruction};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use crate::onion_message::packet::OnionMessageContents;
 use crate::routing::gossip::{NodeId, NodeAlias};
@@ -123,6 +123,7 @@ impl RoutingMessageHandler for IgnoringMessageHandler {
 	}
 	fn processing_queue_high(&self) -> bool { false }
 }
+
 impl OnionMessageHandler for IgnoringMessageHandler {
 	fn handle_onion_message(&self, _their_node_id: &PublicKey, _msg: &msgs::OnionMessage) {}
 	fn next_onion_message_for_peer(&self, _peer_node_id: PublicKey) -> Option<msgs::OnionMessage> { None }
@@ -134,12 +135,15 @@ impl OnionMessageHandler for IgnoringMessageHandler {
  InitFeatures::empty()
 	}
 }
+
 impl OffersMessageHandler for IgnoringMessageHandler {
-	fn handle_message(&self, _msg: OffersMessage) -> Option<OffersMessage> { None }
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ ResponseInstruction::NoResponse
+	}
 }
 impl CustomOnionMessageHandler for IgnoringMessageHandler {
 	type CustomMessage = Infallible;
-	fn handle_custom_message(&self, _msg: Infallible) -> Option<Infallible> {
+	fn handle_custom_message(&self, _message: Self::CustomMessage, _responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
  // Since we always return `None` in the read the handle method should never be called.
  unreachable!();
 	}
@@ -153,6 +157,7 @@ impl CustomOnionMessageHandler for IgnoringMessageHandler {
 
 impl OnionMessageContents for Infallible {
 	fn tlv_type(&self) -> u64 { unreachable!(); }
+	fn msg_type(&self) -> &'static str { unreachable!(); }
 }
 
 impl Deref for IgnoringMessageHandler {

lightning/src/onion_message/functional_tests.rs

@@ -18,7 +18,7 @@ use crate::routing::test_utils::{add_channel, add_or_update_node};
 use crate::sign::{NodeSigner, Recipient};
 use crate::util::ser::{FixedLengthReader, LengthReadable, Writeable, Writer};
 use crate::util::test_utils;
-use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, SendError};
+use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction, SendError};
 use super::offers::{OffersMessage, OffersMessageHandler};
 use super::packet::{OnionMessageContents, Packet};
 
@@ -62,8 +62,8 @@ struct MessengerNode {
 struct TestOffersMessageHandler {}
 
 impl OffersMessageHandler for TestOffersMessageHandler {
-	fn handle_message(&self, _message: OffersMessage) -> Option<OffersMessage> {
-None
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ResponseInstruction::NoResponse
 	}
 }
 
@@ -85,6 +85,9 @@ impl OnionMessageContents for TestCustomMessage {
  TestCustomMessage::Response => CUSTOM_RESPONSE_MESSAGE_TYPE,
  }
 	}
+	fn msg_type(&self) -> &'static str {
+ "Custom Message"
+	}
 }
 
 impl Writeable for TestCustomMessage {
@@ -123,15 +126,19 @@ impl Drop for TestCustomMessageHandler {
 
 impl CustomOnionMessageHandler for TestCustomMessageHandler {
 	type CustomMessage = TestCustomMessage;
-	fn handle_custom_message(&self, msg: Self::CustomMessage) -> Option<Self::CustomMessage> {
+	fn handle_custom_message(&self, msg: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
  match self.expected_messages.lock().unwrap().pop_front() {
  Some(expected_msg) => assert_eq!(expected_msg, msg),
  None => panic!("Unexpected message: {:?}", msg),
  }
-
- match msg {
+ let response_option = match msg {
  TestCustomMessage::Request => Some(TestCustomMessage::Response),
  TestCustomMessage::Response => None,
+ };
+ if let (Some(response), Some(responder)) = (response_option, responder) {
+ responder.respond(response)
+ } else {
+ ResponseInstruction::NoResponse
  }
 	}
 	fn read_custom_message<R: io::Read>(&self, message_type: u64, buffer: &mut R) -> Result<Option<Self::CustomMessage>, DecodeError> where Self: Sized {
@@ -422,6 +429,9 @@ fn invalid_custom_message_type() {
  // Onion message contents must have a TLV >= 64.
  63
  }
+ fn msg_type(&self) -> &'static str {
+ "Invalid Message"
+ }
 	}
 
 	impl Writeable for InvalidCustomMessage {

lightning/src/onion_message/messenger.rs

@@ -135,6 +135,7 @@ pub(super) const MAX_TIMER_TICKS: usize = 2;
 /// # let your_custom_message_type = 42;
 /// your_custom_message_type
 /// }
+/// fn msg_type(&self) -> &'static str { "YourCustomMessageType" }
 /// }
 /// // Send a custom onion message to a node id.
 /// let destination = Destination::Node(destination_node_id);
@@ -246,6 +247,50 @@ impl OnionMessageRecipient {
 	}
 }
 
+
+/// The `Responder` struct creates an appropriate [`ResponseInstruction`]
+/// for responding to a message.
+pub struct Responder {
+	/// The path along which a response can be sent.
+	reply_path: BlindedPath,
+	path_id: Option<[u8; 32]>
+}
+
+impl Responder {
+	/// Creates a new [`Responder`] instance with the provided reply path.
+	fn new(reply_path: BlindedPath, path_id: Option<[u8; 32]>) -> Self {
+ Responder {
+ reply_path,
+ path_id,
+ }
+	}
+
+	/// Creates the appropriate [`ResponseInstruction`] for a given response.
+	pub fn respond<T: OnionMessageContents>(self, response: T) -> ResponseInstruction<T> {
+ ResponseInstruction::WithoutReplyPath(OnionMessageResponse {
+ message: response,
+ reply_path: self.reply_path,
+ path_id: self.path_id,
+ })
+	}
+}
+
+/// This struct contains the information needed to reply to a received message.
+pub struct OnionMessageResponse<T: OnionMessageContents> {
+	message: T,
+	reply_path: BlindedPath,
+	path_id: Option<[u8; 32]>,
+}
+
+/// `ResponseInstruction` represents instructions for responding to received messages.
+pub enum ResponseInstruction<T: OnionMessageContents> {
+	/// Indicates that a response should be sent without including a reply path
+	/// for the recipient to respond back.
+	WithoutReplyPath(OnionMessageResponse<T>),
+	/// Indicates that there's no response to send back.
+	NoResponse,
+}
+
 /// An [`OnionMessage`] for [`OnionMessenger`] to send.
 ///
 /// These are obtained when released from [`OnionMessenger`]'s handlers after which they are
@@ -546,7 +591,7 @@ pub trait CustomOnionMessageHandler {
 	/// Called with the custom message that was received, returning a response to send, if any.
 	///
 	/// The returned [`Self::CustomMessage`], if any, is enqueued to be sent by [`OnionMessenger`].
-	fn handle_custom_message(&self, msg: Self::CustomMessage) -> Option<Self::CustomMessage>;
+	fn handle_custom_message(&self, message: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage>;
 
 	/// Read a custom message of type `message_type` from `buffer`, returning `Ok(None)` if the
 	/// message type is unknown.
@@ -933,19 +978,18 @@ where
 	}
 
 	fn handle_onion_message_response<T: OnionMessageContents>(
- &self, response: Option<T>, reply_path: Option<BlindedPath>, log_suffix: fmt::Arguments
+ &self, response: ResponseInstruction<T>
 	) {
- if let Some(response) = response {
- match reply_path {
- Some(reply_path) => {
- let _ = self.find_path_and_enqueue_onion_message(
- response, Destination::BlindedPath(reply_path), None, log_suffix
- );
- },
- None => {
- log_trace!(self.logger, "Missing reply path {}", log_suffix);
- },
- }
+ if let ResponseInstruction::WithoutReplyPath(response) = response {
+ let message_type = response.message.msg_type();
+ let _ = self.find_path_and_enqueue_onion_message(
+ response.message, Destination::BlindedPath(response.reply_path), None,
+ format_args!(
+ "when responding with {} to an onion message with path_id {:02x?}",
+ message_type,
+ response.path_id
+ )
+ );
  }
 	}
 
@@ -1029,22 +1073,18 @@ where
 
  match message {
  ParsedOnionMessageContents::Offers(msg) => {
- let response = self.offers_handler.handle_message(msg);
- self.handle_onion_message_response(
- response, reply_path, format_args!(
- "when responding to Offers onion message with path_id {:02x?}",
- path_id
- )
+ let responder = reply_path.map(
+ |reply_path| Responder::new(reply_path, path_id)
  );
+ let response_instructions = self.offers_handler.handle_message(msg, responder);
+ self.handle_onion_message_response(response_instructions);
  },
  ParsedOnionMessageContents::Custom(msg) => {
- let response = self.custom_handler.handle_custom_message(msg);
- self.handle_onion_message_response(
- response, reply_path, format_args!(
- "when responding to Custom onion message with path_id {:02x?}",
- path_id
- )
+ let responder = reply_path.map(
+ |reply_path| Responder::new(reply_path, path_id)
  );
+ let response_instructions = self.custom_handler.handle_custom_message(msg, responder);
+ self.handle_onion_message_response(response_instructions);
  },
  }
  },

fuzz/src/onion_message.rs

@@ -16,7 +16,7 @@ use lightning::sign::{Recipient, KeyMaterial, EntropySource, NodeSigner, SignerP
 use lightning::util::test_channel_signer::TestChannelSigner;
 use lightning::util::logger::Logger;
 use lightning::util::ser::{Readable, Writeable, Writer};
-use lightning::onion_message::messenger::{CustomOnionMessageHandler, Destination, MessageRouter, OnionMessagePath, OnionMessenger, PendingOnionMessage};
+use lightning::onion_message::messenger::{CustomOnionMessageHandler, Destination, MessageRouter, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction};
 use lightning::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use lightning::onion_message::packet::OnionMessageContents;
 
@@ -97,8 +97,8 @@ impl MessageRouter for TestMessageRouter {
 struct TestOffersMessageHandler {}
 
 impl OffersMessageHandler for TestOffersMessageHandler {
-	fn handle_message(&self, _message: OffersMessage) -> Option<OffersMessage> {
-None
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ResponseInstruction::NoResponse
 	}
 }
 
@@ -112,6 +112,9 @@ impl OnionMessageContents for TestCustomMessage {
 	fn tlv_type(&self) -> u64 {
  CUSTOM_MESSAGE_TYPE
 	}
+	fn msg_type(&self) -> &'static str {
+ "Custom Message"
+	}
 }
 
 impl Writeable for TestCustomMessage {
@@ -124,8 +127,11 @@ struct TestCustomMessageHandler {}
 
 impl CustomOnionMessageHandler for TestCustomMessageHandler {
 	type CustomMessage = TestCustomMessage;
-	fn handle_custom_message(&self, _msg: Self::CustomMessage) -> Option<Self::CustomMessage> {
- Some(TestCustomMessage {})
+	fn handle_custom_message(&self, message: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
+ match responder {
+ Some(responder) => responder.respond(message),
+ None => ResponseInstruction::NoResponse
+ }
 	}
 	fn read_custom_message<R: io::Read>(&self, _message_type: u64, buffer: &mut R) -> Result<Option<Self::CustomMessage>, msgs::DecodeError> {
  let mut buf = Vec::new();
@@ -280,9 +286,9 @@ mod tests {
  "Received an onion message with path_id None and a reply_path: Custom(TestCustomMessage)"
  .to_string())), Some(&1));
  assert_eq!(log_entries.get(&("lightning::onion_message::messenger".to_string(),
- "Constructing onion message when responding to Custom onion message with path_id None: TestCustomMessage".to_string())), Some(&1));
+ "Constructing onion message when responding with Custom Message to an onion message with path_id None: TestCustomMessage".to_string())), Some(&1));
  assert_eq!(log_entries.get(&("lightning::onion_message::messenger".to_string(),
- "Buffered onion message when responding to Custom onion message with path_id None".to_string())), Some(&1));
+ "Buffered onion message when responding with Custom Message to an onion message with path_id None".to_string())), Some(&1));
  }
 
  let two_unblinded_hops_om = "\

lightning/src/ln/channelmanager.rs

@@ -64,7 +64,7 @@ use crate::offers::invoice_request::{DerivedPayerId, InvoiceRequestBuilder};
 use crate::offers::offer::{Offer, OfferBuilder};
 use crate::offers::parse::Bolt12SemanticError;
 use crate::offers::refund::{Refund, RefundBuilder};
-use crate::onion_message::messenger::{Destination, MessageRouter, PendingOnionMessage, new_pending_onion_message};
+use crate::onion_message::messenger::{new_pending_onion_message, Destination, MessageRouter, PendingOnionMessage, Responder, ResponseInstruction};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use crate::sign::{EntropySource, NodeSigner, Recipient, SignerProvider};
 use crate::sign::ecdsa::WriteableEcdsaChannelSigner;
@@ -75,6 +75,7 @@ use crate::util::string::UntrustedString;
 use crate::util::ser::{BigSize, FixedLengthReader, Readable, ReadableArgs, MaybeReadable, Writeable, Writer, VecWriter};
 use crate::util::logger::{Level, Logger, WithContext};
 use crate::util::errors::APIError;
+
 #[cfg(not(c_bindings))]
 use {
 	crate::offers::offer::DerivedMetadata,
@@ -10332,23 +10333,27 @@ where
 	R::Target: Router,
 	L::Target: Logger,
 {
-	fn handle_message(&self, message: OffersMessage) -> Option<OffersMessage> {
+	fn handle_message(&self, message: OffersMessage, responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
  let secp_ctx = &self.secp_ctx;
  let expanded_key = &self.inbound_payment_key;
 
  match message {
  OffersMessage::InvoiceRequest(invoice_request) => {
+ let responder = match responder {
+ Some(responder) => responder,
+ None => return ResponseInstruction::NoResponse,
+ };
  let amount_msats = match InvoiceBuilder::<DerivedSigningPubkey>::amount_msats(
  &invoice_request
  ) {
  Ok(amount_msats) => amount_msats,
- Err(error) => return Some(OffersMessage::InvoiceError(error.into())),
+ Err(error) => return responder.respond(OffersMessage::InvoiceError(error.into())),
  };
  let invoice_request = match invoice_request.verify(expanded_key, secp_ctx) {
  Ok(invoice_request) => invoice_request,
  Err(()) => {
  let error = Bolt12SemanticError::InvalidMetadata;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10359,7 +10364,7 @@ where
  Ok((payment_hash, payment_secret)) => (payment_hash, payment_secret),
  Err(()) => {
  let error = Bolt12SemanticError::InvalidAmount;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10373,7 +10378,7 @@ where
  Ok(payment_paths) => payment_paths,
  Err(()) => {
  let error = Bolt12SemanticError::MissingPaths;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10418,8 +10423,8 @@ where
  };
 
  match response {
- Ok(invoice) => Some(OffersMessage::Invoice(invoice)),
- Err(error) => Some(OffersMessage::InvoiceError(error.into())),
+ Ok(invoice) => return responder.respond(OffersMessage::Invoice(invoice)),
+ Err(error) => return responder.respond(OffersMessage::InvoiceError(error.into())),
  }
  },
  OffersMessage::Invoice(invoice) => {
@@ -10439,14 +10444,21 @@ where
  }
  });
 
- match response {
- Ok(()) => None,
- Err(e) => Some(OffersMessage::InvoiceError(e)),
+ match (responder, response) {
+ (Some(responder), Err(e)) => responder.respond(OffersMessage::InvoiceError(e)),
+ (None, Err(_)) => {
+ log_trace!(
+ self.logger,
+ "A response was generated, but there is no reply_path specified for sending the response."
+ );
+ return ResponseInstruction::NoResponse;
+ }
+ _ => return ResponseInstruction::NoResponse,
  }
  },
  OffersMessage::InvoiceError(invoice_error) => {
  log_trace!(self.logger, "Received invoice_error: {}", invoice_error);
-None
+return ResponseInstruction::NoResponse;
  },
  }
 	}

lightning/src/ln/peer_handler.rs

@@ -28,7 +28,7 @@ use crate::util::ser::{VecWriter, Writeable, Writer};
 use crate::ln::peer_channel_encryptor::{PeerChannelEncryptor, NextNoiseStep, MessageBuf, MSG_BUF_ALLOC_SIZE};
 use crate::ln::wire;
 use crate::ln::wire::{Encode, Type};
-use crate::onion_message::messenger::{CustomOnionMessageHandler, PendingOnionMessage};
+use crate::onion_message::messenger::{CustomOnionMessageHandler, PendingOnionMessage, Responder, ResponseInstruction};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use crate::onion_message::packet::OnionMessageContents;
 use crate::routing::gossip::{NodeId, NodeAlias};
@@ -123,6 +123,7 @@ impl RoutingMessageHandler for IgnoringMessageHandler {
 	}
 	fn processing_queue_high(&self) -> bool { false }
 }
+
 impl OnionMessageHandler for IgnoringMessageHandler {
 	fn handle_onion_message(&self, _their_node_id: &PublicKey, _msg: &msgs::OnionMessage) {}
 	fn next_onion_message_for_peer(&self, _peer_node_id: PublicKey) -> Option<msgs::OnionMessage> { None }
@@ -134,12 +135,15 @@ impl OnionMessageHandler for IgnoringMessageHandler {
  InitFeatures::empty()
 	}
 }
+
 impl OffersMessageHandler for IgnoringMessageHandler {
-	fn handle_message(&self, _msg: OffersMessage) -> Option<OffersMessage> { None }
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ ResponseInstruction::NoResponse
+	}
 }
 impl CustomOnionMessageHandler for IgnoringMessageHandler {
 	type CustomMessage = Infallible;
-	fn handle_custom_message(&self, _msg: Infallible) -> Option<Infallible> {
+	fn handle_custom_message(&self, _message: Self::CustomMessage, _responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
  // Since we always return `None` in the read the handle method should never be called.
  unreachable!();
 	}
@@ -153,6 +157,7 @@ impl CustomOnionMessageHandler for IgnoringMessageHandler {
 
 impl OnionMessageContents for Infallible {
 	fn tlv_type(&self) -> u64 { unreachable!(); }
+	fn msg_type(&self) -> &'static str { unreachable!(); }
 }
 
 impl Deref for IgnoringMessageHandler {

lightning/src/onion_message/functional_tests.rs

@@ -18,7 +18,7 @@ use crate::routing::test_utils::{add_channel, add_or_update_node};
 use crate::sign::{NodeSigner, Recipient};
 use crate::util::ser::{FixedLengthReader, LengthReadable, Writeable, Writer};
 use crate::util::test_utils;
-use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, SendError};
+use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction, SendError};
 use super::offers::{OffersMessage, OffersMessageHandler};
 use super::packet::{OnionMessageContents, Packet};
 
@@ -62,8 +62,8 @@ struct MessengerNode {
 struct TestOffersMessageHandler {}
 
 impl OffersMessageHandler for TestOffersMessageHandler {
-	fn handle_message(&self, _message: OffersMessage) -> Option<OffersMessage> {
-None
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ResponseInstruction::NoResponse
 	}
 }
 
@@ -85,6 +85,9 @@ impl OnionMessageContents for TestCustomMessage {
  TestCustomMessage::Response => CUSTOM_RESPONSE_MESSAGE_TYPE,
  }
 	}
+	fn msg_type(&self) -> &'static str {
+ "Custom Message"
+	}
 }
 
 impl Writeable for TestCustomMessage {
@@ -123,15 +126,19 @@ impl Drop for TestCustomMessageHandler {
 
 impl CustomOnionMessageHandler for TestCustomMessageHandler {
 	type CustomMessage = TestCustomMessage;
-	fn handle_custom_message(&self, msg: Self::CustomMessage) -> Option<Self::CustomMessage> {
+	fn handle_custom_message(&self, msg: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
  match self.expected_messages.lock().unwrap().pop_front() {
  Some(expected_msg) => assert_eq!(expected_msg, msg),
  None => panic!("Unexpected message: {:?}", msg),
  }
-
- match msg {
+ let response_option = match msg {
  TestCustomMessage::Request => Some(TestCustomMessage::Response),
  TestCustomMessage::Response => None,
+ };
+ if let (Some(response), Some(responder)) = (response_option, responder) {
+ responder.respond(response)
+ } else {
+ ResponseInstruction::NoResponse
  }
 	}
 	fn read_custom_message<R: io::Read>(&self, message_type: u64, buffer: &mut R) -> Result<Option<Self::CustomMessage>, DecodeError> where Self: Sized {
@@ -422,6 +429,9 @@ fn invalid_custom_message_type() {
  // Onion message contents must have a TLV >= 64.
  63
  }
+ fn msg_type(&self) -> &'static str {
+ "Invalid Message"
+ }
 	}
 
 	impl Writeable for InvalidCustomMessage {

lightning/src/onion_message/messenger.rs

@@ -135,6 +135,7 @@ pub(super) const MAX_TIMER_TICKS: usize = 2;
 /// # let your_custom_message_type = 42;
 /// your_custom_message_type
 /// }
+/// fn msg_type(&self) -> &'static str { "YourCustomMessageType" }
 /// }
 /// // Send a custom onion message to a node id.
 /// let destination = Destination::Node(destination_node_id);
@@ -246,6 +247,50 @@ impl OnionMessageRecipient {
 	}
 }
 
+
+/// The `Responder` struct creates an appropriate [`ResponseInstruction`]
+/// for responding to a message.
+pub struct Responder {
+	/// The path along which a response can be sent.
+	reply_path: BlindedPath,
+	path_id: Option<[u8; 32]>
+}
+
+impl Responder {
+	/// Creates a new [`Responder`] instance with the provided reply path.
+	fn new(reply_path: BlindedPath, path_id: Option<[u8; 32]>) -> Self {
+ Responder {
+ reply_path,
+ path_id,
+ }
+	}
+
+	/// Creates the appropriate [`ResponseInstruction`] for a given response.
+	pub fn respond<T: OnionMessageContents>(self, response: T) -> ResponseInstruction<T> {
+ ResponseInstruction::WithoutReplyPath(OnionMessageResponse {
+ message: response,
+ reply_path: self.reply_path,
+ path_id: self.path_id,
+ })
+	}
+}
+
+/// This struct contains the information needed to reply to a received message.
+pub struct OnionMessageResponse<T: OnionMessageContents> {
+	message: T,
+	reply_path: BlindedPath,
+	path_id: Option<[u8; 32]>,
+}
+
+/// `ResponseInstruction` represents instructions for responding to received messages.
+pub enum ResponseInstruction<T: OnionMessageContents> {
+	/// Indicates that a response should be sent without including a reply path
+	/// for the recipient to respond back.
+	WithoutReplyPath(OnionMessageResponse<T>),
+	/// Indicates that there's no response to send back.
+	NoResponse,
+}
+
 /// An [`OnionMessage`] for [`OnionMessenger`] to send.
 ///
 /// These are obtained when released from [`OnionMessenger`]'s handlers after which they are
@@ -546,7 +591,7 @@ pub trait CustomOnionMessageHandler {
 	/// Called with the custom message that was received, returning a response to send, if any.
 	///
 	/// The returned [`Self::CustomMessage`], if any, is enqueued to be sent by [`OnionMessenger`].
-	fn handle_custom_message(&self, msg: Self::CustomMessage) -> Option<Self::CustomMessage>;
+	fn handle_custom_message(&self, message: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage>;
 
 	/// Read a custom message of type `message_type` from `buffer`, returning `Ok(None)` if the
 	/// message type is unknown.
@@ -933,19 +978,18 @@ where
 	}
 
 	fn handle_onion_message_response<T: OnionMessageContents>(
- &self, response: Option<T>, reply_path: Option<BlindedPath>, log_suffix: fmt::Arguments
+ &self, response: ResponseInstruction<T>
 	) {
- if let Some(response) = response {
- match reply_path {
- Some(reply_path) => {
- let _ = self.find_path_and_enqueue_onion_message(
- response, Destination::BlindedPath(reply_path), None, log_suffix
- );
- },
- None => {
- log_trace!(self.logger, "Missing reply path {}", log_suffix);
- },
- }
+ if let ResponseInstruction::WithoutReplyPath(response) = response {
+ let message_type = response.message.msg_type();
+ let _ = self.find_path_and_enqueue_onion_message(
+ response.message, Destination::BlindedPath(response.reply_path), None,
+ format_args!(
+ "when responding with {} to an onion message with path_id {:02x?}",
+ message_type,
+ response.path_id
+ )
+ );
  }
 	}
 
@@ -1029,22 +1073,18 @@ where
 
  match message {
  ParsedOnionMessageContents::Offers(msg) => {
- let response = self.offers_handler.handle_message(msg);
- self.handle_onion_message_response(
- response, reply_path, format_args!(
- "when responding to Offers onion message with path_id {:02x?}",
- path_id
- )
+ let responder = reply_path.map(
+ |reply_path| Responder::new(reply_path, path_id)
  );
+ let response_instructions = self.offers_handler.handle_message(msg, responder);
+ self.handle_onion_message_response(response_instructions);
  },
  ParsedOnionMessageContents::Custom(msg) => {
- let response = self.custom_handler.handle_custom_message(msg);
- self.handle_onion_message_response(
- response, reply_path, format_args!(
- "when responding to Custom onion message with path_id {:02x?}",
- path_id
- )
+ let responder = reply_path.map(
+ |reply_path| Responder::new(reply_path, path_id)
  );
+ let response_instructions = self.custom_handler.handle_custom_message(msg, responder);
+ self.handle_onion_message_response(response_instructions);
  },
  }
  },

fuzz/src/onion_message.rs

@@ -16,7 +16,7 @@ use lightning::sign::{Recipient, KeyMaterial, EntropySource, NodeSigner, SignerP
 use lightning::util::test_channel_signer::TestChannelSigner;
 use lightning::util::logger::Logger;
 use lightning::util::ser::{Readable, Writeable, Writer};
-use lightning::onion_message::messenger::{CustomOnionMessageHandler, Destination, MessageRouter, OnionMessagePath, OnionMessenger, PendingOnionMessage};
+use lightning::onion_message::messenger::{CustomOnionMessageHandler, Destination, MessageRouter, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction};
 use lightning::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use lightning::onion_message::packet::OnionMessageContents;
 
@@ -97,8 +97,8 @@ impl MessageRouter for TestMessageRouter {
 struct TestOffersMessageHandler {}
 
 impl OffersMessageHandler for TestOffersMessageHandler {
-	fn handle_message(&self, _message: OffersMessage) -> Option<OffersMessage> {
-None
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ResponseInstruction::NoResponse
 	}
 }
 
@@ -112,6 +112,9 @@ impl OnionMessageContents for TestCustomMessage {
 	fn tlv_type(&self) -> u64 {
  CUSTOM_MESSAGE_TYPE
 	}
+	fn msg_type(&self) -> &'static str {
+ "Custom Message"
+	}
 }
 
 impl Writeable for TestCustomMessage {
@@ -124,8 +127,11 @@ struct TestCustomMessageHandler {}
 
 impl CustomOnionMessageHandler for TestCustomMessageHandler {
 	type CustomMessage = TestCustomMessage;
-	fn handle_custom_message(&self, _msg: Self::CustomMessage) -> Option<Self::CustomMessage> {
- Some(TestCustomMessage {})
+	fn handle_custom_message(&self, message: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
+ match responder {
+ Some(responder) => responder.respond(message),
+ None => ResponseInstruction::NoResponse
+ }
 	}
 	fn read_custom_message<R: io::Read>(&self, _message_type: u64, buffer: &mut R) -> Result<Option<Self::CustomMessage>, msgs::DecodeError> {
  let mut buf = Vec::new();
@@ -280,9 +286,9 @@ mod tests {
  "Received an onion message with path_id None and a reply_path: Custom(TestCustomMessage)"
  .to_string())), Some(&1));
  assert_eq!(log_entries.get(&("lightning::onion_message::messenger".to_string(),
- "Constructing onion message when responding to Custom onion message with path_id None: TestCustomMessage".to_string())), Some(&1));
+ "Constructing onion message when responding with Custom Message to an onion message with path_id None: TestCustomMessage".to_string())), Some(&1));
  assert_eq!(log_entries.get(&("lightning::onion_message::messenger".to_string(),
- "Buffered onion message when responding to Custom onion message with path_id None".to_string())), Some(&1));
+ "Buffered onion message when responding with Custom Message to an onion message with path_id None".to_string())), Some(&1));
  }
 
  let two_unblinded_hops_om = "\

lightning/src/ln/channelmanager.rs

@@ -64,7 +64,7 @@ use crate::offers::invoice_request::{DerivedPayerId, InvoiceRequestBuilder};
 use crate::offers::offer::{Offer, OfferBuilder};
 use crate::offers::parse::Bolt12SemanticError;
 use crate::offers::refund::{Refund, RefundBuilder};
-use crate::onion_message::messenger::{Destination, MessageRouter, PendingOnionMessage, new_pending_onion_message};
+use crate::onion_message::messenger::{new_pending_onion_message, Destination, MessageRouter, PendingOnionMessage, Responder, ResponseInstruction};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use crate::sign::{EntropySource, NodeSigner, Recipient, SignerProvider};
 use crate::sign::ecdsa::WriteableEcdsaChannelSigner;
@@ -75,6 +75,7 @@ use crate::util::string::UntrustedString;
 use crate::util::ser::{BigSize, FixedLengthReader, Readable, ReadableArgs, MaybeReadable, Writeable, Writer, VecWriter};
 use crate::util::logger::{Level, Logger, WithContext};
 use crate::util::errors::APIError;
+
 #[cfg(not(c_bindings))]
 use {
 	crate::offers::offer::DerivedMetadata,
@@ -10332,23 +10333,27 @@ where
 	R::Target: Router,
 	L::Target: Logger,
 {
-	fn handle_message(&self, message: OffersMessage) -> Option<OffersMessage> {
+	fn handle_message(&self, message: OffersMessage, responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
  let secp_ctx = &self.secp_ctx;
  let expanded_key = &self.inbound_payment_key;
 
  match message {
  OffersMessage::InvoiceRequest(invoice_request) => {
+ let responder = match responder {
+ Some(responder) => responder,
+ None => return ResponseInstruction::NoResponse,
+ };
  let amount_msats = match InvoiceBuilder::<DerivedSigningPubkey>::amount_msats(
  &invoice_request
  ) {
  Ok(amount_msats) => amount_msats,
- Err(error) => return Some(OffersMessage::InvoiceError(error.into())),
+ Err(error) => return responder.respond(OffersMessage::InvoiceError(error.into())),
  };
  let invoice_request = match invoice_request.verify(expanded_key, secp_ctx) {
  Ok(invoice_request) => invoice_request,
  Err(()) => {
  let error = Bolt12SemanticError::InvalidMetadata;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10359,7 +10364,7 @@ where
  Ok((payment_hash, payment_secret)) => (payment_hash, payment_secret),
  Err(()) => {
  let error = Bolt12SemanticError::InvalidAmount;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10373,7 +10378,7 @@ where
  Ok(payment_paths) => payment_paths,
  Err(()) => {
  let error = Bolt12SemanticError::MissingPaths;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10418,8 +10423,8 @@ where
  };
 
  match response {
- Ok(invoice) => Some(OffersMessage::Invoice(invoice)),
- Err(error) => Some(OffersMessage::InvoiceError(error.into())),
+ Ok(invoice) => return responder.respond(OffersMessage::Invoice(invoice)),
+ Err(error) => return responder.respond(OffersMessage::InvoiceError(error.into())),
  }
  },
  OffersMessage::Invoice(invoice) => {
@@ -10439,14 +10444,21 @@ where
  }
  });
 
- match response {
- Ok(()) => None,
- Err(e) => Some(OffersMessage::InvoiceError(e)),
+ match (responder, response) {
+ (Some(responder), Err(e)) => responder.respond(OffersMessage::InvoiceError(e)),
+ (None, Err(_)) => {
+ log_trace!(
+ self.logger,
+ "A response was generated, but there is no reply_path specified for sending the response."
+ );
+ return ResponseInstruction::NoResponse;
+ }
+ _ => return ResponseInstruction::NoResponse,
  }
  },
  OffersMessage::InvoiceError(invoice_error) => {
  log_trace!(self.logger, "Received invoice_error: {}", invoice_error);
-None
+return ResponseInstruction::NoResponse;
  },
  }
 	}

lightning/src/ln/peer_handler.rs

@@ -28,7 +28,7 @@ use crate::util::ser::{VecWriter, Writeable, Writer};
 use crate::ln::peer_channel_encryptor::{PeerChannelEncryptor, NextNoiseStep, MessageBuf, MSG_BUF_ALLOC_SIZE};
 use crate::ln::wire;
 use crate::ln::wire::{Encode, Type};
-use crate::onion_message::messenger::{CustomOnionMessageHandler, PendingOnionMessage};
+use crate::onion_message::messenger::{CustomOnionMessageHandler, PendingOnionMessage, Responder, ResponseInstruction};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use crate::onion_message::packet::OnionMessageContents;
 use crate::routing::gossip::{NodeId, NodeAlias};
@@ -123,6 +123,7 @@ impl RoutingMessageHandler for IgnoringMessageHandler {
 	}
 	fn processing_queue_high(&self) -> bool { false }
 }
+
 impl OnionMessageHandler for IgnoringMessageHandler {
 	fn handle_onion_message(&self, _their_node_id: &PublicKey, _msg: &msgs::OnionMessage) {}
 	fn next_onion_message_for_peer(&self, _peer_node_id: PublicKey) -> Option<msgs::OnionMessage> { None }
@@ -134,12 +135,15 @@ impl OnionMessageHandler for IgnoringMessageHandler {
  InitFeatures::empty()
 	}
 }
+
 impl OffersMessageHandler for IgnoringMessageHandler {
-	fn handle_message(&self, _msg: OffersMessage) -> Option<OffersMessage> { None }
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ ResponseInstruction::NoResponse
+	}
 }
 impl CustomOnionMessageHandler for IgnoringMessageHandler {
 	type CustomMessage = Infallible;
-	fn handle_custom_message(&self, _msg: Infallible) -> Option<Infallible> {
+	fn handle_custom_message(&self, _message: Self::CustomMessage, _responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
  // Since we always return `None` in the read the handle method should never be called.
  unreachable!();
 	}
@@ -153,6 +157,7 @@ impl CustomOnionMessageHandler for IgnoringMessageHandler {
 
 impl OnionMessageContents for Infallible {
 	fn tlv_type(&self) -> u64 { unreachable!(); }
+	fn msg_type(&self) -> &'static str { unreachable!(); }
 }
 
 impl Deref for IgnoringMessageHandler {

lightning/src/onion_message/functional_tests.rs

@@ -18,7 +18,7 @@ use crate::routing::test_utils::{add_channel, add_or_update_node};
 use crate::sign::{NodeSigner, Recipient};
 use crate::util::ser::{FixedLengthReader, LengthReadable, Writeable, Writer};
 use crate::util::test_utils;
-use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, SendError};
+use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction, SendError};
 use super::offers::{OffersMessage, OffersMessageHandler};
 use super::packet::{OnionMessageContents, Packet};
 
@@ -62,8 +62,8 @@ struct MessengerNode {
 struct TestOffersMessageHandler {}
 
 impl OffersMessageHandler for TestOffersMessageHandler {
-	fn handle_message(&self, _message: OffersMessage) -> Option<OffersMessage> {
-None
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ResponseInstruction::NoResponse
 	}
 }
 
@@ -85,6 +85,9 @@ impl OnionMessageContents for TestCustomMessage {
  TestCustomMessage::Response => CUSTOM_RESPONSE_MESSAGE_TYPE,
  }
 	}
+	fn msg_type(&self) -> &'static str {
+ "Custom Message"
+	}
 }
 
 impl Writeable for TestCustomMessage {
@@ -123,15 +126,19 @@ impl Drop for TestCustomMessageHandler {
 
 impl CustomOnionMessageHandler for TestCustomMessageHandler {
 	type CustomMessage = TestCustomMessage;
-	fn handle_custom_message(&self, msg: Self::CustomMessage) -> Option<Self::CustomMessage> {
+	fn handle_custom_message(&self, msg: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
  match self.expected_messages.lock().unwrap().pop_front() {
  Some(expected_msg) => assert_eq!(expected_msg, msg),
  None => panic!("Unexpected message: {:?}", msg),
  }
-
- match msg {
+ let response_option = match msg {
  TestCustomMessage::Request => Some(TestCustomMessage::Response),
  TestCustomMessage::Response => None,
+ };
+ if let (Some(response), Some(responder)) = (response_option, responder) {
+ responder.respond(response)
+ } else {
+ ResponseInstruction::NoResponse
  }
 	}
 	fn read_custom_message<R: io::Read>(&self, message_type: u64, buffer: &mut R) -> Result<Option<Self::CustomMessage>, DecodeError> where Self: Sized {
@@ -422,6 +429,9 @@ fn invalid_custom_message_type() {
  // Onion message contents must have a TLV >= 64.
  63
  }
+ fn msg_type(&self) -> &'static str {
+ "Invalid Message"
+ }
 	}
 
 	impl Writeable for InvalidCustomMessage {

lightning/src/onion_message/messenger.rs

@@ -135,6 +135,7 @@ pub(super) const MAX_TIMER_TICKS: usize = 2;
 /// # let your_custom_message_type = 42;
 /// your_custom_message_type
 /// }
+/// fn msg_type(&self) -> &'static str { "YourCustomMessageType" }
 /// }
 /// // Send a custom onion message to a node id.
 /// let destination = Destination::Node(destination_node_id);
@@ -246,6 +247,50 @@ impl OnionMessageRecipient {
 	}
 }
 
+
+/// The `Responder` struct creates an appropriate [`ResponseInstruction`]
+/// for responding to a message.
+pub struct Responder {
+	/// The path along which a response can be sent.
+	reply_path: BlindedPath,
+	path_id: Option<[u8; 32]>
+}
+
+impl Responder {
+	/// Creates a new [`Responder`] instance with the provided reply path.
+	fn new(reply_path: BlindedPath, path_id: Option<[u8; 32]>) -> Self {
+ Responder {
+ reply_path,
+ path_id,
+ }
+	}
+
+	/// Creates the appropriate [`ResponseInstruction`] for a given response.
+	pub fn respond<T: OnionMessageContents>(self, response: T) -> ResponseInstruction<T> {
+ ResponseInstruction::WithoutReplyPath(OnionMessageResponse {
+ message: response,
+ reply_path: self.reply_path,
+ path_id: self.path_id,
+ })
+	}
+}
+
+/// This struct contains the information needed to reply to a received message.
+pub struct OnionMessageResponse<T: OnionMessageContents> {
+	message: T,
+	reply_path: BlindedPath,
+	path_id: Option<[u8; 32]>,
+}
+
+/// `ResponseInstruction` represents instructions for responding to received messages.
+pub enum ResponseInstruction<T: OnionMessageContents> {
+	/// Indicates that a response should be sent without including a reply path
+	/// for the recipient to respond back.
+	WithoutReplyPath(OnionMessageResponse<T>),
+	/// Indicates that there's no response to send back.
+	NoResponse,
+}
+
 /// An [`OnionMessage`] for [`OnionMessenger`] to send.
 ///
 /// These are obtained when released from [`OnionMessenger`]'s handlers after which they are
@@ -546,7 +591,7 @@ pub trait CustomOnionMessageHandler {
 	/// Called with the custom message that was received, returning a response to send, if any.
 	///
 	/// The returned [`Self::CustomMessage`], if any, is enqueued to be sent by [`OnionMessenger`].
-	fn handle_custom_message(&self, msg: Self::CustomMessage) -> Option<Self::CustomMessage>;
+	fn handle_custom_message(&self, message: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage>;
 
 	/// Read a custom message of type `message_type` from `buffer`, returning `Ok(None)` if the
 	/// message type is unknown.
@@ -933,19 +978,18 @@ where
 	}
 
 	fn handle_onion_message_response<T: OnionMessageContents>(
- &self, response: Option<T>, reply_path: Option<BlindedPath>, log_suffix: fmt::Arguments
+ &self, response: ResponseInstruction<T>
 	) {
- if let Some(response) = response {
- match reply_path {
- Some(reply_path) => {
- let _ = self.find_path_and_enqueue_onion_message(
- response, Destination::BlindedPath(reply_path), None, log_suffix
- );
- },
- None => {
- log_trace!(self.logger, "Missing reply path {}", log_suffix);
- },
- }
+ if let ResponseInstruction::WithoutReplyPath(response) = response {
+ let message_type = response.message.msg_type();
+ let _ = self.find_path_and_enqueue_onion_message(
+ response.message, Destination::BlindedPath(response.reply_path), None,
+ format_args!(
+ "when responding with {} to an onion message with path_id {:02x?}",
+ message_type,
+ response.path_id
+ )
+ );
  }
 	}
 
@@ -1029,22 +1073,18 @@ where
 
  match message {
  ParsedOnionMessageContents::Offers(msg) => {
- let response = self.offers_handler.handle_message(msg);
- self.handle_onion_message_response(
- response, reply_path, format_args!(
- "when responding to Offers onion message with path_id {:02x?}",
- path_id
- )
+ let responder = reply_path.map(
+ |reply_path| Responder::new(reply_path, path_id)
  );
+ let response_instructions = self.offers_handler.handle_message(msg, responder);
+ self.handle_onion_message_response(response_instructions);
  },
  ParsedOnionMessageContents::Custom(msg) => {
- let response = self.custom_handler.handle_custom_message(msg);
- self.handle_onion_message_response(
- response, reply_path, format_args!(
- "when responding to Custom onion message with path_id {:02x?}",
- path_id
- )
+ let responder = reply_path.map(
+ |reply_path| Responder::new(reply_path, path_id)
  );
+ let response_instructions = self.custom_handler.handle_custom_message(msg, responder);
+ self.handle_onion_message_response(response_instructions);
  },
  }
  },

fuzz/src/onion_message.rs

@@ -16,7 +16,7 @@ use lightning::sign::{Recipient, KeyMaterial, EntropySource, NodeSigner, SignerP
 use lightning::util::test_channel_signer::TestChannelSigner;
 use lightning::util::logger::Logger;
 use lightning::util::ser::{Readable, Writeable, Writer};
-use lightning::onion_message::messenger::{CustomOnionMessageHandler, Destination, MessageRouter, OnionMessagePath, OnionMessenger, PendingOnionMessage};
+use lightning::onion_message::messenger::{CustomOnionMessageHandler, Destination, MessageRouter, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction};
 use lightning::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use lightning::onion_message::packet::OnionMessageContents;
 
@@ -97,8 +97,8 @@ impl MessageRouter for TestMessageRouter {
 struct TestOffersMessageHandler {}
 
 impl OffersMessageHandler for TestOffersMessageHandler {
-	fn handle_message(&self, _message: OffersMessage) -> Option<OffersMessage> {
-None
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ResponseInstruction::NoResponse
 	}
 }
 
@@ -112,6 +112,9 @@ impl OnionMessageContents for TestCustomMessage {
 	fn tlv_type(&self) -> u64 {
  CUSTOM_MESSAGE_TYPE
 	}
+	fn msg_type(&self) -> &'static str {
+ "Custom Message"
+	}
 }
 
 impl Writeable for TestCustomMessage {
@@ -124,8 +127,11 @@ struct TestCustomMessageHandler {}
 
 impl CustomOnionMessageHandler for TestCustomMessageHandler {
 	type CustomMessage = TestCustomMessage;
-	fn handle_custom_message(&self, _msg: Self::CustomMessage) -> Option<Self::CustomMessage> {
- Some(TestCustomMessage {})
+	fn handle_custom_message(&self, message: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
+ match responder {
+ Some(responder) => responder.respond(message),
+ None => ResponseInstruction::NoResponse
+ }
 	}
 	fn read_custom_message<R: io::Read>(&self, _message_type: u64, buffer: &mut R) -> Result<Option<Self::CustomMessage>, msgs::DecodeError> {
  let mut buf = Vec::new();
@@ -280,9 +286,9 @@ mod tests {
  "Received an onion message with path_id None and a reply_path: Custom(TestCustomMessage)"
  .to_string())), Some(&1));
  assert_eq!(log_entries.get(&("lightning::onion_message::messenger".to_string(),
- "Constructing onion message when responding to Custom onion message with path_id None: TestCustomMessage".to_string())), Some(&1));
+ "Constructing onion message when responding with Custom Message to an onion message with path_id None: TestCustomMessage".to_string())), Some(&1));
  assert_eq!(log_entries.get(&("lightning::onion_message::messenger".to_string(),
- "Buffered onion message when responding to Custom onion message with path_id None".to_string())), Some(&1));
+ "Buffered onion message when responding with Custom Message to an onion message with path_id None".to_string())), Some(&1));
  }
 
  let two_unblinded_hops_om = "\

lightning/src/ln/channelmanager.rs

@@ -64,7 +64,7 @@ use crate::offers::invoice_request::{DerivedPayerId, InvoiceRequestBuilder};
 use crate::offers::offer::{Offer, OfferBuilder};
 use crate::offers::parse::Bolt12SemanticError;
 use crate::offers::refund::{Refund, RefundBuilder};
-use crate::onion_message::messenger::{Destination, MessageRouter, PendingOnionMessage, new_pending_onion_message};
+use crate::onion_message::messenger::{new_pending_onion_message, Destination, MessageRouter, PendingOnionMessage, Responder, ResponseInstruction};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use crate::sign::{EntropySource, NodeSigner, Recipient, SignerProvider};
 use crate::sign::ecdsa::WriteableEcdsaChannelSigner;
@@ -75,6 +75,7 @@ use crate::util::string::UntrustedString;
 use crate::util::ser::{BigSize, FixedLengthReader, Readable, ReadableArgs, MaybeReadable, Writeable, Writer, VecWriter};
 use crate::util::logger::{Level, Logger, WithContext};
 use crate::util::errors::APIError;
+
 #[cfg(not(c_bindings))]
 use {
 	crate::offers::offer::DerivedMetadata,
@@ -10332,23 +10333,27 @@ where
 	R::Target: Router,
 	L::Target: Logger,
 {
-	fn handle_message(&self, message: OffersMessage) -> Option<OffersMessage> {
+	fn handle_message(&self, message: OffersMessage, responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
  let secp_ctx = &self.secp_ctx;
  let expanded_key = &self.inbound_payment_key;
 
  match message {
  OffersMessage::InvoiceRequest(invoice_request) => {
+ let responder = match responder {
+ Some(responder) => responder,
+ None => return ResponseInstruction::NoResponse,
+ };
  let amount_msats = match InvoiceBuilder::<DerivedSigningPubkey>::amount_msats(
  &invoice_request
  ) {
  Ok(amount_msats) => amount_msats,
- Err(error) => return Some(OffersMessage::InvoiceError(error.into())),
+ Err(error) => return responder.respond(OffersMessage::InvoiceError(error.into())),
  };
  let invoice_request = match invoice_request.verify(expanded_key, secp_ctx) {
  Ok(invoice_request) => invoice_request,
  Err(()) => {
  let error = Bolt12SemanticError::InvalidMetadata;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10359,7 +10364,7 @@ where
  Ok((payment_hash, payment_secret)) => (payment_hash, payment_secret),
  Err(()) => {
  let error = Bolt12SemanticError::InvalidAmount;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10373,7 +10378,7 @@ where
  Ok(payment_paths) => payment_paths,
  Err(()) => {
  let error = Bolt12SemanticError::MissingPaths;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10418,8 +10423,8 @@ where
  };
 
  match response {
- Ok(invoice) => Some(OffersMessage::Invoice(invoice)),
- Err(error) => Some(OffersMessage::InvoiceError(error.into())),
+ Ok(invoice) => return responder.respond(OffersMessage::Invoice(invoice)),
+ Err(error) => return responder.respond(OffersMessage::InvoiceError(error.into())),
  }
  },
  OffersMessage::Invoice(invoice) => {
@@ -10439,14 +10444,21 @@ where
  }
  });
 
- match response {
- Ok(()) => None,
- Err(e) => Some(OffersMessage::InvoiceError(e)),
+ match (responder, response) {
+ (Some(responder), Err(e)) => responder.respond(OffersMessage::InvoiceError(e)),
+ (None, Err(_)) => {
+ log_trace!(
+ self.logger,
+ "A response was generated, but there is no reply_path specified for sending the response."
+ );
+ return ResponseInstruction::NoResponse;
+ }
+ _ => return ResponseInstruction::NoResponse,
  }
  },
  OffersMessage::InvoiceError(invoice_error) => {
  log_trace!(self.logger, "Received invoice_error: {}", invoice_error);
-None
+return ResponseInstruction::NoResponse;
  },
  }
 	}

lightning/src/ln/peer_handler.rs

@@ -28,7 +28,7 @@ use crate::util::ser::{VecWriter, Writeable, Writer};
 use crate::ln::peer_channel_encryptor::{PeerChannelEncryptor, NextNoiseStep, MessageBuf, MSG_BUF_ALLOC_SIZE};
 use crate::ln::wire;
 use crate::ln::wire::{Encode, Type};
-use crate::onion_message::messenger::{CustomOnionMessageHandler, PendingOnionMessage};
+use crate::onion_message::messenger::{CustomOnionMessageHandler, PendingOnionMessage, Responder, ResponseInstruction};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use crate::onion_message::packet::OnionMessageContents;
 use crate::routing::gossip::{NodeId, NodeAlias};
@@ -123,6 +123,7 @@ impl RoutingMessageHandler for IgnoringMessageHandler {
 	}
 	fn processing_queue_high(&self) -> bool { false }
 }
+
 impl OnionMessageHandler for IgnoringMessageHandler {
 	fn handle_onion_message(&self, _their_node_id: &PublicKey, _msg: &msgs::OnionMessage) {}
 	fn next_onion_message_for_peer(&self, _peer_node_id: PublicKey) -> Option<msgs::OnionMessage> { None }
@@ -134,12 +135,15 @@ impl OnionMessageHandler for IgnoringMessageHandler {
  InitFeatures::empty()
 	}
 }
+
 impl OffersMessageHandler for IgnoringMessageHandler {
-	fn handle_message(&self, _msg: OffersMessage) -> Option<OffersMessage> { None }
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ ResponseInstruction::NoResponse
+	}
 }
 impl CustomOnionMessageHandler for IgnoringMessageHandler {
 	type CustomMessage = Infallible;
-	fn handle_custom_message(&self, _msg: Infallible) -> Option<Infallible> {
+	fn handle_custom_message(&self, _message: Self::CustomMessage, _responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
  // Since we always return `None` in the read the handle method should never be called.
  unreachable!();
 	}
@@ -153,6 +157,7 @@ impl CustomOnionMessageHandler for IgnoringMessageHandler {
 
 impl OnionMessageContents for Infallible {
 	fn tlv_type(&self) -> u64 { unreachable!(); }
+	fn msg_type(&self) -> &'static str { unreachable!(); }
 }
 
 impl Deref for IgnoringMessageHandler {

lightning/src/onion_message/functional_tests.rs

@@ -18,7 +18,7 @@ use crate::routing::test_utils::{add_channel, add_or_update_node};
 use crate::sign::{NodeSigner, Recipient};
 use crate::util::ser::{FixedLengthReader, LengthReadable, Writeable, Writer};
 use crate::util::test_utils;
-use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, SendError};
+use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction, SendError};
 use super::offers::{OffersMessage, OffersMessageHandler};
 use super::packet::{OnionMessageContents, Packet};
 
@@ -62,8 +62,8 @@ struct MessengerNode {
 struct TestOffersMessageHandler {}
 
 impl OffersMessageHandler for TestOffersMessageHandler {
-	fn handle_message(&self, _message: OffersMessage) -> Option<OffersMessage> {
-None
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ResponseInstruction::NoResponse
 	}
 }
 
@@ -85,6 +85,9 @@ impl OnionMessageContents for TestCustomMessage {
  TestCustomMessage::Response => CUSTOM_RESPONSE_MESSAGE_TYPE,
  }
 	}
+	fn msg_type(&self) -> &'static str {
+ "Custom Message"
+	}
 }
 
 impl Writeable for TestCustomMessage {
@@ -123,15 +126,19 @@ impl Drop for TestCustomMessageHandler {
 
 impl CustomOnionMessageHandler for TestCustomMessageHandler {
 	type CustomMessage = TestCustomMessage;
-	fn handle_custom_message(&self, msg: Self::CustomMessage) -> Option<Self::CustomMessage> {
+	fn handle_custom_message(&self, msg: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
  match self.expected_messages.lock().unwrap().pop_front() {
  Some(expected_msg) => assert_eq!(expected_msg, msg),
  None => panic!("Unexpected message: {:?}", msg),
  }
-
- match msg {
+ let response_option = match msg {
  TestCustomMessage::Request => Some(TestCustomMessage::Response),
  TestCustomMessage::Response => None,
+ };
+ if let (Some(response), Some(responder)) = (response_option, responder) {
+ responder.respond(response)
+ } else {
+ ResponseInstruction::NoResponse
  }
 	}
 	fn read_custom_message<R: io::Read>(&self, message_type: u64, buffer: &mut R) -> Result<Option<Self::CustomMessage>, DecodeError> where Self: Sized {
@@ -422,6 +429,9 @@ fn invalid_custom_message_type() {
  // Onion message contents must have a TLV >= 64.
  63
  }
+ fn msg_type(&self) -> &'static str {
+ "Invalid Message"
+ }
 	}
 
 	impl Writeable for InvalidCustomMessage {

lightning/src/onion_message/messenger.rs

@@ -135,6 +135,7 @@ pub(super) const MAX_TIMER_TICKS: usize = 2;
 /// # let your_custom_message_type = 42;
 /// your_custom_message_type
 /// }
+/// fn msg_type(&self) -> &'static str { "YourCustomMessageType" }
 /// }
 /// // Send a custom onion message to a node id.
 /// let destination = Destination::Node(destination_node_id);
@@ -246,6 +247,50 @@ impl OnionMessageRecipient {
 	}
 }
 
+
+/// The `Responder` struct creates an appropriate [`ResponseInstruction`]
+/// for responding to a message.
+pub struct Responder {
+	/// The path along which a response can be sent.
+	reply_path: BlindedPath,
+	path_id: Option<[u8; 32]>
+}
+
+impl Responder {
+	/// Creates a new [`Responder`] instance with the provided reply path.
+	fn new(reply_path: BlindedPath, path_id: Option<[u8; 32]>) -> Self {
+ Responder {
+ reply_path,
+ path_id,
+ }
+	}
+
+	/// Creates the appropriate [`ResponseInstruction`] for a given response.
+	pub fn respond<T: OnionMessageContents>(self, response: T) -> ResponseInstruction<T> {
+ ResponseInstruction::WithoutReplyPath(OnionMessageResponse {
+ message: response,
+ reply_path: self.reply_path,
+ path_id: self.path_id,
+ })
+	}
+}
+
+/// This struct contains the information needed to reply to a received message.
+pub struct OnionMessageResponse<T: OnionMessageContents> {
+	message: T,
+	reply_path: BlindedPath,
+	path_id: Option<[u8; 32]>,
+}
+
+/// `ResponseInstruction` represents instructions for responding to received messages.
+pub enum ResponseInstruction<T: OnionMessageContents> {
+	/// Indicates that a response should be sent without including a reply path
+	/// for the recipient to respond back.
+	WithoutReplyPath(OnionMessageResponse<T>),
+	/// Indicates that there's no response to send back.
+	NoResponse,
+}
+
 /// An [`OnionMessage`] for [`OnionMessenger`] to send.
 ///
 /// These are obtained when released from [`OnionMessenger`]'s handlers after which they are
@@ -546,7 +591,7 @@ pub trait CustomOnionMessageHandler {
 	/// Called with the custom message that was received, returning a response to send, if any.
 	///
 	/// The returned [`Self::CustomMessage`], if any, is enqueued to be sent by [`OnionMessenger`].
-	fn handle_custom_message(&self, msg: Self::CustomMessage) -> Option<Self::CustomMessage>;
+	fn handle_custom_message(&self, message: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage>;
 
 	/// Read a custom message of type `message_type` from `buffer`, returning `Ok(None)` if the
 	/// message type is unknown.
@@ -933,19 +978,18 @@ where
 	}
 
 	fn handle_onion_message_response<T: OnionMessageContents>(
- &self, response: Option<T>, reply_path: Option<BlindedPath>, log_suffix: fmt::Arguments
+ &self, response: ResponseInstruction<T>
 	) {
- if let Some(response) = response {
- match reply_path {
- Some(reply_path) => {
- let _ = self.find_path_and_enqueue_onion_message(
- response, Destination::BlindedPath(reply_path), None, log_suffix
- );
- },
- None => {
- log_trace!(self.logger, "Missing reply path {}", log_suffix);
- },
- }
+ if let ResponseInstruction::WithoutReplyPath(response) = response {
+ let message_type = response.message.msg_type();
+ let _ = self.find_path_and_enqueue_onion_message(
+ response.message, Destination::BlindedPath(response.reply_path), None,
+ format_args!(
+ "when responding with {} to an onion message with path_id {:02x?}",
+ message_type,
+ response.path_id
+ )
+ );
  }
 	}
 
@@ -1029,22 +1073,18 @@ where
 
  match message {
  ParsedOnionMessageContents::Offers(msg) => {
- let response = self.offers_handler.handle_message(msg);
- self.handle_onion_message_response(
- response, reply_path, format_args!(
- "when responding to Offers onion message with path_id {:02x?}",
- path_id
- )
+ let responder = reply_path.map(
+ |reply_path| Responder::new(reply_path, path_id)
  );
+ let response_instructions = self.offers_handler.handle_message(msg, responder);
+ self.handle_onion_message_response(response_instructions);
  },
  ParsedOnionMessageContents::Custom(msg) => {
- let response = self.custom_handler.handle_custom_message(msg);
- self.handle_onion_message_response(
- response, reply_path, format_args!(
- "when responding to Custom onion message with path_id {:02x?}",
- path_id
- )
+ let responder = reply_path.map(
+ |reply_path| Responder::new(reply_path, path_id)
  );
+ let response_instructions = self.custom_handler.handle_custom_message(msg, responder);
+ self.handle_onion_message_response(response_instructions);
  },
  }
  },

fuzz/src/onion_message.rs

@@ -16,7 +16,7 @@ use lightning::sign::{Recipient, KeyMaterial, EntropySource, NodeSigner, SignerP
 use lightning::util::test_channel_signer::TestChannelSigner;
 use lightning::util::logger::Logger;
 use lightning::util::ser::{Readable, Writeable, Writer};
-use lightning::onion_message::messenger::{CustomOnionMessageHandler, Destination, MessageRouter, OnionMessagePath, OnionMessenger, PendingOnionMessage};
+use lightning::onion_message::messenger::{CustomOnionMessageHandler, Destination, MessageRouter, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction};
 use lightning::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use lightning::onion_message::packet::OnionMessageContents;
 
@@ -97,8 +97,8 @@ impl MessageRouter for TestMessageRouter {
 struct TestOffersMessageHandler {}
 
 impl OffersMessageHandler for TestOffersMessageHandler {
-	fn handle_message(&self, _message: OffersMessage) -> Option<OffersMessage> {
-None
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ResponseInstruction::NoResponse
 	}
 }
 
@@ -112,6 +112,9 @@ impl OnionMessageContents for TestCustomMessage {
 	fn tlv_type(&self) -> u64 {
  CUSTOM_MESSAGE_TYPE
 	}
+	fn msg_type(&self) -> &'static str {
+ "Custom Message"
+	}
 }
 
 impl Writeable for TestCustomMessage {
@@ -124,8 +127,11 @@ struct TestCustomMessageHandler {}
 
 impl CustomOnionMessageHandler for TestCustomMessageHandler {
 	type CustomMessage = TestCustomMessage;
-	fn handle_custom_message(&self, _msg: Self::CustomMessage) -> Option<Self::CustomMessage> {
- Some(TestCustomMessage {})
+	fn handle_custom_message(&self, message: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
+ match responder {
+ Some(responder) => responder.respond(message),
+ None => ResponseInstruction::NoResponse
+ }
 	}
 	fn read_custom_message<R: io::Read>(&self, _message_type: u64, buffer: &mut R) -> Result<Option<Self::CustomMessage>, msgs::DecodeError> {
  let mut buf = Vec::new();
@@ -280,9 +286,9 @@ mod tests {
  "Received an onion message with path_id None and a reply_path: Custom(TestCustomMessage)"
  .to_string())), Some(&1));
  assert_eq!(log_entries.get(&("lightning::onion_message::messenger".to_string(),
- "Constructing onion message when responding to Custom onion message with path_id None: TestCustomMessage".to_string())), Some(&1));
+ "Constructing onion message when responding with Custom Message to an onion message with path_id None: TestCustomMessage".to_string())), Some(&1));
  assert_eq!(log_entries.get(&("lightning::onion_message::messenger".to_string(),
- "Buffered onion message when responding to Custom onion message with path_id None".to_string())), Some(&1));
+ "Buffered onion message when responding with Custom Message to an onion message with path_id None".to_string())), Some(&1));
  }
 
  let two_unblinded_hops_om = "\

lightning/src/ln/channelmanager.rs

@@ -64,7 +64,7 @@ use crate::offers::invoice_request::{DerivedPayerId, InvoiceRequestBuilder};
 use crate::offers::offer::{Offer, OfferBuilder};
 use crate::offers::parse::Bolt12SemanticError;
 use crate::offers::refund::{Refund, RefundBuilder};
-use crate::onion_message::messenger::{Destination, MessageRouter, PendingOnionMessage, new_pending_onion_message};
+use crate::onion_message::messenger::{new_pending_onion_message, Destination, MessageRouter, PendingOnionMessage, Responder, ResponseInstruction};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use crate::sign::{EntropySource, NodeSigner, Recipient, SignerProvider};
 use crate::sign::ecdsa::WriteableEcdsaChannelSigner;
@@ -75,6 +75,7 @@ use crate::util::string::UntrustedString;
 use crate::util::ser::{BigSize, FixedLengthReader, Readable, ReadableArgs, MaybeReadable, Writeable, Writer, VecWriter};
 use crate::util::logger::{Level, Logger, WithContext};
 use crate::util::errors::APIError;
+
 #[cfg(not(c_bindings))]
 use {
 	crate::offers::offer::DerivedMetadata,
@@ -10332,23 +10333,27 @@ where
 	R::Target: Router,
 	L::Target: Logger,
 {
-	fn handle_message(&self, message: OffersMessage) -> Option<OffersMessage> {
+	fn handle_message(&self, message: OffersMessage, responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
  let secp_ctx = &self.secp_ctx;
  let expanded_key = &self.inbound_payment_key;
 
  match message {
  OffersMessage::InvoiceRequest(invoice_request) => {
+ let responder = match responder {
+ Some(responder) => responder,
+ None => return ResponseInstruction::NoResponse,
+ };
  let amount_msats = match InvoiceBuilder::<DerivedSigningPubkey>::amount_msats(
  &invoice_request
  ) {
  Ok(amount_msats) => amount_msats,
- Err(error) => return Some(OffersMessage::InvoiceError(error.into())),
+ Err(error) => return responder.respond(OffersMessage::InvoiceError(error.into())),
  };
  let invoice_request = match invoice_request.verify(expanded_key, secp_ctx) {
  Ok(invoice_request) => invoice_request,
  Err(()) => {
  let error = Bolt12SemanticError::InvalidMetadata;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10359,7 +10364,7 @@ where
  Ok((payment_hash, payment_secret)) => (payment_hash, payment_secret),
  Err(()) => {
  let error = Bolt12SemanticError::InvalidAmount;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10373,7 +10378,7 @@ where
  Ok(payment_paths) => payment_paths,
  Err(()) => {
  let error = Bolt12SemanticError::MissingPaths;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10418,8 +10423,8 @@ where
  };
 
  match response {
- Ok(invoice) => Some(OffersMessage::Invoice(invoice)),
- Err(error) => Some(OffersMessage::InvoiceError(error.into())),
+ Ok(invoice) => return responder.respond(OffersMessage::Invoice(invoice)),
+ Err(error) => return responder.respond(OffersMessage::InvoiceError(error.into())),
  }
  },
  OffersMessage::Invoice(invoice) => {
@@ -10439,14 +10444,21 @@ where
  }
  });
 
- match response {
- Ok(()) => None,
- Err(e) => Some(OffersMessage::InvoiceError(e)),
+ match (responder, response) {
+ (Some(responder), Err(e)) => responder.respond(OffersMessage::InvoiceError(e)),
+ (None, Err(_)) => {
+ log_trace!(
+ self.logger,
+ "A response was generated, but there is no reply_path specified for sending the response."
+ );
+ return ResponseInstruction::NoResponse;
+ }
+ _ => return ResponseInstruction::NoResponse,
  }
  },
  OffersMessage::InvoiceError(invoice_error) => {
  log_trace!(self.logger, "Received invoice_error: {}", invoice_error);
-None
+return ResponseInstruction::NoResponse;
  },
  }
 	}

lightning/src/ln/peer_handler.rs

@@ -28,7 +28,7 @@ use crate::util::ser::{VecWriter, Writeable, Writer};
 use crate::ln::peer_channel_encryptor::{PeerChannelEncryptor, NextNoiseStep, MessageBuf, MSG_BUF_ALLOC_SIZE};
 use crate::ln::wire;
 use crate::ln::wire::{Encode, Type};
-use crate::onion_message::messenger::{CustomOnionMessageHandler, PendingOnionMessage};
+use crate::onion_message::messenger::{CustomOnionMessageHandler, PendingOnionMessage, Responder, ResponseInstruction};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use crate::onion_message::packet::OnionMessageContents;
 use crate::routing::gossip::{NodeId, NodeAlias};
@@ -123,6 +123,7 @@ impl RoutingMessageHandler for IgnoringMessageHandler {
 	}
 	fn processing_queue_high(&self) -> bool { false }
 }
+
 impl OnionMessageHandler for IgnoringMessageHandler {
 	fn handle_onion_message(&self, _their_node_id: &PublicKey, _msg: &msgs::OnionMessage) {}
 	fn next_onion_message_for_peer(&self, _peer_node_id: PublicKey) -> Option<msgs::OnionMessage> { None }
@@ -134,12 +135,15 @@ impl OnionMessageHandler for IgnoringMessageHandler {
  InitFeatures::empty()
 	}
 }
+
 impl OffersMessageHandler for IgnoringMessageHandler {
-	fn handle_message(&self, _msg: OffersMessage) -> Option<OffersMessage> { None }
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ ResponseInstruction::NoResponse
+	}
 }
 impl CustomOnionMessageHandler for IgnoringMessageHandler {
 	type CustomMessage = Infallible;
-	fn handle_custom_message(&self, _msg: Infallible) -> Option<Infallible> {
+	fn handle_custom_message(&self, _message: Self::CustomMessage, _responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
  // Since we always return `None` in the read the handle method should never be called.
  unreachable!();
 	}
@@ -153,6 +157,7 @@ impl CustomOnionMessageHandler for IgnoringMessageHandler {
 
 impl OnionMessageContents for Infallible {
 	fn tlv_type(&self) -> u64 { unreachable!(); }
+	fn msg_type(&self) -> &'static str { unreachable!(); }
 }
 
 impl Deref for IgnoringMessageHandler {

lightning/src/onion_message/functional_tests.rs

@@ -18,7 +18,7 @@ use crate::routing::test_utils::{add_channel, add_or_update_node};
 use crate::sign::{NodeSigner, Recipient};
 use crate::util::ser::{FixedLengthReader, LengthReadable, Writeable, Writer};
 use crate::util::test_utils;
-use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, SendError};
+use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction, SendError};
 use super::offers::{OffersMessage, OffersMessageHandler};
 use super::packet::{OnionMessageContents, Packet};
 
@@ -62,8 +62,8 @@ struct MessengerNode {
 struct TestOffersMessageHandler {}
 
 impl OffersMessageHandler for TestOffersMessageHandler {
-	fn handle_message(&self, _message: OffersMessage) -> Option<OffersMessage> {
-None
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ResponseInstruction::NoResponse
 	}
 }
 
@@ -85,6 +85,9 @@ impl OnionMessageContents for TestCustomMessage {
  TestCustomMessage::Response => CUSTOM_RESPONSE_MESSAGE_TYPE,
  }
 	}
+	fn msg_type(&self) -> &'static str {
+ "Custom Message"
+	}
 }
 
 impl Writeable for TestCustomMessage {
@@ -123,15 +126,19 @@ impl Drop for TestCustomMessageHandler {
 
 impl CustomOnionMessageHandler for TestCustomMessageHandler {
 	type CustomMessage = TestCustomMessage;
-	fn handle_custom_message(&self, msg: Self::CustomMessage) -> Option<Self::CustomMessage> {
+	fn handle_custom_message(&self, msg: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
  match self.expected_messages.lock().unwrap().pop_front() {
  Some(expected_msg) => assert_eq!(expected_msg, msg),
  None => panic!("Unexpected message: {:?}", msg),
  }
-
- match msg {
+ let response_option = match msg {
  TestCustomMessage::Request => Some(TestCustomMessage::Response),
  TestCustomMessage::Response => None,
+ };
+ if let (Some(response), Some(responder)) = (response_option, responder) {
+ responder.respond(response)
+ } else {
+ ResponseInstruction::NoResponse
  }
 	}
 	fn read_custom_message<R: io::Read>(&self, message_type: u64, buffer: &mut R) -> Result<Option<Self::CustomMessage>, DecodeError> where Self: Sized {
@@ -422,6 +429,9 @@ fn invalid_custom_message_type() {
  // Onion message contents must have a TLV >= 64.
  63
  }
+ fn msg_type(&self) -> &'static str {
+ "Invalid Message"
+ }
 	}
 
 	impl Writeable for InvalidCustomMessage {

lightning/src/onion_message/messenger.rs

@@ -135,6 +135,7 @@ pub(super) const MAX_TIMER_TICKS: usize = 2;
 /// # let your_custom_message_type = 42;
 /// your_custom_message_type
 /// }
+/// fn msg_type(&self) -> &'static str { "YourCustomMessageType" }
 /// }
 /// // Send a custom onion message to a node id.
 /// let destination = Destination::Node(destination_node_id);
@@ -246,6 +247,50 @@ impl OnionMessageRecipient {
 	}
 }
 
+
+/// The `Responder` struct creates an appropriate [`ResponseInstruction`]
+/// for responding to a message.
+pub struct Responder {
+	/// The path along which a response can be sent.
+	reply_path: BlindedPath,
+	path_id: Option<[u8; 32]>
+}
+
+impl Responder {
+	/// Creates a new [`Responder`] instance with the provided reply path.
+	fn new(reply_path: BlindedPath, path_id: Option<[u8; 32]>) -> Self {
+ Responder {
+ reply_path,
+ path_id,
+ }
+	}
+
+	/// Creates the appropriate [`ResponseInstruction`] for a given response.
+	pub fn respond<T: OnionMessageContents>(self, response: T) -> ResponseInstruction<T> {
+ ResponseInstruction::WithoutReplyPath(OnionMessageResponse {
+ message: response,
+ reply_path: self.reply_path,
+ path_id: self.path_id,
+ })
+	}
+}
+
+/// This struct contains the information needed to reply to a received message.
+pub struct OnionMessageResponse<T: OnionMessageContents> {
+	message: T,
+	reply_path: BlindedPath,
+	path_id: Option<[u8; 32]>,
+}
+
+/// `ResponseInstruction` represents instructions for responding to received messages.
+pub enum ResponseInstruction<T: OnionMessageContents> {
+	/// Indicates that a response should be sent without including a reply path
+	/// for the recipient to respond back.
+	WithoutReplyPath(OnionMessageResponse<T>),
+	/// Indicates that there's no response to send back.
+	NoResponse,
+}
+
 /// An [`OnionMessage`] for [`OnionMessenger`] to send.
 ///
 /// These are obtained when released from [`OnionMessenger`]'s handlers after which they are
@@ -546,7 +591,7 @@ pub trait CustomOnionMessageHandler {
 	/// Called with the custom message that was received, returning a response to send, if any.
 	///
 	/// The returned [`Self::CustomMessage`], if any, is enqueued to be sent by [`OnionMessenger`].
-	fn handle_custom_message(&self, msg: Self::CustomMessage) -> Option<Self::CustomMessage>;
+	fn handle_custom_message(&self, message: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage>;
 
 	/// Read a custom message of type `message_type` from `buffer`, returning `Ok(None)` if the
 	/// message type is unknown.
@@ -933,19 +978,18 @@ where
 	}
 
 	fn handle_onion_message_response<T: OnionMessageContents>(
- &self, response: Option<T>, reply_path: Option<BlindedPath>, log_suffix: fmt::Arguments
+ &self, response: ResponseInstruction<T>
 	) {
- if let Some(response) = response {
- match reply_path {
- Some(reply_path) => {
- let _ = self.find_path_and_enqueue_onion_message(
- response, Destination::BlindedPath(reply_path), None, log_suffix
- );
- },
- None => {
- log_trace!(self.logger, "Missing reply path {}", log_suffix);
- },
- }
+ if let ResponseInstruction::WithoutReplyPath(response) = response {
+ let message_type = response.message.msg_type();
+ let _ = self.find_path_and_enqueue_onion_message(
+ response.message, Destination::BlindedPath(response.reply_path), None,
+ format_args!(
+ "when responding with {} to an onion message with path_id {:02x?}",
+ message_type,
+ response.path_id
+ )
+ );
  }
 	}
 
@@ -1029,22 +1073,18 @@ where
 
  match message {
  ParsedOnionMessageContents::Offers(msg) => {
- let response = self.offers_handler.handle_message(msg);
- self.handle_onion_message_response(
- response, reply_path, format_args!(
- "when responding to Offers onion message with path_id {:02x?}",
- path_id
- )
+ let responder = reply_path.map(
+ |reply_path| Responder::new(reply_path, path_id)
  );
+ let response_instructions = self.offers_handler.handle_message(msg, responder);
+ self.handle_onion_message_response(response_instructions);
  },
  ParsedOnionMessageContents::Custom(msg) => {
- let response = self.custom_handler.handle_custom_message(msg);
- self.handle_onion_message_response(
- response, reply_path, format_args!(
- "when responding to Custom onion message with path_id {:02x?}",
- path_id
- )
+ let responder = reply_path.map(
+ |reply_path| Responder::new(reply_path, path_id)
  );
+ let response_instructions = self.custom_handler.handle_custom_message(msg, responder);
+ self.handle_onion_message_response(response_instructions);
  },
  }
  },

fuzz/src/onion_message.rs

@@ -16,7 +16,7 @@ use lightning::sign::{Recipient, KeyMaterial, EntropySource, NodeSigner, SignerP
 use lightning::util::test_channel_signer::TestChannelSigner;
 use lightning::util::logger::Logger;
 use lightning::util::ser::{Readable, Writeable, Writer};
-use lightning::onion_message::messenger::{CustomOnionMessageHandler, Destination, MessageRouter, OnionMessagePath, OnionMessenger, PendingOnionMessage};
+use lightning::onion_message::messenger::{CustomOnionMessageHandler, Destination, MessageRouter, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction};
 use lightning::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use lightning::onion_message::packet::OnionMessageContents;
 
@@ -97,8 +97,8 @@ impl MessageRouter for TestMessageRouter {
 struct TestOffersMessageHandler {}
 
 impl OffersMessageHandler for TestOffersMessageHandler {
-	fn handle_message(&self, _message: OffersMessage) -> Option<OffersMessage> {
-None
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ResponseInstruction::NoResponse
 	}
 }
 
@@ -112,6 +112,9 @@ impl OnionMessageContents for TestCustomMessage {
 	fn tlv_type(&self) -> u64 {
  CUSTOM_MESSAGE_TYPE
 	}
+	fn msg_type(&self) -> &'static str {
+ "Custom Message"
+	}
 }
 
 impl Writeable for TestCustomMessage {
@@ -124,8 +127,11 @@ struct TestCustomMessageHandler {}
 
 impl CustomOnionMessageHandler for TestCustomMessageHandler {
 	type CustomMessage = TestCustomMessage;
-	fn handle_custom_message(&self, _msg: Self::CustomMessage) -> Option<Self::CustomMessage> {
- Some(TestCustomMessage {})
+	fn handle_custom_message(&self, message: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
+ match responder {
+ Some(responder) => responder.respond(message),
+ None => ResponseInstruction::NoResponse
+ }
 	}
 	fn read_custom_message<R: io::Read>(&self, _message_type: u64, buffer: &mut R) -> Result<Option<Self::CustomMessage>, msgs::DecodeError> {
  let mut buf = Vec::new();
@@ -280,9 +286,9 @@ mod tests {
  "Received an onion message with path_id None and a reply_path: Custom(TestCustomMessage)"
  .to_string())), Some(&1));
  assert_eq!(log_entries.get(&("lightning::onion_message::messenger".to_string(),
- "Constructing onion message when responding to Custom onion message with path_id None: TestCustomMessage".to_string())), Some(&1));
+ "Constructing onion message when responding with Custom Message to an onion message with path_id None: TestCustomMessage".to_string())), Some(&1));
  assert_eq!(log_entries.get(&("lightning::onion_message::messenger".to_string(),
- "Buffered onion message when responding to Custom onion message with path_id None".to_string())), Some(&1));
+ "Buffered onion message when responding with Custom Message to an onion message with path_id None".to_string())), Some(&1));
  }
 
  let two_unblinded_hops_om = "\

lightning/src/ln/channelmanager.rs

@@ -64,7 +64,7 @@ use crate::offers::invoice_request::{DerivedPayerId, InvoiceRequestBuilder};
 use crate::offers::offer::{Offer, OfferBuilder};
 use crate::offers::parse::Bolt12SemanticError;
 use crate::offers::refund::{Refund, RefundBuilder};
-use crate::onion_message::messenger::{Destination, MessageRouter, PendingOnionMessage, new_pending_onion_message};
+use crate::onion_message::messenger::{new_pending_onion_message, Destination, MessageRouter, PendingOnionMessage, Responder, ResponseInstruction};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use crate::sign::{EntropySource, NodeSigner, Recipient, SignerProvider};
 use crate::sign::ecdsa::WriteableEcdsaChannelSigner;
@@ -75,6 +75,7 @@ use crate::util::string::UntrustedString;
 use crate::util::ser::{BigSize, FixedLengthReader, Readable, ReadableArgs, MaybeReadable, Writeable, Writer, VecWriter};
 use crate::util::logger::{Level, Logger, WithContext};
 use crate::util::errors::APIError;
+
 #[cfg(not(c_bindings))]
 use {
 	crate::offers::offer::DerivedMetadata,
@@ -10332,23 +10333,27 @@ where
 	R::Target: Router,
 	L::Target: Logger,
 {
-	fn handle_message(&self, message: OffersMessage) -> Option<OffersMessage> {
+	fn handle_message(&self, message: OffersMessage, responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
  let secp_ctx = &self.secp_ctx;
  let expanded_key = &self.inbound_payment_key;
 
  match message {
  OffersMessage::InvoiceRequest(invoice_request) => {
+ let responder = match responder {
+ Some(responder) => responder,
+ None => return ResponseInstruction::NoResponse,
+ };
  let amount_msats = match InvoiceBuilder::<DerivedSigningPubkey>::amount_msats(
  &invoice_request
  ) {
  Ok(amount_msats) => amount_msats,
- Err(error) => return Some(OffersMessage::InvoiceError(error.into())),
+ Err(error) => return responder.respond(OffersMessage::InvoiceError(error.into())),
  };
  let invoice_request = match invoice_request.verify(expanded_key, secp_ctx) {
  Ok(invoice_request) => invoice_request,
  Err(()) => {
  let error = Bolt12SemanticError::InvalidMetadata;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10359,7 +10364,7 @@ where
  Ok((payment_hash, payment_secret)) => (payment_hash, payment_secret),
  Err(()) => {
  let error = Bolt12SemanticError::InvalidAmount;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10373,7 +10378,7 @@ where
  Ok(payment_paths) => payment_paths,
  Err(()) => {
  let error = Bolt12SemanticError::MissingPaths;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10418,8 +10423,8 @@ where
  };
 
  match response {
- Ok(invoice) => Some(OffersMessage::Invoice(invoice)),
- Err(error) => Some(OffersMessage::InvoiceError(error.into())),
+ Ok(invoice) => return responder.respond(OffersMessage::Invoice(invoice)),
+ Err(error) => return responder.respond(OffersMessage::InvoiceError(error.into())),
  }
  },
  OffersMessage::Invoice(invoice) => {
@@ -10439,14 +10444,21 @@ where
  }
  });
 
- match response {
- Ok(()) => None,
- Err(e) => Some(OffersMessage::InvoiceError(e)),
+ match (responder, response) {
+ (Some(responder), Err(e)) => responder.respond(OffersMessage::InvoiceError(e)),
+ (None, Err(_)) => {
+ log_trace!(
+ self.logger,
+ "A response was generated, but there is no reply_path specified for sending the response."
+ );
+ return ResponseInstruction::NoResponse;
+ }
+ _ => return ResponseInstruction::NoResponse,
  }
  },
  OffersMessage::InvoiceError(invoice_error) => {
  log_trace!(self.logger, "Received invoice_error: {}", invoice_error);
-None
+return ResponseInstruction::NoResponse;
  },
  }
 	}

lightning/src/ln/peer_handler.rs

@@ -28,7 +28,7 @@ use crate::util::ser::{VecWriter, Writeable, Writer};
 use crate::ln::peer_channel_encryptor::{PeerChannelEncryptor, NextNoiseStep, MessageBuf, MSG_BUF_ALLOC_SIZE};
 use crate::ln::wire;
 use crate::ln::wire::{Encode, Type};
-use crate::onion_message::messenger::{CustomOnionMessageHandler, PendingOnionMessage};
+use crate::onion_message::messenger::{CustomOnionMessageHandler, PendingOnionMessage, Responder, ResponseInstruction};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use crate::onion_message::packet::OnionMessageContents;
 use crate::routing::gossip::{NodeId, NodeAlias};
@@ -123,6 +123,7 @@ impl RoutingMessageHandler for IgnoringMessageHandler {
 	}
 	fn processing_queue_high(&self) -> bool { false }
 }
+
 impl OnionMessageHandler for IgnoringMessageHandler {
 	fn handle_onion_message(&self, _their_node_id: &PublicKey, _msg: &msgs::OnionMessage) {}
 	fn next_onion_message_for_peer(&self, _peer_node_id: PublicKey) -> Option<msgs::OnionMessage> { None }
@@ -134,12 +135,15 @@ impl OnionMessageHandler for IgnoringMessageHandler {
  InitFeatures::empty()
 	}
 }
+
 impl OffersMessageHandler for IgnoringMessageHandler {
-	fn handle_message(&self, _msg: OffersMessage) -> Option<OffersMessage> { None }
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ ResponseInstruction::NoResponse
+	}
 }
 impl CustomOnionMessageHandler for IgnoringMessageHandler {
 	type CustomMessage = Infallible;
-	fn handle_custom_message(&self, _msg: Infallible) -> Option<Infallible> {
+	fn handle_custom_message(&self, _message: Self::CustomMessage, _responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
  // Since we always return `None` in the read the handle method should never be called.
  unreachable!();
 	}
@@ -153,6 +157,7 @@ impl CustomOnionMessageHandler for IgnoringMessageHandler {
 
 impl OnionMessageContents for Infallible {
 	fn tlv_type(&self) -> u64 { unreachable!(); }
+	fn msg_type(&self) -> &'static str { unreachable!(); }
 }
 
 impl Deref for IgnoringMessageHandler {

lightning/src/onion_message/functional_tests.rs

@@ -18,7 +18,7 @@ use crate::routing::test_utils::{add_channel, add_or_update_node};
 use crate::sign::{NodeSigner, Recipient};
 use crate::util::ser::{FixedLengthReader, LengthReadable, Writeable, Writer};
 use crate::util::test_utils;
-use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, SendError};
+use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction, SendError};
 use super::offers::{OffersMessage, OffersMessageHandler};
 use super::packet::{OnionMessageContents, Packet};
 
@@ -62,8 +62,8 @@ struct MessengerNode {
 struct TestOffersMessageHandler {}
 
 impl OffersMessageHandler for TestOffersMessageHandler {
-	fn handle_message(&self, _message: OffersMessage) -> Option<OffersMessage> {
-None
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ResponseInstruction::NoResponse
 	}
 }
 
@@ -85,6 +85,9 @@ impl OnionMessageContents for TestCustomMessage {
  TestCustomMessage::Response => CUSTOM_RESPONSE_MESSAGE_TYPE,
  }
 	}
+	fn msg_type(&self) -> &'static str {
+ "Custom Message"
+	}
 }
 
 impl Writeable for TestCustomMessage {
@@ -123,15 +126,19 @@ impl Drop for TestCustomMessageHandler {
 
 impl CustomOnionMessageHandler for TestCustomMessageHandler {
 	type CustomMessage = TestCustomMessage;
-	fn handle_custom_message(&self, msg: Self::CustomMessage) -> Option<Self::CustomMessage> {
+	fn handle_custom_message(&self, msg: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
  match self.expected_messages.lock().unwrap().pop_front() {
  Some(expected_msg) => assert_eq!(expected_msg, msg),
  None => panic!("Unexpected message: {:?}", msg),
  }
-
- match msg {
+ let response_option = match msg {
  TestCustomMessage::Request => Some(TestCustomMessage::Response),
  TestCustomMessage::Response => None,
+ };
+ if let (Some(response), Some(responder)) = (response_option, responder) {
+ responder.respond(response)
+ } else {
+ ResponseInstruction::NoResponse
  }
 	}
 	fn read_custom_message<R: io::Read>(&self, message_type: u64, buffer: &mut R) -> Result<Option<Self::CustomMessage>, DecodeError> where Self: Sized {
@@ -422,6 +429,9 @@ fn invalid_custom_message_type() {
  // Onion message contents must have a TLV >= 64.
  63
  }
+ fn msg_type(&self) -> &'static str {
+ "Invalid Message"
+ }
 	}
 
 	impl Writeable for InvalidCustomMessage {

lightning/src/onion_message/messenger.rs

@@ -135,6 +135,7 @@ pub(super) const MAX_TIMER_TICKS: usize = 2;
 /// # let your_custom_message_type = 42;
 /// your_custom_message_type
 /// }
+/// fn msg_type(&self) -> &'static str { "YourCustomMessageType" }
 /// }
 /// // Send a custom onion message to a node id.
 /// let destination = Destination::Node(destination_node_id);
@@ -246,6 +247,50 @@ impl OnionMessageRecipient {
 	}
 }
 
+
+/// The `Responder` struct creates an appropriate [`ResponseInstruction`]
+/// for responding to a message.
+pub struct Responder {
+	/// The path along which a response can be sent.
+	reply_path: BlindedPath,
+	path_id: Option<[u8; 32]>
+}
+
+impl Responder {
+	/// Creates a new [`Responder`] instance with the provided reply path.
+	fn new(reply_path: BlindedPath, path_id: Option<[u8; 32]>) -> Self {
+ Responder {
+ reply_path,
+ path_id,
+ }
+	}
+
+	/// Creates the appropriate [`ResponseInstruction`] for a given response.
+	pub fn respond<T: OnionMessageContents>(self, response: T) -> ResponseInstruction<T> {
+ ResponseInstruction::WithoutReplyPath(OnionMessageResponse {
+ message: response,
+ reply_path: self.reply_path,
+ path_id: self.path_id,
+ })
+	}
+}
+
+/// This struct contains the information needed to reply to a received message.
+pub struct OnionMessageResponse<T: OnionMessageContents> {
+	message: T,
+	reply_path: BlindedPath,
+	path_id: Option<[u8; 32]>,
+}
+
+/// `ResponseInstruction` represents instructions for responding to received messages.
+pub enum ResponseInstruction<T: OnionMessageContents> {
+	/// Indicates that a response should be sent without including a reply path
+	/// for the recipient to respond back.
+	WithoutReplyPath(OnionMessageResponse<T>),
+	/// Indicates that there's no response to send back.
+	NoResponse,
+}
+
 /// An [`OnionMessage`] for [`OnionMessenger`] to send.
 ///
 /// These are obtained when released from [`OnionMessenger`]'s handlers after which they are
@@ -546,7 +591,7 @@ pub trait CustomOnionMessageHandler {
 	/// Called with the custom message that was received, returning a response to send, if any.
 	///
 	/// The returned [`Self::CustomMessage`], if any, is enqueued to be sent by [`OnionMessenger`].
-	fn handle_custom_message(&self, msg: Self::CustomMessage) -> Option<Self::CustomMessage>;
+	fn handle_custom_message(&self, message: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage>;
 
 	/// Read a custom message of type `message_type` from `buffer`, returning `Ok(None)` if the
 	/// message type is unknown.
@@ -933,19 +978,18 @@ where
 	}
 
 	fn handle_onion_message_response<T: OnionMessageContents>(
- &self, response: Option<T>, reply_path: Option<BlindedPath>, log_suffix: fmt::Arguments
+ &self, response: ResponseInstruction<T>
 	) {
- if let Some(response) = response {
- match reply_path {
- Some(reply_path) => {
- let _ = self.find_path_and_enqueue_onion_message(
- response, Destination::BlindedPath(reply_path), None, log_suffix
- );
- },
- None => {
- log_trace!(self.logger, "Missing reply path {}", log_suffix);
- },
- }
+ if let ResponseInstruction::WithoutReplyPath(response) = response {
+ let message_type = response.message.msg_type();
+ let _ = self.find_path_and_enqueue_onion_message(
+ response.message, Destination::BlindedPath(response.reply_path), None,
+ format_args!(
+ "when responding with {} to an onion message with path_id {:02x?}",
+ message_type,
+ response.path_id
+ )
+ );
  }
 	}
 
@@ -1029,22 +1073,18 @@ where
 
  match message {
  ParsedOnionMessageContents::Offers(msg) => {
- let response = self.offers_handler.handle_message(msg);
- self.handle_onion_message_response(
- response, reply_path, format_args!(
- "when responding to Offers onion message with path_id {:02x?}",
- path_id
- )
+ let responder = reply_path.map(
+ |reply_path| Responder::new(reply_path, path_id)
  );
+ let response_instructions = self.offers_handler.handle_message(msg, responder);
+ self.handle_onion_message_response(response_instructions);
  },
  ParsedOnionMessageContents::Custom(msg) => {
- let response = self.custom_handler.handle_custom_message(msg);
- self.handle_onion_message_response(
- response, reply_path, format_args!(
- "when responding to Custom onion message with path_id {:02x?}",
- path_id
- )
+ let responder = reply_path.map(
+ |reply_path| Responder::new(reply_path, path_id)
  );
+ let response_instructions = self.custom_handler.handle_custom_message(msg, responder);
+ self.handle_onion_message_response(response_instructions);
  },
  }
  },

fuzz/src/onion_message.rs

@@ -16,7 +16,7 @@ use lightning::sign::{Recipient, KeyMaterial, EntropySource, NodeSigner, SignerP
 use lightning::util::test_channel_signer::TestChannelSigner;
 use lightning::util::logger::Logger;
 use lightning::util::ser::{Readable, Writeable, Writer};
-use lightning::onion_message::messenger::{CustomOnionMessageHandler, Destination, MessageRouter, OnionMessagePath, OnionMessenger, PendingOnionMessage};
+use lightning::onion_message::messenger::{CustomOnionMessageHandler, Destination, MessageRouter, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction};
 use lightning::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use lightning::onion_message::packet::OnionMessageContents;
 
@@ -97,8 +97,8 @@ impl MessageRouter for TestMessageRouter {
 struct TestOffersMessageHandler {}
 
 impl OffersMessageHandler for TestOffersMessageHandler {
-	fn handle_message(&self, _message: OffersMessage) -> Option<OffersMessage> {
-None
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ResponseInstruction::NoResponse
 	}
 }
 
@@ -112,6 +112,9 @@ impl OnionMessageContents for TestCustomMessage {
 	fn tlv_type(&self) -> u64 {
  CUSTOM_MESSAGE_TYPE
 	}
+	fn msg_type(&self) -> &'static str {
+ "Custom Message"
+	}
 }
 
 impl Writeable for TestCustomMessage {
@@ -124,8 +127,11 @@ struct TestCustomMessageHandler {}
 
 impl CustomOnionMessageHandler for TestCustomMessageHandler {
 	type CustomMessage = TestCustomMessage;
-	fn handle_custom_message(&self, _msg: Self::CustomMessage) -> Option<Self::CustomMessage> {
- Some(TestCustomMessage {})
+	fn handle_custom_message(&self, message: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
+ match responder {
+ Some(responder) => responder.respond(message),
+ None => ResponseInstruction::NoResponse
+ }
 	}
 	fn read_custom_message<R: io::Read>(&self, _message_type: u64, buffer: &mut R) -> Result<Option<Self::CustomMessage>, msgs::DecodeError> {
  let mut buf = Vec::new();
@@ -280,9 +286,9 @@ mod tests {
  "Received an onion message with path_id None and a reply_path: Custom(TestCustomMessage)"
  .to_string())), Some(&1));
  assert_eq!(log_entries.get(&("lightning::onion_message::messenger".to_string(),
- "Constructing onion message when responding to Custom onion message with path_id None: TestCustomMessage".to_string())), Some(&1));
+ "Constructing onion message when responding with Custom Message to an onion message with path_id None: TestCustomMessage".to_string())), Some(&1));
  assert_eq!(log_entries.get(&("lightning::onion_message::messenger".to_string(),
- "Buffered onion message when responding to Custom onion message with path_id None".to_string())), Some(&1));
+ "Buffered onion message when responding with Custom Message to an onion message with path_id None".to_string())), Some(&1));
  }
 
  let two_unblinded_hops_om = "\

lightning/src/ln/channelmanager.rs

@@ -64,7 +64,7 @@ use crate::offers::invoice_request::{DerivedPayerId, InvoiceRequestBuilder};
 use crate::offers::offer::{Offer, OfferBuilder};
 use crate::offers::parse::Bolt12SemanticError;
 use crate::offers::refund::{Refund, RefundBuilder};
-use crate::onion_message::messenger::{Destination, MessageRouter, PendingOnionMessage, new_pending_onion_message};
+use crate::onion_message::messenger::{new_pending_onion_message, Destination, MessageRouter, PendingOnionMessage, Responder, ResponseInstruction};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use crate::sign::{EntropySource, NodeSigner, Recipient, SignerProvider};
 use crate::sign::ecdsa::WriteableEcdsaChannelSigner;
@@ -75,6 +75,7 @@ use crate::util::string::UntrustedString;
 use crate::util::ser::{BigSize, FixedLengthReader, Readable, ReadableArgs, MaybeReadable, Writeable, Writer, VecWriter};
 use crate::util::logger::{Level, Logger, WithContext};
 use crate::util::errors::APIError;
+
 #[cfg(not(c_bindings))]
 use {
 	crate::offers::offer::DerivedMetadata,
@@ -10332,23 +10333,27 @@ where
 	R::Target: Router,
 	L::Target: Logger,
 {
-	fn handle_message(&self, message: OffersMessage) -> Option<OffersMessage> {
+	fn handle_message(&self, message: OffersMessage, responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
  let secp_ctx = &self.secp_ctx;
  let expanded_key = &self.inbound_payment_key;
 
  match message {
  OffersMessage::InvoiceRequest(invoice_request) => {
+ let responder = match responder {
+ Some(responder) => responder,
+ None => return ResponseInstruction::NoResponse,
+ };
  let amount_msats = match InvoiceBuilder::<DerivedSigningPubkey>::amount_msats(
  &invoice_request
  ) {
  Ok(amount_msats) => amount_msats,
- Err(error) => return Some(OffersMessage::InvoiceError(error.into())),
+ Err(error) => return responder.respond(OffersMessage::InvoiceError(error.into())),
  };
  let invoice_request = match invoice_request.verify(expanded_key, secp_ctx) {
  Ok(invoice_request) => invoice_request,
  Err(()) => {
  let error = Bolt12SemanticError::InvalidMetadata;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10359,7 +10364,7 @@ where
  Ok((payment_hash, payment_secret)) => (payment_hash, payment_secret),
  Err(()) => {
  let error = Bolt12SemanticError::InvalidAmount;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10373,7 +10378,7 @@ where
  Ok(payment_paths) => payment_paths,
  Err(()) => {
  let error = Bolt12SemanticError::MissingPaths;
- return Some(OffersMessage::InvoiceError(error.into()));
+ return responder.respond(OffersMessage::InvoiceError(error.into()));
  },
  };
 
@@ -10418,8 +10423,8 @@ where
  };
 
  match response {
- Ok(invoice) => Some(OffersMessage::Invoice(invoice)),
- Err(error) => Some(OffersMessage::InvoiceError(error.into())),
+ Ok(invoice) => return responder.respond(OffersMessage::Invoice(invoice)),
+ Err(error) => return responder.respond(OffersMessage::InvoiceError(error.into())),
  }
  },
  OffersMessage::Invoice(invoice) => {
@@ -10439,14 +10444,21 @@ where
  }
  });
 
- match response {
- Ok(()) => None,
- Err(e) => Some(OffersMessage::InvoiceError(e)),
+ match (responder, response) {
+ (Some(responder), Err(e)) => responder.respond(OffersMessage::InvoiceError(e)),
+ (None, Err(_)) => {
+ log_trace!(
+ self.logger,
+ "A response was generated, but there is no reply_path specified for sending the response."
+ );
+ return ResponseInstruction::NoResponse;
+ }
+ _ => return ResponseInstruction::NoResponse,
  }
  },
  OffersMessage::InvoiceError(invoice_error) => {
  log_trace!(self.logger, "Received invoice_error: {}", invoice_error);
-None
+return ResponseInstruction::NoResponse;
  },
  }
 	}

lightning/src/ln/peer_handler.rs

@@ -28,7 +28,7 @@ use crate::util::ser::{VecWriter, Writeable, Writer};
 use crate::ln::peer_channel_encryptor::{PeerChannelEncryptor, NextNoiseStep, MessageBuf, MSG_BUF_ALLOC_SIZE};
 use crate::ln::wire;
 use crate::ln::wire::{Encode, Type};
-use crate::onion_message::messenger::{CustomOnionMessageHandler, PendingOnionMessage};
+use crate::onion_message::messenger::{CustomOnionMessageHandler, PendingOnionMessage, Responder, ResponseInstruction};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
 use crate::onion_message::packet::OnionMessageContents;
 use crate::routing::gossip::{NodeId, NodeAlias};
@@ -123,6 +123,7 @@ impl RoutingMessageHandler for IgnoringMessageHandler {
 	}
 	fn processing_queue_high(&self) -> bool { false }
 }
+
 impl OnionMessageHandler for IgnoringMessageHandler {
 	fn handle_onion_message(&self, _their_node_id: &PublicKey, _msg: &msgs::OnionMessage) {}
 	fn next_onion_message_for_peer(&self, _peer_node_id: PublicKey) -> Option<msgs::OnionMessage> { None }
@@ -134,12 +135,15 @@ impl OnionMessageHandler for IgnoringMessageHandler {
  InitFeatures::empty()
 	}
 }
+
 impl OffersMessageHandler for IgnoringMessageHandler {
-	fn handle_message(&self, _msg: OffersMessage) -> Option<OffersMessage> { None }
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ ResponseInstruction::NoResponse
+	}
 }
 impl CustomOnionMessageHandler for IgnoringMessageHandler {
 	type CustomMessage = Infallible;
-	fn handle_custom_message(&self, _msg: Infallible) -> Option<Infallible> {
+	fn handle_custom_message(&self, _message: Self::CustomMessage, _responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
  // Since we always return `None` in the read the handle method should never be called.
  unreachable!();
 	}
@@ -153,6 +157,7 @@ impl CustomOnionMessageHandler for IgnoringMessageHandler {
 
 impl OnionMessageContents for Infallible {
 	fn tlv_type(&self) -> u64 { unreachable!(); }
+	fn msg_type(&self) -> &'static str { unreachable!(); }
 }
 
 impl Deref for IgnoringMessageHandler {

lightning/src/onion_message/functional_tests.rs

@@ -18,7 +18,7 @@ use crate::routing::test_utils::{add_channel, add_or_update_node};
 use crate::sign::{NodeSigner, Recipient};
 use crate::util::ser::{FixedLengthReader, LengthReadable, Writeable, Writer};
 use crate::util::test_utils;
-use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, SendError};
+use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction, SendError};
 use super::offers::{OffersMessage, OffersMessageHandler};
 use super::packet::{OnionMessageContents, Packet};
 
@@ -62,8 +62,8 @@ struct MessengerNode {
 struct TestOffersMessageHandler {}
 
 impl OffersMessageHandler for TestOffersMessageHandler {
-	fn handle_message(&self, _message: OffersMessage) -> Option<OffersMessage> {
-None
+	fn handle_message(&self, _message: OffersMessage, _responder: Option<Responder>) -> ResponseInstruction<OffersMessage> {
+ResponseInstruction::NoResponse
 	}
 }
 
@@ -85,6 +85,9 @@ impl OnionMessageContents for TestCustomMessage {
  TestCustomMessage::Response => CUSTOM_RESPONSE_MESSAGE_TYPE,
  }
 	}
+	fn msg_type(&self) -> &'static str {
+ "Custom Message"
+	}
 }
 
 impl Writeable for TestCustomMessage {
@@ -123,15 +126,19 @@ impl Drop for TestCustomMessageHandler {
 
 impl CustomOnionMessageHandler for TestCustomMessageHandler {
 	type CustomMessage = TestCustomMessage;
-	fn handle_custom_message(&self, msg: Self::CustomMessage) -> Option<Self::CustomMessage> {
+	fn handle_custom_message(&self, msg: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage> {
  match self.expected_messages.lock().unwrap().pop_front() {
  Some(expected_msg) => assert_eq!(expected_msg, msg),
  None => panic!("Unexpected message: {:?}", msg),
  }
-
- match msg {
+ let response_option = match msg {
  TestCustomMessage::Request => Some(TestCustomMessage::Response),
  TestCustomMessage::Response => None,
+ };
+ if let (Some(response), Some(responder)) = (response_option, responder) {
+ responder.respond(response)
+ } else {
+ ResponseInstruction::NoResponse
  }
 	}
 	fn read_custom_message<R: io::Read>(&self, message_type: u64, buffer: &mut R) -> Result<Option<Self::CustomMessage>, DecodeError> where Self: Sized {
@@ -422,6 +429,9 @@ fn invalid_custom_message_type() {
  // Onion message contents must have a TLV >= 64.
  63
  }
+ fn msg_type(&self) -> &'static str {
+ "Invalid Message"
+ }
 	}
 
 	impl Writeable for InvalidCustomMessage {

lightning/src/onion_message/messenger.rs

@@ -135,6 +135,7 @@ pub(super) const MAX_TIMER_TICKS: usize = 2;
 /// # let your_custom_message_type = 42;
 /// your_custom_message_type
 /// }
+/// fn msg_type(&self) -> &'static str { "YourCustomMessageType" }
 /// }
 /// // Send a custom onion message to a node id.
 /// let destination = Destination::Node(destination_node_id);
@@ -246,6 +247,50 @@ impl OnionMessageRecipient {
 	}
 }
 
+
+/// The `Responder` struct creates an appropriate [`ResponseInstruction`]
+/// for responding to a message.
+pub struct Responder {
+	/// The path along which a response can be sent.
+	reply_path: BlindedPath,
+	path_id: Option<[u8; 32]>
+}
+
+impl Responder {
+	/// Creates a new [`Responder`] instance with the provided reply path.
+	fn new(reply_path: BlindedPath, path_id: Option<[u8; 32]>) -> Self {
+ Responder {
+ reply_path,
+ path_id,
+ }
+	}
+
+	/// Creates the appropriate [`ResponseInstruction`] for a given response.
+	pub fn respond<T: OnionMessageContents>(self, response: T) -> ResponseInstruction<T> {
+ ResponseInstruction::WithoutReplyPath(OnionMessageResponse {
+ message: response,
+ reply_path: self.reply_path,
+ path_id: self.path_id,
+ })
+	}
+}
+
+/// This struct contains the information needed to reply to a received message.
+pub struct OnionMessageResponse<T: OnionMessageContents> {
+	message: T,
+	reply_path: BlindedPath,
+	path_id: Option<[u8; 32]>,
+}
+
+/// `ResponseInstruction` represents instructions for responding to received messages.
+pub enum ResponseInstruction<T: OnionMessageContents> {
+	/// Indicates that a response should be sent without including a reply path
+	/// for the recipient to respond back.
+	WithoutReplyPath(OnionMessageResponse<T>),
+	/// Indicates that there's no response to send back.
+	NoResponse,
+}
+
 /// An [`OnionMessage`] for [`OnionMessenger`] to send.
 ///
 /// These are obtained when released from [`OnionMessenger`]'s handlers after which they are
@@ -546,7 +591,7 @@ pub trait CustomOnionMessageHandler {
 	/// Called with the custom message that was received, returning a response to send, if any.
 	///
 	/// The returned [`Self::CustomMessage`], if any, is enqueued to be sent by [`OnionMessenger`].
-	fn handle_custom_message(&self, msg: Self::CustomMessage) -> Option<Self::CustomMessage>;
+	fn handle_custom_message(&self, message: Self::CustomMessage, responder: Option<Responder>) -> ResponseInstruction<Self::CustomMessage>;
 
 	/// Read a custom message of type `message_type` from `buffer`, returning `Ok(None)` if the
 	/// message type is unknown.
@@ -933,19 +978,18 @@ where
 	}
 
 	fn handle_onion_message_response<T: OnionMessageContents>(
- &self, response: Option<T>, reply_path: Option<BlindedPath>, log_suffix: fmt::Arguments
+ &self, response: ResponseInstruction<T>
 	) {
- if let Some(response) = response {
- match reply_path {
- Some(reply_path) => {
- let _ = self.find_path_and_enqueue_onion_message(
- response, Destination::BlindedPath(reply_path), None, log_suffix
- );
- },
- None => {
- log_trace!(self.logger, "Missing reply path {}", log_suffix);
- },
- }
+ if let ResponseInstruction::WithoutReplyPath(response) = response {
+ let message_type = response.message.msg_type();
+ let _ = self.find_path_and_enqueue_onion_message(
+ response.message, Destination::BlindedPath(response.reply_path), None,
+ format_args!(
+ "when responding with {} to an onion message with path_id {:02x?}",
+ message_type,
+ response.path_id
+ )
+ );
  }
 	}
 
@@ -1029,22 +1073,18 @@ where
 
  match message {
  ParsedOnionMessageContents::Offers(msg) => {
- let response = self.offers_handler.handle_message(msg);
- self.handle_onion_message_response(
- response, reply_path, format_args!(
- "when responding to Offers onion message with path_id {:02x?}",
- path_id
- )
+ let responder = reply_path.map(
+ |reply_path| Responder::new(reply_path, path_id)
  );
+ let response_instructions = self.offers_handler.handle_message(msg, responder);
+ self.handle_onion_message_response(response_instructions);
  },
  ParsedOnionMessageContents::Custom(msg) => {
- let response = self.custom_handler.handle_custom_message(msg);
- self.handle_onion_message_response(
- response, reply_path, format_args!(
- "when responding to Custom onion message with path_id {:02x?}",
- path_id
- )
+ let responder = reply_path.map(
+ |reply_path| Responder::new(reply_path, path_id)
  );
+ let response_instructions = self.custom_handler.handle_custom_message(msg, responder);
+ self.handle_onion_message_response(response_instructions);
  },
  }
  },