Introduce ResponseInstruction::WithReplyPath variant.

And expand handle_onion_message_response return Type

1. Introduce a new function in OnionMessenger to create blinded paths.
2. Use it in handle_onion_message_response to create a reply_path for
   the right variant and use it in onion_message.
3. Expand the return type of handle_onion_message_response to handle three cases:
	1. Ok(None) in case of no response to be sent.
	2. Ok(Some(SendSuccess) and Err(SendError) in case of successful and
   	unsuccessful queueing up of response messages respectively.

This allows the user to get access to the Success/Failure status of the sending
of response and handle it accordingly.

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, Responder, ResponseInstruction, SendError};
+use super::messenger::{CustomOnionMessageHandler, DefaultMessageRouter, Destination, OnionMessagePath, OnionMessenger, PendingOnionMessage, Responder, ResponseInstruction, SendError, SendSuccess};
 use super::offers::{OffersMessage, OffersMessageHandler};
 use super::packet::{OnionMessageContents, Packet};
 
@@ -391,7 +391,11 @@ fn async_response_over_one_blinded_hop() {
 	let response_instruction = nodes[0].custom_message_handler.handle_custom_message(message, responder);
 
 	// 6. Simulate Alice asynchronously responding back to Bob with a response.
-	nodes[0].messenger.handle_onion_message_response(response_instruction);
+	assert_eq!(
+		nodes[0].messenger.handle_onion_message_response(response_instruction),
+		Ok(Some(SendSuccess::Buffered)),
+	);
+
 	bob.custom_message_handler.expect_message(TestCustomMessage::Response);
 
 	pass_along_path(&nodes);

lightning/src/onion_message/messenger.rs

@@ -267,14 +267,27 @@ impl Responder {
 		}
 	}
 
-	/// Creates the appropriate [`ResponseInstruction`] for a given response.
+	/// Creates a [`ResponseInstruction::WithoutReplyPath`] for a given response.
+	///
+	/// Use when the recipient doesn't need to send back a reply to us.
 	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,
 		})
 	}
+
+	/// Creates a [`ResponseInstruction::WithReplyPath`] for a given response.
+	///
+	/// Use when the recipient need to send back a reply to us.
+	pub fn respond_with_reply_path<T: OnionMessageContents>(self, response: T) -> ResponseInstruction<T> {
+		ResponseInstruction::WithReplyPath(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.
@@ -286,6 +299,9 @@ pub struct OnionMessageResponse<T: OnionMessageContents> {
 
 /// `ResponseInstruction` represents instructions for responding to received messages.
 pub enum ResponseInstruction<T: OnionMessageContents> {
+	/// Indicates that a response should be sent including a reply path for
+	/// the recipient to respond back.
+	WithReplyPath(OnionMessageResponse<T>),
 	/// Indicates that a response should be sent without including a reply path
 	/// for the recipient to respond back.
 	WithoutReplyPath(OnionMessageResponse<T>),
@@ -971,6 +987,24 @@ where
 			.map_err(|_| SendError::PathNotFound)
 	}
 
+	fn create_blinded_path(&self) -> Result<BlindedPath, SendError> {
+		let recipient = self.node_signer
+			.get_node_id(Recipient::Node)
+			.map_err(|_| SendError::GetNodeIdFailed)?;
+		let secp_ctx = &self.secp_ctx;
+
+		let peers = self.message_recipients.lock().unwrap()
+			.iter()
+			.filter(|(_, peer)| matches!(peer, OnionMessageRecipient::ConnectedPeer(_)))
+			.map(|(node_id, _)| *node_id)
+			.collect();
+
+		self.message_router
+			.create_blinded_paths(recipient, peers, secp_ctx)
+			.and_then(|paths| paths.into_iter().next().ok_or(()))
+			.map_err(|_| SendError::PathNotFound)
+	}
+
 	fn enqueue_onion_message<T: OnionMessageContents>(
 		&self, path: OnionMessagePath, contents: T, reply_path: Option<BlindedPath>,
 		log_suffix: fmt::Arguments
@@ -1047,18 +1081,36 @@ where
 	/// enqueueing any response for sending.
 	pub fn handle_onion_message_response<T: OnionMessageContents>(
 		&self, response: ResponseInstruction<T>
-	) {
-		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
-				)
-			);
-		}
+	) -> Result<Option<SendSuccess>, SendError> {
+		let (response, create_reply_path) = match response {
+			ResponseInstruction::WithReplyPath(response) => (response, true),
+			ResponseInstruction::WithoutReplyPath(response) => (response, false),
+			ResponseInstruction::NoResponse => return Ok(None),
+		};
+
+		let message_type = response.message.msg_type();
+		let log_suffix = format!(
+			"when responding with {} to an onion message with path_id {:02x?}",
+			message_type,
+			response.path_id
+		);
+
+		let reply_path = if create_reply_path {
+			match self.create_blinded_path() {
+				Ok(reply_path) => Some(reply_path),
+				Err(e) => {
+					log_trace!(
+						self.logger, "Failed to create reply_path {}: {:?}", log_suffix, e
+					);
+					return Err(e);
+				}
+			}
+		} else { None };
+
+		self.find_path_and_enqueue_onion_message(
+			response.message, Destination::BlindedPath(response.reply_path), reply_path,
+			format_args!("{}", log_suffix)
+		).map(|result| Some(result))
 	}
 
 	#[cfg(test)]
@@ -1164,14 +1216,14 @@ where
 							|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);
+						let _ = self.handle_onion_message_response(response_instructions);
 					},
 					ParsedOnionMessageContents::Custom(msg) => {
 						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);
+						let _ = self.handle_onion_message_response(response_instructions);
 					},
 				}
 			},