Expand create_blinded_path Functionality for Enhanced Path Diversification

- Previously, the `create_blinded_path` function was limited to
  returning a single `BlindedPath`, which restricted the usage of
  `blinded_paths`.
- This commit extends the `create_blinded_path` function to return
  the entire blinded path vector generated by the `MessageRouter`'s
  `create_blinded_paths`.
- The updated functionality is integrated across the codebase, enabling
  the sending of Offers Response messages, such as `InvoiceRequest`
  (in `pay_for_offer`) and `Invoice` (in `request_refund_payment`),
  utilizing multiple reply paths.

Author: shaavan

lightning/src/ln/channelmanager.rs

@@ -8376,8 +8376,10 @@ macro_rules! create_offer_builder { ($self: ident, $builder: ty) => {
 		let entropy = &*$self.entropy_source;
 		let secp_ctx = &$self.secp_ctx;
 
-		let path = $self.create_blinded_path_using_absolute_expiry(absolute_expiry)
+		let path = $self.create_blinded_paths_using_absolute_expiry(absolute_expiry)
+			.and_then(|paths| paths.into_iter().next().ok_or(()))
 			.map_err(|_| Bolt12SemanticError::MissingPaths)?;
+
 		let builder = OfferBuilder::deriving_signing_pubkey(
 			node_id, expanded_key, entropy, secp_ctx
 		)
@@ -8448,8 +8450,10 @@ macro_rules! create_refund_builder { ($self: ident, $builder: ty) => {
 		let entropy = &*$self.entropy_source;
 		let secp_ctx = &$self.secp_ctx;
 
-		let path = $self.create_blinded_path_using_absolute_expiry(Some(absolute_expiry))
+		let path = $self.create_blinded_paths_using_absolute_expiry(Some(absolute_expiry))
+			.and_then(|paths| paths.into_iter().next().ok_or(()))
 			.map_err(|_| Bolt12SemanticError::MissingPaths)?;
+
 		let builder = RefundBuilder::deriving_payer_id(
 			node_id, expanded_key, entropy, secp_ctx, amount_msats, payment_id
 		)?
@@ -8470,6 +8474,12 @@ macro_rules! create_refund_builder { ($self: ident, $builder: ty) => {
 	}
 } }
 
+/// Defines the maximum number of [`OffersMessage`] to be sent along different reply paths.
+/// Sending multiple requests increases the chances of successful delivery in case some
+/// paths are unavailable. However, only one invoice for a given [`PaymentId`] will be paid,
+/// even if multiple invoices are received.
+const OFFERS_MESSAGE_REQUEST_LIMIT: usize = 10;
+
 impl<M: Deref, T: Deref, ES: Deref, NS: Deref, SP: Deref, F: Deref, R: Deref, L: Deref> ChannelManager<M, T, ES, NS, SP, F, R, L>
 where
 	M::Target: chain::Watch<<SP::Target as SignerProvider>::EcdsaSigner>,
@@ -8571,7 +8581,7 @@ where
 			Some(payer_note) => builder.payer_note(payer_note),
 		};
 		let invoice_request = builder.build_and_sign()?;
-		let reply_path = self.create_blinded_path().map_err(|_| Bolt12SemanticError::MissingPaths)?;
+		let reply_paths = self.create_blinded_paths().map_err(|_| Bolt12SemanticError::MissingPaths)?;
 
 		let _persistence_guard = PersistenceNotifierGuard::notify_on_drop(self);
 
@@ -8584,25 +8594,27 @@ where
 
 		let mut pending_offers_messages = self.pending_offers_messages.lock().unwrap();
 		if !offer.paths().is_empty() {
-			// Send as many invoice requests as there are paths in the offer (with an upper bound).
-			// Using only one path could result in a failure if the path no longer exists. But only
-			// one invoice for a given payment id will be paid, even if more than one is received.
-			const REQUEST_LIMIT: usize = 10;
-			for path in offer.paths().into_iter().take(REQUEST_LIMIT) {
+			reply_paths
+				.iter()
+				.flat_map(|reply_path| offer.paths().iter().map(move |path| (path, reply_path)))
+				.take(OFFERS_MESSAGE_REQUEST_LIMIT)
+				.for_each(|(path, reply_path)| {
+					let message = new_pending_onion_message(
+						OffersMessage::InvoiceRequest(invoice_request.clone()),
+						Destination::BlindedPath(path.clone()),
+						Some(reply_path.clone()),
+					);
+					pending_offers_messages.push(message);
+				});
+		} else if let Some(signing_pubkey) = offer.signing_pubkey() {
+			for reply_path in reply_paths {
 				let message = new_pending_onion_message(
 					OffersMessage::InvoiceRequest(invoice_request.clone()),
-					Destination::BlindedPath(path.clone()),
-					Some(reply_path.clone()),
+					Destination::Node(signing_pubkey),
+					Some(reply_path),
 				);
 				pending_offers_messages.push(message);
 			}
-		} else if let Some(signing_pubkey) = offer.signing_pubkey() {
-			let message = new_pending_onion_message(
-				OffersMessage::InvoiceRequest(invoice_request),
-				Destination::Node(signing_pubkey),
-				Some(reply_path),
-			);
-			pending_offers_messages.push(message);
 		} else {
 			debug_assert!(false);
 			return Err(Bolt12SemanticError::MissingSigningPubkey);
@@ -8671,26 +8683,32 @@ where
 				)?;
 				let builder: InvoiceBuilder<DerivedSigningPubkey> = builder.into();
 				let invoice = builder.allow_mpp().build_and_sign(secp_ctx)?;
-				let reply_path = self.create_blinded_path()
+				let reply_paths = self.create_blinded_paths()
 					.map_err(|_| Bolt12SemanticError::MissingPaths)?;
 
 				let mut pending_offers_messages = self.pending_offers_messages.lock().unwrap();
 				if refund.paths().is_empty() {
-					let message = new_pending_onion_message(
-						OffersMessage::Invoice(invoice.clone()),
-						Destination::Node(refund.payer_id()),
-						Some(reply_path),
-					);
-					pending_offers_messages.push(message);
-				} else {
-					for path in refund.paths() {
+					for reply_path in reply_paths {
 						let message = new_pending_onion_message(
 							OffersMessage::Invoice(invoice.clone()),
-							Destination::BlindedPath(path.clone()),
-							Some(reply_path.clone()),
+							Destination::Node(refund.payer_id()),
+							Some(reply_path),
 						);
 						pending_offers_messages.push(message);
 					}
+				} else {
+					reply_paths
+						.iter()
+						.flat_map(|reply_path| refund.paths().iter().map(move |path| (path, reply_path)))
+						.take(OFFERS_MESSAGE_REQUEST_LIMIT)
+						.for_each(|(path, reply_path)| {
+							let message = new_pending_onion_message(
+								OffersMessage::Invoice(invoice.clone()),
+								Destination::BlindedPath(path.clone()),
+								Some(reply_path.clone()),
+							);
+							pending_offers_messages.push(message);
+						});
 				}
 
 				Ok(invoice)
@@ -8797,22 +8815,22 @@ where
 		inbound_payment::get_payment_preimage(payment_hash, payment_secret, &self.inbound_payment_key)
 	}
 
-	/// Creates a blinded path by delegating to [`MessageRouter`] based on the path's intended
-	/// lifetime.
+	/// Creates a collection of blinded paths by delegating to [`MessageRouter`] based on
+	/// the path's intended lifetime.
 	///
 	/// Whether or not the path is compact depends on whether the path is short-lived or long-lived,
 	/// respectively, based on the given `absolute_expiry` as seconds since the Unix epoch. See
 	/// [`MAX_SHORT_LIVED_RELATIVE_EXPIRY`].
-	fn create_blinded_path_using_absolute_expiry(
+	fn create_blinded_paths_using_absolute_expiry(
 		&self, absolute_expiry: Option<Duration>
-	) -> Result<BlindedPath, ()> {
+	) -> Result<Vec<BlindedPath>, ()> {
 		let now = self.duration_since_epoch();
 		let max_short_lived_absolute_expiry = now.saturating_add(MAX_SHORT_LIVED_RELATIVE_EXPIRY);
 
 		if absolute_expiry.unwrap_or(Duration::MAX) <= max_short_lived_absolute_expiry {
-			self.create_compact_blinded_path()
+			self.create_compact_blinded_paths()
 		} else {
-			self.create_blinded_path()
+			self.create_blinded_paths()
 		}
 	}
 
@@ -8829,10 +8847,11 @@ where
 		now
 	}
 
-	/// Creates a blinded path by delegating to [`MessageRouter::create_blinded_paths`].
+	/// Creates a collection of blinded paths by delegating to
+	/// [`MessageRouter::create_blinded_paths`].
 	///
-	/// Errors if the `MessageRouter` errors or returns an empty `Vec`.
-	fn create_blinded_path(&self) -> Result<BlindedPath, ()> {
+	/// Errors if the `MessageRouter` errors.
+	fn create_blinded_paths(&self) -> Result<Vec<BlindedPath>, ()> {
 		let recipient = self.get_our_node_id();
 		let secp_ctx = &self.secp_ctx;
 
@@ -8846,13 +8865,14 @@ where
 
 		self.router
 			.create_blinded_paths(recipient, peers, secp_ctx)
-			.and_then(|paths| paths.into_iter().next().ok_or(()))
+			.and_then(|paths| (!paths.is_empty()).then(|| paths).ok_or(()))
 	}
 
-	/// Creates a blinded path by delegating to [`MessageRouter::create_compact_blinded_paths`].
+	/// Creates a collection of blinded paths by delegating to
+	/// [`MessageRouter::create_compact_blinded_paths`].
 	///
-	/// Errors if the `MessageRouter` errors or returns an empty `Vec`.
-	fn create_compact_blinded_path(&self) -> Result<BlindedPath, ()> {
+	/// Errors if the `MessageRouter` errors.
+	fn create_compact_blinded_paths(&self) -> Result<Vec<BlindedPath>, ()> {
 		let recipient = self.get_our_node_id();
 		let secp_ctx = &self.secp_ctx;
 
@@ -8873,7 +8893,7 @@ where
 
 		self.router
 			.create_compact_blinded_paths(recipient, peers, secp_ctx)
-			.and_then(|paths| paths.into_iter().next().ok_or(()))
+			.and_then(|paths| (!paths.is_empty()).then(|| paths).ok_or(()))
 	}
 
 	/// Creates multi-hop blinded payment paths for the given `amount_msats` by delegating to