Introduce create_{offer,refund}_builder_using_router

Introduced two new builder functions that accept a custom
`MessageRouter` as input.

This gives users the flexibility to plug in their own router
implementation, enabling custom logic for blinded path creation
when building Offers or Refunds.
It’s a step toward greater extensibility and control in how paths
are constructed.

Author: shaavan

lightning/src/ln/channelmanager.rs

@@ -10221,6 +10221,57 @@ macro_rules! create_offer_builder { ($self: ident, $builder: ty) => {
 
 		Ok(builder.into())
 	}
+
+	/// Creates an [`OfferBuilder`] such that the [`Offer`] it builds is recognized by the
+	/// [`ChannelManager`] when handling [`InvoiceRequest`] messages for the offer.
+	///
+	/// # Privacy
+	///
+	/// Uses [`MessageRouter`] to construct a [`BlindedMessagePath`] for the offer based on the given
+	/// `absolute_expiry` according to [`MAX_SHORT_LIVED_RELATIVE_EXPIRY`]. See those docs for
+	/// privacy implications as well as those of the parameterized [`Router`], which implements
+	/// [`MessageRouter`].
+	///
+	/// Also, uses a derived signing pubkey in the offer for recipient privacy.
+	///
+	/// # Limitations
+	///
+	/// Requires a direct connection to the introduction node in the responding [`InvoiceRequest`]'s
+	/// reply path.
+	///
+	/// # Errors
+	///
+	/// Errors if the provided [`Router`] is unable to create a blinded path for the offer.
+	///
+	/// [`Offer`]: crate::offers::offer::Offer
+	/// [`InvoiceRequest`]: crate::offers::invoice_request::InvoiceRequest
+	pub fn create_offer_builder_using_router<ME: MessageRouter>(
+		&$self,
+		router: ME,
+	) -> Result<$builder, Bolt12SemanticError> {
+		let node_id = $self.get_our_node_id();
+		let expanded_key = &$self.inbound_payment_key;
+		let entropy = &*$self.entropy_source;
+		let secp_ctx = &$self.secp_ctx;
+
+		let nonce = Nonce::from_entropy_source(entropy);
+		let context = MessageContext::Offers(OffersContext::InvoiceRequest { nonce });
+
+		let peers = $self.get_peers_for_blinded_path();
+
+		let path = router.create_blinded_paths(node_id, context, peers, secp_ctx)
+			.map_err(|_| Bolt12SemanticError::MissingPaths)?
+			.into_iter().next();
+
+		let mut builder = OfferBuilder::deriving_signing_pubkey(node_id, expanded_key, nonce, secp_ctx)
+			.chain_hash($self.chain_hash);
+
+		if let Some(path) = path {
+			builder = builder.path(path)
+		}
+
+		Ok(builder.into())
+	}
 } }
 
 macro_rules! create_refund_builder { ($self: ident, $builder: ty) => {
@@ -10302,6 +10353,91 @@ macro_rules! create_refund_builder { ($self: ident, $builder: ty) => {
 
 		Ok(builder.into())
 	}
+
+	/// Creates a [`RefundBuilder`] such that the [`Refund`] it builds is recognized by the
+	/// [`ChannelManager`] when handling [`Bolt12Invoice`] messages for the refund.
+	///
+	/// # Payment
+	///
+	/// The provided `payment_id` is used to ensure that only one invoice is paid for the refund.
+	/// See [Avoiding Duplicate Payments] for other requirements once the payment has been sent.
+	///
+	/// The builder will have the provided expiration set. Any changes to the expiration on the
+	/// returned builder will not be honored by [`ChannelManager`]. For non-`std`, the highest seen
+	/// block time minus two hours is used for the current time when determining if the refund has
+	/// expired.
+	///
+	/// To revoke the refund, use [`ChannelManager::abandon_payment`] prior to receiving the
+	/// invoice. If abandoned, or an invoice isn't received before expiration, the payment will fail
+	/// with an [`Event::PaymentFailed`].
+	///
+	/// If `max_total_routing_fee_msat` is not specified, The default from
+	/// [`RouteParameters::from_payment_params_and_value`] is applied.
+	///
+	/// # Privacy
+	///
+	/// Constructs a [`BlindedMessagePath`] for the refund using a custom [`MessageRouter`].
+	/// Users can implement a custom [`MessageRouter`] to define properties of the
+	/// [`BlindedMessagePath`] as required or opt not to create any `BlindedMessagePath`.
+	///
+	/// Also, uses a derived payer id in the refund for payer privacy.
+	///
+	/// # Limitations
+	///
+	/// Requires a direct connection to an introduction node in the responding
+	/// [`Bolt12Invoice::payment_paths`].
+	///
+	/// # Errors
+	///
+	/// Errors if:
+	/// - a duplicate `payment_id` is provided given the caveats in the aforementioned link,
+	/// - `amount_msats` is invalid, or
+	/// - the provided [`Router`] is unable to create a blinded path for the refund.
+	///
+	/// [`Refund`]: crate::offers::refund::Refund
+	/// [`Bolt12Invoice`]: crate::offers::invoice::Bolt12Invoice
+	/// [`Bolt12Invoice::payment_paths`]: crate::offers::invoice::Bolt12Invoice::payment_paths
+	/// [Avoiding Duplicate Payments]: #avoiding-duplicate-payments
+	pub fn create_refund_builder_using_router<ME: MessageRouter>(
+		&$self, router: ME, amount_msats: u64, absolute_expiry: Duration, payment_id: PaymentId,
+		retry_strategy: Retry, route_params_config: RouteParametersConfig
+	) -> Result<$builder, Bolt12SemanticError> {
+		let node_id = $self.get_our_node_id();
+		let expanded_key = &$self.inbound_payment_key;
+		let entropy = &*$self.entropy_source;
+		let secp_ctx = &$self.secp_ctx;
+
+		let nonce = Nonce::from_entropy_source(entropy);
+		let context = MessageContext::Offers(
+			OffersContext::OutboundPayment { payment_id, nonce, hmac: None }
+		);
+
+		let peers = $self.get_peers_for_blinded_path();
+		let path = router.create_blinded_paths(node_id, context, peers, secp_ctx)
+			.map_err(|_| Bolt12SemanticError::MissingPaths)?
+			.into_iter().next();
+
+		let mut builder = RefundBuilder::deriving_signing_pubkey(
+			node_id, expanded_key, nonce, secp_ctx, amount_msats, payment_id
+		)?
+			.chain_hash($self.chain_hash)
+			.absolute_expiry(absolute_expiry);
+
+		if let Some(path) = path {
+			builder = builder.path(path)
+		}
+
+		let _persistence_guard = PersistenceNotifierGuard::notify_on_drop($self);
+
+		let expiration = StaleExpiration::AbsoluteTimeout(absolute_expiry);
+		$self.pending_outbound_payments
+			.add_new_awaiting_invoice(
+				payment_id, expiration, retry_strategy, route_params_config, None,
+			)
+			.map_err(|_| Bolt12SemanticError::DuplicatePaymentId)?;
+
+		Ok(builder.into())
+	}
 } }
 
 /// Defines the maximum number of [`OffersMessage`] including different reply paths to be sent