Utility for creating and sending InvoiceRequests

Add a utility to ChannelManager for creating an InvoiceRequest for an
Offer such that derived keys are used for the payer id. This allows for
stateless verification of any Invoice messages before it is paid.

Also tracks future payments using the given PaymentId such that the
corresponding Invoice is paid only once.

Author: jkczyz

lightning/src/ln/channelmanager.rs

@@ -56,10 +56,10 @@ use crate::ln::msgs::{ChannelMessageHandler, DecodeError, LightningError};
 use crate::ln::outbound_payment;
 use crate::ln::outbound_payment::{OutboundPayments, PaymentAttempts, PendingOutboundPayment, SendAlongPathArgs};
 use crate::ln::wire::Encode;
-use crate::offers::offer::{DerivedMetadata, OfferBuilder};
+use crate::offers::offer::{DerivedMetadata, Offer, OfferBuilder};
 use crate::offers::parse::Bolt12SemanticError;
 use crate::offers::refund::RefundBuilder;
-use crate::onion_message::{OffersMessage, PendingOnionMessage};
+use crate::onion_message::{Destination, OffersMessage, PendingOnionMessage};
 use crate::sign::{EntropySource, KeysManager, NodeSigner, Recipient, SignerProvider, WriteableEcdsaChannelSigner};
 use crate::util::config::{UserConfig, ChannelConfig, ChannelConfigUpdate};
 use crate::util::wakers::{Future, Notifier};
@@ -7158,6 +7158,91 @@ where
 		Ok(builder)
 	}
 
+	/// Pays for an [`Offer`] using the given parameters by creating an [`InvoiceRequest`] and
+	/// enqueuing it to be sent via an onion message. [`ChannelManager`] will pay the actual
+	/// [`Bolt12Invoice`] once it is received.
+	///
+	/// Uses [`InvoiceRequestBuilder`] such that the [`InvoiceRequest`] it builds is recognized by
+	/// the [`ChannelManager`] when handling a [`Bolt12Invoice`] message in response to the request.
+	/// The optional parameters are used in the builder, if `Some`:
+	/// - `quantity` for [`InvoiceRequest::quantity`] which must be set if
+	///   [`Offer::expects_quantity`] is `true`.
+	/// - `amount_msats` if overpaying what is required for the given `quantity` is desired, and
+	/// - `payer_note` for [`InvoiceRequest::payer_note`].
+	///
+	/// The provided `payment_id` is used to ensure that only one invoice is paid for the request
+	/// when received. See [Avoiding Duplicate Payments] for other requirements once the payment has
+	/// been sent. Errors if a duplicate `payment_id` is provided given the caveats in the
+	/// aforementioned link.
+	///
+	/// [`InvoiceRequest`]: crate::offers::invoice_request::InvoiceRequest
+	/// [`InvoiceRequest::quantity`]: crate::offers::invoice_request::InvoiceRequest::quantity
+	/// [`InvoiceRequest::payer_note`]: crate::offers::invoice_request::InvoiceRequest::payer_note
+	/// [`InvoiceRequestBuilder`]: crate::offers::invoice_request::InvoiceRequestBuilder
+	/// [`Bolt12Invoice`]: crate::offers::invoice::Bolt12Invoice
+	/// [Avoiding Duplicate Payments]: #avoiding-duplicate-payments
+	pub fn pay_for_offer(
+		&self, offer: &Offer, quantity: Option<u64>, amount_msats: Option<u64>,
+		payer_note: Option<String>, payment_id: PaymentId, retry_strategy: Retry,
+		max_total_routing_fee_msat: Option<u64>
+	) -> Result<(), Bolt12SemanticError> {
+		let expanded_key = &self.inbound_payment_key;
+		let entropy = &*self.entropy_source;
+		let secp_ctx = &self.secp_ctx;
+
+		if !offer.supports_chain(self.chain_hash) {
+			return Err(Bolt12SemanticError::UnsupportedChain);
+		}
+
+		let builder = offer.request_invoice_deriving_payer_id(
+			expanded_key, entropy, secp_ctx, payment_id
+		)?;
+		let builder = match quantity {
+			None => builder,
+			Some(quantity) => builder.quantity(quantity)?,
+		};
+		let builder = match amount_msats {
+			None => builder,
+			Some(amount_msats) => builder.amount_msats(amount_msats)?,
+		};
+		let builder = match payer_note {
+			None => builder,
+			Some(payer_note) => builder.payer_note(payer_note),
+		};
+
+		let invoice_request = builder.build_and_sign()?;
+		let reply_path = self.create_one_hop_blinded_path();
+
+		self.pending_outbound_payments
+			.add_new_awaiting_invoice(payment_id, retry_strategy, max_total_routing_fee_msat)
+			.map_err(|_| Bolt12SemanticError::DuplicatePaymentId)?;
+
+		let mut pending_offers_messages = self.pending_offers_messages.lock().unwrap();
+		if offer.paths().is_empty() {
+			let message = PendingOnionMessage {
+				contents: OffersMessage::InvoiceRequest(invoice_request),
+				destination: Destination::Node(offer.signing_pubkey()),
+				reply_path: Some(reply_path),
+			};
+			pending_offers_messages.push(message);
+		} else {
+			// 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) {
+				let message = PendingOnionMessage {
+					contents: OffersMessage::InvoiceRequest(invoice_request.clone()),
+					destination: Destination::BlindedPath(path.clone()),
+					reply_path: Some(reply_path.clone()),
+				};
+				pending_offers_messages.push(message);
+			}
+		}
+
+		Ok(())
+	}
+
 	/// Gets a payment secret and payment hash for use in an invoice given to a third party wishing
 	/// to pay us.
 	///