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.

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};
@@ -6880,6 +6880,69 @@ where
 		Ok(builder)
 	}
 
+	/// Creates an [`InvoiceRequest`] for an [`Offer`] from the given parameters and enqueues it to
+	/// be sent via an onion message.
+	///
+	/// Use [`InvoiceRequestBuilder`] such that the [`InvoiceRequest`] it builds is recognized by
+	/// the [`ChannelManager`] when handling [`Bolt12Invoice`] messages for the request.
+	///
+	/// The provided `payment_id` is used to ensure that only one invoice is paid for the request.
+	///
+	/// [`InvoiceRequest`]: crate::offers::invoice_request::InvoiceRequest
+	/// [`Bolt12Invoice`]: crate::offers::invoice::Bolt12Invoice
+	pub fn request_invoice(
+		&self, offer: &Offer, quantity: Option<u64>, amount_msats: Option<u64>,
+		payer_note: Option<String>, payment_id: PaymentId, retry_strategy: Retry
+	) -> Result<(), Bolt12SemanticError> {
+		let expanded_key = &self.inbound_payment_key;
+		let entropy = &*self.entropy_source;
+		let secp_ctx = &self.secp_ctx;
+
+		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)
+			.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 {
+			for path in offer.paths() {
+				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.
 	///