Introduce enqueue_invoice_error API

Adds an API to send an `InvoiceError` to the counterparty via the flow.

This becomes useful with the introduction of Flow events in upcoming
commits, where the user can choose to either respond to Offers Messages
or return an `InvoiceError`.

Note:
Given the small scope of changes in this commit, we also take the
opportunity to perform minor documentation cleanups in `flow.rs`.

Author: shaavan

lightning/src/offers/flow.rs

@@ -39,6 +39,7 @@ use crate::offers::invoice::{
 	Bolt12Invoice, DerivedSigningPubkey, ExplicitSigningPubkey, InvoiceBuilder,
 	DEFAULT_RELATIVE_EXPIRY,
 };
+use crate::offers::invoice_error::InvoiceError;
 use crate::offers::invoice_request::{
 	CurrencyConversion, InvoiceRequest, InvoiceRequestBuilder, InvoiceRequestVerifiedFromOffer,
 	VerifiedInvoiceRequest,
@@ -1095,9 +1096,6 @@ where
 	/// The user must provide a list of [`MessageForwardNode`] that will be used to generate
 	/// valid reply paths for the counterparty to send back the corresponding [`Bolt12Invoice`]
 	/// or [`InvoiceError`].
-	///
-	/// [`InvoiceError`]: crate::offers::invoice_error::InvoiceError
-	/// [`supports_onion_messages`]: crate::types::features::Features::supports_onion_messages
 	pub fn enqueue_invoice_request(
 		&self, invoice_request: InvoiceRequest, payment_id: PaymentId, nonce: Nonce,
 		peers: Vec<MessageForwardNode>,
@@ -1144,8 +1142,6 @@ where
 	/// Reply paths are generated from the given `peers` to allow the counterparty to return
 	/// an [`InvoiceError`] in case they fail to process the invoice. If valid reply paths
 	/// cannot be constructed, this method returns a [`Bolt12SemanticError::MissingPaths`].
-	///
-	/// [`InvoiceError`]: crate::offers::invoice_error::InvoiceError
 	pub fn enqueue_invoice_using_node_id(
 		&self, invoice: Bolt12Invoice, destination: PublicKey, peers: Vec<MessageForwardNode>,
 	) -> Result<(), Bolt12SemanticError> {
@@ -1200,6 +1196,26 @@ where
 		Ok(())
 	}
 
+	/// Enqueues an [`InvoiceError`] to be sent to the counterparty via a specified
+	/// [`BlindedMessagePath`].
+	///
+	/// Since this method returns the invoice error to the counterparty without
+	/// expecting back a response, we enqueue it without a reply path.
+	pub fn enqueue_invoice_error(
+		&self, invoice_error: InvoiceError, path: BlindedMessagePath,
+	) -> Result<(), Bolt12SemanticError> {
+		let mut pending_offers_messages = self.pending_offers_messages.lock().unwrap();
+
+		let instructions = MessageSendInstructions::WithoutReplyPath {
+			destination: Destination::BlindedPath(path),
+		};
+
+		let message = OffersMessage::InvoiceError(invoice_error);
+		pending_offers_messages.push((message, instructions));
+
+		Ok(())
+	}
+
 	/// Forwards a [`StaticInvoice`] over the provided [`Responder`] in response to an
 	/// [`InvoiceRequest`] that we as a static invoice server received on behalf of an often-offline
 	/// recipient.
@@ -1252,7 +1268,6 @@ where
 	/// reply paths for the recipient to send back the corresponding [`ReleaseHeldHtlc`] onion message.
 	///
 	/// [`ReleaseHeldHtlc`]: crate::onion_message::async_payments::ReleaseHeldHtlc
-	/// [`supports_onion_messages`]: crate::types::features::Features::supports_onion_messages
 	pub fn enqueue_held_htlc_available(
 		&self, invoice: &StaticInvoice, payment_id: PaymentId, peers: Vec<MessageForwardNode>,
 	) -> Result<(), Bolt12SemanticError> {
@@ -1311,8 +1326,6 @@ where
 	/// The user must provide a list of [`MessageForwardNode`] that will be used to generate
 	/// valid reply paths for the counterparty to send back the corresponding response for
 	/// the [`DNSSECQuery`] message.
-	///
-	/// [`supports_onion_messages`]: crate::types::features::Features::supports_onion_messages
 	#[cfg(feature = "dnssec")]
 	pub fn enqueue_dns_onion_message(
 		&self, message: DNSSECQuery, context: DNSResolverContext, dns_resolvers: Vec<Destination>,