Add issuer_id to Bolt12Invoice and StaticInvoice

Useful for determining if the signing_pubkey is the issuer_id or from a
blinded path.

lightning/src/offers/invoice.rs

@@ -705,6 +705,16 @@ macro_rules! invoice_accessors { ($self: ident, $contents: expr) => {
 		$contents.supported_quantity()
 	}
 
+	/// A possibly transient id of the recipient.
+	///
+	/// From [`Offer::issuer_id`]; `None` if the invoice was created in response to a [`Refund`].
+	///
+	/// [`Offer`]: crate::offers::offer::Offer
+	/// [`Offer::issuer_id`]: crate::offers::offer::Offer::issuer_id
+	pub fn issuer_id(&$self) -> Option<PublicKey> {
+		$contents.issuer_id()
+	}
+
 	/// An unpredictable series of bytes from the payer.
 	///
 	/// From [`InvoiceRequest::payer_metadata`] or [`Refund::payer_metadata`].
@@ -936,6 +946,15 @@ impl InvoiceContents {
 		}
 	}
 
+	fn issuer_id(&self) -> Option<PublicKey> {
+		match self {
+			InvoiceContents::ForOffer { invoice_request, .. } => {
+				invoice_request.inner.offer.issuer_id()
+			},
+			InvoiceContents::ForRefund { .. } => None,
+		}
+	}
+
 	fn payer_metadata(&self) -> &[u8] {
 		match self {
 			InvoiceContents::ForOffer { invoice_request, .. } => invoice_request.metadata(),

lightning/src/offers/invoice_macros.rs

@@ -126,6 +126,18 @@ macro_rules! invoice_accessors_common { ($self: ident, $contents: expr, $invoice
 	}
 
 	/// The public key corresponding to the key used to sign the invoice.
+	///
+	/// If the invoices was created in response to an [`Offer`], then will be:
+	/// - [`Offer::issuer_id`] if `Some`, otherwise
+	/// - the final blinded node id from a [`BlindedPath`] in [`Offer::paths`] if `None`.
+	///
+	/// If the invoice was created in response to a [`Refund`], then may be transient id chosen by
+	/// the recipient.
+	///
+	/// [`Offer`]: crate::offers::offer::Offer
+	/// [`Offer::issuer_id`]: crate::offers::offer::Offer::issuer_id
+	/// [`Offer::paths`]: crate::offers::offer::Offer::paths
+	/// [`Refund`]: crate::offers::refund::Refund
 	pub fn signing_pubkey(&$self) -> PublicKey {
 		$contents.signing_pubkey()
 	}

lightning/src/offers/static_invoice.rs

@@ -238,6 +238,13 @@ macro_rules! invoice_accessors { ($self: ident, $contents: expr) => {
 	pub fn supported_quantity(&$self) -> Quantity {
 		$contents.supported_quantity()
 	}
+
+	/// A possibly transient id of the recipient, from [`Offer::issuer_id`].
+	///
+	/// [`Offer::issuer_id`]: crate::offers::offer::Offer::issuer_id
+	pub fn issuer_id(&$self) -> Option<PublicKey> {
+		$contents.issuer_id()
+	}
 } }
 
 impl UnsignedStaticInvoice {
@@ -405,6 +412,10 @@ impl InvoiceContents {
 		self.offer.supported_quantity()
 	}
 
+	fn issuer_id(&self) -> Option<PublicKey> {
+		self.offer.issuer_id()
+	}
+
 	fn payment_paths(&self) -> &[(BlindedPayInfo, BlindedPath)] {
 		&self.payment_paths[..]
 	}