Fix amount overflow in Invoice building

An overflow can occur when multiplying the offer amount by the requested
quantity when no amount is given in the request. Return an error instead
of overflowing.

lightning/src/offers/invoice.rs

@@ -148,7 +148,8 @@ impl<'a> InvoiceBuilder<'a> {
 			Some(amount_msats) => amount_msats,
 			None => match invoice_request.contents.offer.amount() {
 				Some(Amount::Bitcoin { amount_msats }) => {
-					amount_msats * invoice_request.quantity().unwrap_or(1)
+					amount_msats.checked_mul(invoice_request.quantity().unwrap_or(1))
+						.ok_or(SemanticError::InvalidAmount)?
 				},
 				Some(Amount::Currency { .. }) => return Err(SemanticError::UnsupportedCurrency),
 				None => return Err(SemanticError::MissingAmount),
@@ -787,7 +788,7 @@ mod tests {
 	use crate::ln::features::{BlindedHopFeatures, Bolt12InvoiceFeatures};
 	use crate::offers::invoice_request::InvoiceRequestTlvStreamRef;
 	use crate::offers::merkle::{SignError, SignatureTlvStreamRef, self};
-	use crate::offers::offer::{OfferBuilder, OfferTlvStreamRef};
+	use crate::offers::offer::{OfferBuilder, OfferTlvStreamRef, Quantity};
 	use crate::offers::parse::{ParseError, SemanticError};
 	use crate::offers::payer::PayerTlvStreamRef;
 	use crate::offers::refund::RefundBuilder;
@@ -1177,6 +1178,38 @@ mod tests {
 		assert_eq!(tlv_stream.amount, Some(1001));
 	}
 
+	#[test]
+	fn builds_invoice_with_quantity_from_request() {
+		let invoice = OfferBuilder::new("foo".into(), recipient_pubkey())
+			.amount_msats(1000)
+			.supported_quantity(Quantity::Unbounded)
+			.build().unwrap()
+			.request_invoice(vec![1; 32], payer_pubkey()).unwrap()
+			.quantity(2).unwrap()
+			.build().unwrap()
+			.sign(payer_sign).unwrap()
+			.respond_with_no_std(payment_paths(), payment_hash(), now()).unwrap()
+			.build().unwrap()
+			.sign(recipient_sign).unwrap();
+		let (_, _, _, tlv_stream, _) = invoice.as_tlv_stream();
+		assert_eq!(invoice.amount_msats(), 2000);
+		assert_eq!(tlv_stream.amount, Some(2000));
+
+		match OfferBuilder::new("foo".into(), recipient_pubkey())
+			.amount_msats(1000)
+			.supported_quantity(Quantity::Unbounded)
+			.build().unwrap()
+			.request_invoice(vec![1; 32], payer_pubkey()).unwrap()
+			.quantity(u64::max_value()).unwrap()
+			.build_unchecked()
+			.sign(payer_sign).unwrap()
+			.respond_with_no_std(payment_paths(), payment_hash(), now())
+		{
+			Ok(_) => panic!("expected error"),
+			Err(e) => assert_eq!(e, SemanticError::InvalidAmount),
+		}
+	}
+
 	#[test]
 	fn builds_invoice_with_fallback_address() {
 		let script = Script::new();