Refactor amount checks and fix currency check

Modified check_amount_msats_for_quantity to trust the
user-provided msats amount and moved/fixed the currency
check to the InvoiceRequest parsing code. We check if
the user attempts to use an Amount:Currency and return
an error if so.

Author: slanesuke

lightning/src/offers/invoice_request.rs

@@ -71,7 +71,7 @@ use crate::ln::msgs::DecodeError;
 use crate::offers::invoice::BlindedPayInfo;
 use crate::offers::merkle::{SignError, SignFn, SignatureTlvStream, SignatureTlvStreamRef, TaggedHash, self};
 use crate::offers::nonce::Nonce;
-use crate::offers::offer::{Offer, OfferContents, OfferId, OfferTlvStream, OfferTlvStreamRef};
+use crate::offers::offer::{Amount, Offer, OfferContents, OfferId, OfferTlvStream, OfferTlvStreamRef};
 use crate::offers::parse::{Bolt12ParseError, ParsedMessage, Bolt12SemanticError};
 use crate::offers::payer::{PayerContents, PayerTlvStream, PayerTlvStreamRef};
 use crate::offers::signer::{Metadata, MetadataMaterial};
@@ -1158,8 +1158,13 @@ impl TryFrom<PartialInvoiceRequestTlvStream> for InvoiceRequestContents {
 			return Err(Bolt12SemanticError::MissingAmount);
 		}
 
-		offer.check_quantity(quantity)?;
-		offer.check_amount_msats_for_quantity(amount, quantity)?;
+		match offer.amount() {
+			Some(Amount::Currency {..}) => return Err(Bolt12SemanticError::UnsupportedCurrency),
+			_ => {
+				offer.check_amount_msats_for_quantity(amount, quantity)?;
+				offer.check_quantity(quantity)?;
+			},
+		};
 
 		let features = features.unwrap_or_else(InvoiceRequestFeatures::empty);
 

lightning/src/offers/offer.rs

@@ -857,9 +857,8 @@ impl OfferContents {
 		&self, amount_msats: Option<u64>, quantity: Option<u64>
 	) -> Result<(), Bolt12SemanticError> {
 		let offer_amount_msats = match self.amount {
-			None => 0,
 			Some(Amount::Bitcoin { amount_msats }) => amount_msats,
-			Some(Amount::Currency { .. }) => return Err(Bolt12SemanticError::UnsupportedCurrency),
+			_ => 0,
 		};
 
 		if !self.expects_quantity() || quantity.is_some() {