Abort if insufficient liquidity in pay_for_offer

This commit introduces a check to abort early if
there's insufficient liquidity when fulfilling
the payment in `pay_for_offer`

Author: slanesuke

lightning/src/ln/channelmanager.rs

@@ -8816,8 +8816,15 @@ where
 			None => builder,
 			Some(payer_note) => builder.payer_note(payer_note),
 		};
+
 		let invoice_request = builder.build_and_sign()?;
 
+		let total_liquidity: u64 = self.list_usable_channels().iter().map(|channel| channel.next_outbound_htlc_limit_msat).sum();
+		if invoice_request.amount_msats() > Some(total_liquidity) {
+			log_error!(self.logger, "Insufficient liquidity for payment with payment id: {}", payment_id);
+			return Err(Bolt12SemanticError::InsufficientLiquidity);
+		}
+
 		let context = OffersContext::OutboundPayment { payment_id };
 		let reply_path = self.create_blinded_path(context).map_err(|_| Bolt12SemanticError::MissingPaths)?;
 

lightning/src/offers/parse.rs

@@ -193,6 +193,8 @@ pub enum Bolt12SemanticError {
 	UnexpectedPaymentHash,
 	/// A signature was expected but was missing.
 	MissingSignature,
+	/// There is insufficient liquidity to complete the payment.
+	InsufficientLiquidity,
 }
 
 impl From<bech32::Error> for Bolt12ParseError {