Clean up compute_fees and avoid the scorer with inf fees

If a channel's fee calculation overflows, don't bother calling the
scorer for the channel, as we can't use it anyway.

Author: TheBlueMatt

lightning/src/routing/router.rs

@@ -885,18 +885,11 @@ impl<'a> PaymentPath<'a> {
 	}
 }
 
+#[inline(always)]
+/// Calculate the fees required to route the given amount over a channel with the given fees.
 fn compute_fees(amount_msat: u64, channel_fees: RoutingFees) -> Option<u64> {
-	let proportional_fee_millions =
-		amount_msat.checked_mul(channel_fees.proportional_millionths as u64);
-	if let Some(new_fee) = proportional_fee_millions.and_then(|part| {
-			(channel_fees.base_msat as u64).checked_add(part / 1_000_000) }) {
-
-		Some(new_fee)
-	} else {
-		// This function may be (indirectly) called without any verification,
-		// with channel_fees provided by a caller. We should handle it gracefully.
-		None
-	}
+	amount_msat.checked_mul(channel_fees.proportional_millionths as u64)
+		.and_then(|part| (channel_fees.base_msat as u64).checked_add(part / 1_000_000))
 }
 
 /// The default `features` we assume for a node in a route, when no `features` are known about that
@@ -1289,7 +1282,7 @@ where L::Target: Logger {
 							if !should_process { should_process = true; }
 						}
 
-						if should_process {
+						'processing_node: while should_process {
 							let mut hop_use_fee_msat = 0;
 							let mut total_fee_msat = $next_hops_fee_msat;
 
@@ -1299,7 +1292,7 @@ where L::Target: Logger {
 								match compute_fees(amount_to_transfer_over_msat, $candidate.fees()) {
 									// max_value means we'll always fail
 									// the old_entry.total_fee_msat > total_fee_msat check
-									None => total_fee_msat = u64::max_value(),
+									None => break 'processing_node,
 									Some(fee_msat) => {
 										hop_use_fee_msat = fee_msat;
 										total_fee_msat += hop_use_fee_msat;
@@ -1392,6 +1385,7 @@ where L::Target: Logger {
 									);
 								}
 							}
+							break 'processing_node;
 						}
 					}
 				}