f add comment and fuzz coverage

Author: valentinewallace

fuzz/src/router.rs

@@ -289,7 +289,7 @@ pub fn do_test<Out: test_logger::Output>(data: &[u8], out: Out) {
 							},
 							cltv_expiry_delta: slice_to_be16(get_slice!(2)),
 							htlc_minimum_msat: Some(slice_to_be64(get_slice!(8))),
-							htlc_maximum_msat: None,
+							htlc_maximum_msat: Some(slice_to_be64(get_slice!(8))),
 						}]));
 					}
 				}

lightning/src/routing/router.rs

@@ -968,6 +968,8 @@ fn max_htlc_from_capacity(capacity: EffectiveCapacity, max_channel_saturation_po
 		EffectiveCapacity::Unknown => EffectiveCapacity::Unknown.as_msat(),
 		EffectiveCapacity::AdvertisedMaxHTLC { amount_msat } =>
 			amount_msat.checked_shr(saturation_shift).unwrap_or(0),
+		// Treat max_htlc from a route hint as an exact liquidity amount rather than right shifting,
+		// since the invoice is expected to have been generated from up-to-date capacity information.
 		EffectiveCapacity::HintMaxHTLC { amount_msat } => amount_msat,
 		EffectiveCapacity::Total { capacity_msat, htlc_maximum_msat } =>
 			cmp::min(capacity_msat.checked_shr(saturation_shift).unwrap_or(0), htlc_maximum_msat),