Reduce hold time resolution

In the spec process, it was agreed to report hold time in multiples of
100 ms.

Author: joostjager

lightning/src/ln/channel.rs

@@ -62,7 +62,9 @@ use crate::ln::interactivetxs::{
 };
 use crate::ln::msgs;
 use crate::ln::msgs::{ClosingSigned, ClosingSignedFeeRange, DecodeError, OnionErrorPacket};
-use crate::ln::onion_utils::{AttributionData, HTLCFailReason, LocalHTLCFailureReason};
+use crate::ln::onion_utils::{
+	AttributionData, HTLCFailReason, LocalHTLCFailureReason, HOLD_TIME_UNIT_MILLIS,
+};
 use crate::ln::script::{self, ShutdownScript};
 use crate::ln::types::ChannelId;
 use crate::routing::gossip::NodeId;
@@ -7168,9 +7170,9 @@ where
 					// We really want take() here, but, again, non-mut ref :(
 					if let OutboundHTLCOutcome::Failure(mut reason) = outcome.clone() {
 						if let (Some(timestamp), Some(now)) = (htlc.send_timestamp, now) {
-							let hold_time =
-								u32::try_from(now.saturating_sub(timestamp).as_millis())
-									.unwrap_or(u32::MAX);
+							let elapsed_millis = now.saturating_sub(timestamp).as_millis();
+							let elapsed_units = elapsed_millis / HOLD_TIME_UNIT_MILLIS;
+							let hold_time = u32::try_from(elapsed_units).unwrap_or(u32::MAX);
 							reason.set_hold_time(hold_time);
 						}
 

lightning/src/ln/onion_utils.rs

@@ -44,6 +44,9 @@ use crate::prelude::*;
 
 const DEFAULT_MIN_FAILURE_PACKET_LEN: usize = 256;
 
+/// The unit size of the hold time. This is used to reduce the hold time resolution to improve privacy.
+pub(crate) const HOLD_TIME_UNIT_MILLIS: u128 = 100;
+
 pub(crate) struct OnionKeys {
 	#[cfg(test)]
 	pub(crate) shared_secret: SharedSecret,
@@ -1223,7 +1226,7 @@ where
 							logger,
 							"Htlc hold time at pos {}: {} ms",
 							route_hop_idx,
-							hold_time
+							hold_time * HOLD_TIME_UNIT_MILLIS as u32
 						);
 
 						// Shift attribution data to prepare for processing the next hop.