Add a scoring decay method to the ScoreUpdate trait

Rather than relying on fetching the current time during
routefinding, here we introduce a new trait method to `ScoreUpdate`
to do so. This largely mirrors what we do with the `NetworkGraph`,
and allows us to take on much more expensive operations (floating
point exponentiation) in our decaying.

Author: TheBlueMatt

lightning-background-processor/src/lib.rs

@@ -294,6 +294,7 @@ macro_rules! define_run_body {
 		let mut last_scorer_persist_call = $get_timer(SCORER_PERSIST_TIMER);
 		let mut last_rebroadcast_call = $get_timer(REBROADCAST_TIMER);
 		let mut have_pruned = false;
+		let mut have_decayed_scorer = false;
 
 		loop {
 			$process_channel_manager_events;
@@ -401,9 +402,21 @@ macro_rules! define_run_body {
 				last_prune_call = $get_timer(prune_timer);
 			}
 
+			if !have_decayed_scorer {
+				if let Some(ref scorer) = $scorer {
+					if let Some(duration_since_epoch) = $time_fetch() {
+						scorer.write_lock().decay_liquidity_certainty(duration_since_epoch);
+					}
+				}
+				have_decayed_scorer = true;
+			}
+
 			if $timer_elapsed(&mut last_scorer_persist_call, SCORER_PERSIST_TIMER) {
 				if let Some(ref scorer) = $scorer {
-					log_trace!($logger, "Persisting scorer");
+					log_trace!($logger, "Decaying and persisting scorer");
+					if let Some(duration_since_epoch) = $time_fetch() {
+						scorer.write_lock().decay_liquidity_certainty(duration_since_epoch);
+					}
 					if let Err(e) = $persister.persist_scorer(&scorer) {
 						log_error!($logger, "Error: Failed to persist scorer, check your disk and permissions {}", e)
 					}
@@ -1208,6 +1221,7 @@ mod tests {
 				}
 			}
 		}
+		fn decay_liquidity_certainty(&mut self, _: Duration) {}
 	}
 
 	#[cfg(c_bindings)]
@@ -1616,7 +1630,7 @@ mod tests {
 
 		loop {
 			let log_entries = nodes[0].logger.lines.lock().unwrap();
-			let expected_log = "Persisting scorer".to_string();
+			let expected_log = "Decaying and persisting scorer".to_string();
 			if log_entries.get(&("lightning_background_processor", expected_log)).is_some() {
 				break
 			}

lightning/src/routing/scoring.rs

@@ -120,6 +120,12 @@ pub trait ScoreUpdate {
 
 	/// Handles updating channel penalties after a probe over the given path succeeded.
 	fn probe_successful(&mut self, path: &Path, duration_since_epoch: Duration);
+
+	/// Scorers may wich to reduce their certainty of channel liquidity information over time.
+	/// Thus, this method is provided to allow scorers to observe the passage of time - the holder
+	/// of this object should call this method regularly (generally via the
+	/// `lightning-background-processor` trait).
+	fn decay_liquidity_certainty(&mut self, duration_since_epoch: Duration);
 }
 
 /// A trait which can both lookup and update routing channel penalty scores.
@@ -160,6 +166,10 @@ impl<S: ScoreUpdate, T: DerefMut<Target=S>> ScoreUpdate for T {
 	fn probe_successful(&mut self, path: &Path, duration_since_epoch: Duration) {
 		self.deref_mut().probe_successful(path, duration_since_epoch)
 	}
+
+	fn decay_liquidity_certainty(&mut self, duration_since_epoch: Duration) {
+		self.deref_mut().decay_liquidity_certainty(duration_since_epoch)
+	}
 }
 } }
 
@@ -361,6 +371,10 @@ impl<'a, T: Score> ScoreUpdate for MultiThreadedScoreLockWrite<'a, T> {
 	fn probe_successful(&mut self, path: &Path, duration_since_epoch: Duration) {
 		self.0.probe_successful(path, duration_since_epoch)
 	}
+
+	fn decay_liquidity_certainty(&mut self, duration_since_epoch: Duration) {
+		self.0.decay_liquidity_certainty(duration_since_epoch)
+	}
 }
 
 
@@ -406,6 +420,8 @@ impl ScoreUpdate for FixedPenaltyScorer {
 	fn probe_failed(&mut self, _path: &Path, _short_channel_id: u64, _duration_since_epoch: Duration) {}
 
 	fn probe_successful(&mut self, _path: &Path, _duration_since_epoch: Duration) {}
+
+	fn decay_liquidity_certainty(&mut self, _duration_since_epoch: Duration) {}
 }
 
 impl Writeable for FixedPenaltyScorer {
@@ -1463,6 +1479,8 @@ impl<G: Deref<Target = NetworkGraph<L>>, L: Deref, T: Time> ScoreUpdate for Prob
 	fn probe_successful(&mut self, path: &Path, duration_since_epoch: Duration) {
 		self.payment_path_failed(path, u64::max_value(), duration_since_epoch)
 	}
+
+	fn decay_liquidity_certainty(&mut self, _duration_since_epoch: Duration) {}
 }
 
 #[cfg(c_bindings)]

lightning/src/util/test_utils.rs

@@ -1357,6 +1357,8 @@ impl ScoreUpdate for TestScorer {
 	fn probe_failed(&mut self, _actual_path: &Path, _: u64, _duration_since_epoch: Duration) {}
 
 	fn probe_successful(&mut self, _actual_path: &Path, _duration_since_epoch: Duration) {}
+
+	fn decay_liquidity_certainty(&mut self, _duration_since_epoch: Duration) {}
 }
 
 impl Drop for TestScorer {