Add SinceEpoch time to test Scorer hermetically

In order to test Scorer hermetically, sleeps must be avoided. Add a
SinceEpoch abstraction for manually advancing time. Implement the Time
trait for SinceEpoch so that it can be used with ScorerUsingTime in
tests.

Author: jkczyz

lightning/src/routing/scorer.rs

@@ -247,6 +247,7 @@ impl Time for std::time::Instant {
 }
 
 /// A state in which time has no meaning.
+#[derive(Debug, PartialEq, Eq)]
 pub struct Eternity;
 
 impl Time for Eternity {
@@ -320,3 +321,77 @@ impl<T: Time> Readable for ChannelFailure<T> {
 		})
 	}
 }
+
+#[cfg(test)]
+mod tests {
+	use super::{Eternity, ScoringParameters, ScorerUsingTime, Time};
+
+	use routing::Score;
+	use routing::network_graph::NodeId;
+
+	use bitcoin::secp256k1::PublicKey;
+	use core::cell::Cell;
+	use core::ops::Sub;
+	use core::time::Duration;
+
+	/// Time that can be advanced manually in tests.
+	#[derive(Debug, PartialEq, Eq)]
+	struct SinceEpoch(Duration);
+
+	impl SinceEpoch {
+		thread_local! {
+			static ELAPSED: Cell<Duration> = core::cell::Cell::new(Duration::from_secs(0));
+		}
+
+		fn advance(duration: Duration) {
+			Self::ELAPSED.with(|elapsed| elapsed.set(elapsed.get() + duration))
+		}
+	}
+
+	impl Time for SinceEpoch {
+		fn now() -> Self {
+			Self(Self::duration_since_epoch())
+		}
+
+		fn duration_since_epoch() -> Duration {
+			Self::ELAPSED.with(|elapsed| elapsed.get())
+		}
+
+		fn elapsed(&self) -> Duration {
+			Self::duration_since_epoch() - self.0
+		}
+	}
+
+	impl Sub<Duration> for SinceEpoch {
+		type Output = Self;
+
+		fn sub(self, other: Duration) -> Self {
+			Self(self.0 - other)
+		}
+	}
+
+	#[test]
+	fn time_passes_when_advanced() {
+		let now = SinceEpoch::now();
+		assert_eq!(now.elapsed(), Duration::from_secs(0));
+
+		SinceEpoch::advance(Duration::from_secs(1));
+		SinceEpoch::advance(Duration::from_secs(1));
+
+		let elapsed = now.elapsed();
+		let later = SinceEpoch::now();
+
+		assert_eq!(elapsed, Duration::from_secs(2));
+		assert_eq!(later - elapsed, now);
+	}
+
+	#[test]
+	fn time_never_passes_in_an_eternity() {
+		let now = Eternity::now();
+		let elapsed = now.elapsed();
+		let later = Eternity::now();
+
+		assert_eq!(now.elapsed(), Duration::from_secs(0));
+		assert_eq!(later - elapsed, now);
+	}
+}