Add MockInstant time utility for testing

To avoid sleeping in tests when using `Instant`s, we introduce a new
`MockInstant` that relies on a fake time which can be advanced in tests
deterministically.

Author: dunxen

lightning/src/util/time.rs

@@ -125,6 +125,27 @@ pub mod tests {
 		}
 	}
 
+	#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+	pub struct MockInstant(Duration);
+
+	impl MockInstant {
+		thread_local! {
+			static TIME: Cell<Duration> = Cell::new(Duration::default());
+		}
+	
+		pub fn now() -> Self {
+			Self(Self::TIME.with(|t| t.get()))
+		}
+
+		pub fn advance(duration: Duration) {
+			Self::TIME.with(|t| t.set(t.get() + duration))	
+		}
+
+		pub fn elapsed(&self) -> Duration {
+			Self::TIME.with(|t| t.get()) - self.0
+		}
+	}
+
 	#[test]
 	fn time_passes_when_advanced() {
 		let now = SinceEpoch::now();
@@ -149,4 +170,18 @@ pub mod tests {
 		assert_eq!(now.elapsed(), Duration::from_secs(0));
 		assert_eq!(later - elapsed, now);
 	}
+
+	#[test]
+	fn time_passes_when_advanced_for_mock_instant() {
+		let now = MockInstant::now();
+		
+		MockInstant::advance(Duration::from_secs(1));
+		MockInstant::advance(Duration::from_secs(1));
+
+		let elapsed = now.elapsed();
+		let later = MockInstant::now();
+
+		assert_eq!(elapsed, Duration::from_secs(2));
+		assert_eq!(later.elapsed().as_secs(), 0);
+	}
 }