f - Add duration_since_epoch to Time and note undefined behavior

lightning/src/routing/scorer.rs

@@ -44,6 +44,12 @@
 //! # }
 //! ```
 //!
+//! # Note
+//!
+//! If persisting [`Scorer`], it must be restored using the same [`Time`] parameterization. Using a
+//! different type results in undefined behavior. Specifically, persisting when built with feature
+//! `no-std` and restoring without it, or vice versa, uses different types and thus is undefined.
+//!
 //! [`find_route`]: crate::routing::router::find_route
 
 use routing;
@@ -79,6 +85,10 @@ pub type DefaultTime = Eternity;
 /// [`routing::Score`] implementation parameterized by [`Time`].
 ///
 /// See [`Scorer`] for details.
+///
+/// # Note
+///
+/// Mixing [`Time`] types between serialization and deserialization results in undefined behavior.
 pub struct ScorerUsingTime<T: Time> {
 	params: ScoringParameters,
 	// TODO: Remove entries of closed channels.
@@ -131,6 +141,11 @@ pub trait Time: Sub<Duration, Output = Self> where Self: Sized {
 
 	/// Returns the amount of time elapsed since created.
 	fn elapsed(&self) -> Duration;
+
+	/// Returns the amount of time passed since the beginning of [`Time`].
+	///
+	/// Used during (de-)serialization.
+	fn duration_since_epoch() -> Duration;
 }
 
 impl<T: Time> ScorerUsingTime<T> {
@@ -218,6 +233,11 @@ impl Time for std::time::Instant {
 		std::time::Instant::now()
 	}
 
+	fn duration_since_epoch() -> Duration {
+		use std::time::SystemTime;
+		SystemTime::now().duration_since(SystemTime::UNIX_EPOCH).unwrap()
+	}
+
 	fn elapsed(&self) -> Duration {
 		std::time::Instant::elapsed(self)
 	}
@@ -231,6 +251,10 @@ impl Time for Eternity {
 		Self
 	}
 
+	fn duration_since_epoch() -> Duration {
+		Duration::from_secs(0)
+	}
+
 	fn elapsed(&self) -> Duration {
 		Duration::from_secs(0)
 	}
@@ -266,7 +290,7 @@ impl<T: Time> Writeable for ChannelFailure<T> {
 	#[inline]
 	fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
 		self.undecayed_penalty_msat.write(w)?;
-		(duration_since_epoch() - self.last_failed.elapsed()).write(w)
+		(T::duration_since_epoch() - self.last_failed.elapsed()).write(w)
 	}
 }
 
@@ -275,17 +299,7 @@ impl<T: Time> Readable for ChannelFailure<T> {
 	fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
 		Ok(Self {
 			undecayed_penalty_msat: Readable::read(r)?,
-			last_failed: T::now() - (duration_since_epoch() - Readable::read(r)?),
+			last_failed: T::now() - (T::duration_since_epoch() - Readable::read(r)?),
 		})
 	}
 }
-
-fn duration_since_epoch() -> Duration {
-	#[cfg(not(feature = "no-std"))]
-	{
-		use std::time::SystemTime;
-		SystemTime::now().duration_since(SystemTime::UNIX_EPOCH).unwrap()
-	}
-	#[cfg(feature = "no-std")]
-	return Duration::from_secs(0);
-}