Seal scoring::Time and only use Instant or Eternity publicly

`scoring::Time` exists in part to make testing the passage of time
in `Scorer` practical. To allow no-std users to provide a time
source it was exposed as a trait as well. However, it seems
somewhat unlikely that a no-std user is going to have a use for
providing their own time source (otherwise they wouldn't be a
no-std user), and likely they won't have a graph in memory either.

`scoring::Time` as currently written is also exceptionally hard to
write C bindings for - the C bindings trait mappings relies on the
ability to construct trait implementations at runtime with function
pointers (i.e. `dyn Trait`s). `scoring::Time`, on the other hand,
is a supertrait of `core::ops::Sub` which requires a `sub` method
which takes a type parameter and returns a type parameter. Both of
which aren't practical in bindings, especially given the
`Sub::Output` associated type is not bound by any trait bounds at
all (implying we cannot simply map the `sub` function to return an
opaque trait object).

Thus, for simplicity, we here simply seal `scoring::Time` and make
it effectively-private, ensuring the bindings don't need to bother
with it.

Author: TheBlueMatt

lightning/src/routing/scoring.rs

@@ -46,9 +46,8 @@
 //!
 //! # 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.
+//! 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
 
@@ -59,9 +58,10 @@ use util::ser::{Readable, Writeable, Writer};
 
 use prelude::*;
 use core::cell::{RefCell, RefMut};
-use core::ops::{DerefMut, Sub};
+use core::ops::DerefMut;
 use core::time::Duration;
-use io::{self, Read}; use sync::{Mutex, MutexGuard};
+use io::{self, Read};
+use sync::{Mutex, MutexGuard};
 
 /// We define Score ever-so-slightly differently based on whether we are being build for C bindings
 /// or not. For users, `LockableScore` must somehow be writeable to disk. For Rust users, this is
@@ -185,23 +185,33 @@ impl<'a, S: Writeable> Writeable for MutexGuard<'a, S> {
 /// See [module-level documentation] for usage.
 ///
 /// [module-level documentation]: crate::routing::scoring
-pub type Scorer = ScorerUsingTime::<DefaultTime>;
-
-/// Time used by [`Scorer`].
 #[cfg(not(feature = "no-std"))]
-pub type DefaultTime = std::time::Instant;
-
-/// Time used by [`Scorer`].
+pub type Scorer = ScorerUsingTime::<std::time::Instant>;
+/// [`Score`] implementation that provides reasonable default behavior.
+///
+/// Used to apply a fixed penalty to each channel, thus avoiding long paths when shorter paths with
+/// slightly higher fees are available. Will further penalize channels that fail to relay payments.
+///
+/// See [module-level documentation] for usage.
+///
+/// [module-level documentation]: crate::routing::scoring
 #[cfg(feature = "no-std")]
-pub type DefaultTime = Eternity;
+pub type Scorer = ScorerUsingTime::<time::Eternity>;
+
+// Note that ideally we'd hide ScorerUsingTime from public view by sealing it as well, but rustdoc
+// doesn't handle this well - instead exposing a `Scorer` which has no trait implementation(s) or
+// methods at all.
 
-/// [`Score`] implementation parameterized by [`Time`].
+/// [`Score`] implementation.
 ///
 /// See [`Scorer`] for details.
 ///
 /// # Note
 ///
-/// Mixing [`Time`] types between serialization and deserialization results in undefined behavior.
+/// Mixing the `no-std` feature between serialization and deserialization results in undefined
+/// behavior.
+///
+/// (C-not exported) generally all users should use the [`Scorer`] type alias.
 pub struct ScorerUsingTime<T: Time> {
 	params: ScoringParameters,
 	// TODO: Remove entries of closed channels.
@@ -247,8 +257,8 @@ pub struct ScoringParameters {
 	///
 	/// # Note
 	///
-	/// When time is an [`Eternity`], as is default when enabling feature `no-std`, it will never
-	/// elapse. Therefore, this penalty will never decay.
+	/// When built with the `no-std` feature, time will never elapse. Therefore, this penalty will
+	/// never decay.
 	///
 	/// [`failure_penalty_msat`]: Self::failure_penalty_msat
 	pub failure_penalty_half_life: Duration,
@@ -273,20 +283,6 @@ struct ChannelFailure<T: Time> {
 	last_failed: T,
 }
 
-/// A measurement of time.
-pub trait Time: Sub<Duration, Output = Self> where Self: Sized {
-	/// Returns an instance corresponding to the current moment.
-	fn now() -> Self;
-
-	/// Returns the amount of time elapsed since `self` was 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> {
 	/// Creates a new scorer using the given scoring parameters.
 	pub fn new(params: ScoringParameters) -> Self {
@@ -383,48 +379,6 @@ impl<T: Time> Score for ScorerUsingTime<T> {
 	}
 }
 
-#[cfg(not(feature = "no-std"))]
-impl Time for std::time::Instant {
-	fn now() -> Self {
-		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)
-	}
-}
-
-/// A state in which time has no meaning.
-#[derive(Debug, PartialEq, Eq)]
-pub struct Eternity;
-
-impl Time for Eternity {
-	fn now() -> Self {
-		Self
-	}
-
-	fn duration_since_epoch() -> Duration {
-		Duration::from_secs(0)
-	}
-
-	fn elapsed(&self) -> Duration {
-		Duration::from_secs(0)
-	}
-}
-
-impl Sub<Duration> for Eternity {
-	type Output = Self;
-
-	fn sub(self, _other: Duration) -> Self {
-		self
-	}
-}
-
 impl<T: Time> Writeable for ScorerUsingTime<T> {
 	#[inline]
 	fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
@@ -475,9 +429,72 @@ impl<T: Time> Readable for ChannelFailure<T> {
 	}
 }
 
+pub(crate) mod time {
+	use core::ops::Sub;
+	use core::time::Duration;
+	/// A measurement of time.
+	pub trait Time: Sub<Duration, Output = Self> where Self: Sized {
+		/// Returns an instance corresponding to the current moment.
+		fn now() -> Self;
+
+		/// Returns the amount of time elapsed since `self` was 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;
+	}
+
+	/// A state in which time has no meaning.
+	#[derive(Debug, PartialEq, Eq)]
+	pub struct Eternity;
+
+	#[cfg(not(feature = "no-std"))]
+	impl Time for std::time::Instant {
+		fn now() -> Self {
+			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)
+		}
+	}
+
+	impl Time for Eternity {
+		fn now() -> Self {
+			Self
+		}
+
+		fn duration_since_epoch() -> Duration {
+			Duration::from_secs(0)
+		}
+
+		fn elapsed(&self) -> Duration {
+			Duration::from_secs(0)
+		}
+	}
+
+	impl Sub<Duration> for Eternity {
+		type Output = Self;
+
+		fn sub(self, _other: Duration) -> Self {
+			self
+		}
+	}
+}
+
+pub(crate) use self::time::Time;
+
 #[cfg(test)]
 mod tests {
-	use super::{Eternity, ScoringParameters, ScorerUsingTime, Time};
+	use super::{ScoringParameters, ScorerUsingTime, Time};
+	use super::time::Eternity;
 
 	use routing::scoring::Score;
 	use routing::network_graph::NodeId;

lightning/src/util/test_utils.rs

@@ -21,7 +21,8 @@ use ln::features::{ChannelFeatures, InitFeatures};
 use ln::msgs;
 use ln::msgs::OptionalField;
 use ln::script::ShutdownScript;
-use routing::scoring::{Eternity, ScorerUsingTime};
+use routing::scoring::ScorerUsingTime;
+use routing::scoring::time::Eternity;
 use util::enforcing_trait_impls::{EnforcingSigner, EnforcementState};
 use util::events;
 use util::logger::{Logger, Level, Record};