Avoid panicking on wallclock time going backwards across restart

Because we serialize `Instant`s using wallclock time in
`ProbabilisticScorer`, if time goes backwards across restarts we
may end up with `Instant`s in the future, which causes rustc prior
to 1.60 to panic when calculating durations. Here we simply avoid
this by setting the time to `now` if we get a time in the future.

diff --git a/lightning/src/routing/scoring.rs b/lightning/src/routing/scoring.rs
index 524f0ed3158b40dabae37d9516009a2588b6cd08..95bb2559023daf474c7f462f0f15227dd4250a65 100644 (file)
--- a/lightning/src/routing/scoring.rs
+++ b/lightning/src/routing/scoring.rs
@@ -1177,10 +1177,22 @@ impl<T: Time> Readable for ChannelLiquidity<T> {
                        (2, max_liquidity_offset_msat, required),
                        (4, duration_since_epoch, required),
                });
+               // On rust prior to 1.60 `Instant::duration_since` will panic if time goes backwards.
+               // We write `last_updated` as wallclock time even though its ultimately an `Instant` (which
+               // is a time from a monotonic clock usually represented as an offset against boot time).
+               // Thus, we have to construct an `Instant` by subtracting the difference in wallclock time
+               // from the one that was written. However, because `Instant` can panic if we construct one
+               // in the future, we must handle wallclock time jumping backwards, which we do by simply
+               // using `Instant::now()` in that case.
+               let wall_clock_now = T::duration_since_epoch();
+               let now = T::now();
+               let last_updated = if wall_clock_now > duration_since_epoch {
+                       now - (wall_clock_now - duration_since_epoch)
+               } else { now };
                Ok(Self {
                        min_liquidity_offset_msat,
                        max_liquidity_offset_msat,
-                       last_updated: T::now() - (T::duration_since_epoch() - duration_since_epoch),
+                       last_updated,
                })
        }
 }