authority_round: Fix next_step_time_duration. (#11379)

It's supposed to find the first step at or after `time`.
To compute that step's timestamp, it needs to add the total length
of all steps since the previous transitions to the timestamp of the
previous transition, not to `time`.

ethcore/engines/authority-round/src/lib.rs

@@ -215,7 +215,7 @@ impl From<ethjson::spec::AuthorityRoundParams> for AuthorityRoundParams {
 }
 
 /// A triple containing the first step number and the starting timestamp of the given step duration.
-#[derive(Clone, Debug)]
+#[derive(Clone, Copy, Debug)]
 struct StepDurationInfo {
 	transition_step: u64,
 	transition_timestamp: u64,
@@ -1896,9 +1896,10 @@ fn next_step_time_duration(info: StepDurationInfo, time: u64) -> Option<(u64, u6
 		.checked_sub(1)?
 		.checked_sub(info.transition_timestamp)?
 		.checked_div(info.step_duration)?;
+	let time_diff = step_diff.checked_mul(info.step_duration)?;
 	Some((
 		info.transition_step.checked_add(step_diff)?,
-		step_diff.checked_mul(info.step_duration)?.checked_add(time)?,
+		info.transition_timestamp.checked_add(time_diff)?,
 	))
 }
 
@@ -1940,7 +1941,7 @@ mod tests {
 
 	use super::{
 		AuthorityRoundParams, AuthorityRound, EmptyStep, SealedEmptyStep, StepDurationInfo,
-		calculate_score, util::BoundContract,
+		calculate_score, util::BoundContract, next_step_time_duration,
 	};
 
 	fn build_aura<F>(f: F) -> Arc<AuthorityRound> where
@@ -2304,6 +2305,22 @@ mod tests {
 		step.duration_remaining();
 	}
 
+	#[test]
+	fn test_next_step_time_duration() {
+		// At step 7 (time 1000), we transitioned to step duration 10.
+		let info = StepDurationInfo {
+			step_duration: 10,
+			transition_step: 7,
+			transition_timestamp: 1000,
+		};
+		// So the next transition can happen e.g. at step 12 (time 1050) or 13 (time 1060).
+		assert_eq!(Some((12, 1050)), next_step_time_duration(info, 1050));
+		assert_eq!(Some((13, 1060)), next_step_time_duration(info, 1051));
+		assert_eq!(Some((13, 1060)), next_step_time_duration(info, 1055));
+		// The next transition could also happen immediately.
+		assert_eq!(Some((7, 1000)), next_step_time_duration(info, 1000));
+	}
+
 	#[test]
 	fn test_change_step_duration() {
 		use super::Step;