Problem: AuRa's unsafeties around step duration

Firstly, `Step.duration_remaining` casts it to u32, unnecesarily
limiting it to 2^32. While theoretically this is "good enough" (at 3
seconds steps it provides room for a little over 400 years), it is
still a lossy way to calculate the remaining time until the next step.

Secondly, step duration might be zero, triggering division by zero
in `Step.calibrate`

Solution: rework the code around the fact that duration is
typically in single digits and never grows, hence, it can be represented
by a much narrower range (u16) and this highlights the fact that
multiplying u64 by u16 will only result in an overflow in even further
future, at which point we should panic informatively (if anybody's
still around)

Similarly, panic when it is detected that incrementing the step
counter wrapped around on the overflow of usize.

As for the division by zero, prevent it by making zero an invalid
value for step duration. This will make AuRa log the constraint
mismatch and panic (after all, what purpose would zero step duration
serve? it makes no sense within the definition of the protocol,
as finality can only be achieved as per the specification
if messages are received within the step duration, which would violate
the speed of light and other physical laws in this case).

ethcore/src/engines/authority_round/mod.rs

@@ -51,8 +51,12 @@ mod finality;
 
 /// `AuthorityRound` params.
 pub struct AuthorityRoundParams {
-	/// Time to wait before next block or authority switching.
-	pub step_duration: Duration,
+	/// Time to wait before next block or authority switching,
+	/// in seconds.
+	///
+	/// Deliberately typed as u16 as too high of a value leads
+	/// to slow block issuance.
+	pub step_duration: u16,
 	/// Starting step,
 	pub start_step: Option<u64>,
 	/// Valid validators.
@@ -71,10 +75,17 @@ pub struct AuthorityRoundParams {
 	pub maximum_uncle_count: usize,
 }
 
+const U16_MAX: usize = ::std::u16::MAX as usize;
+
 impl From<ethjson::spec::AuthorityRoundParams> for AuthorityRoundParams {
 	fn from(p: ethjson::spec::AuthorityRoundParams) -> Self {
+		let mut step_duration_usize: usize = p.step_duration.into();
+		if step_duration_usize > U8_MAX {
+			step_duration_usize = U8_MAX;
+			warn!(target: "engine", "step_duration is too high ({}), setting it to {}", step_duration_usize, U8_MAX);
+		}
 		AuthorityRoundParams {
-			step_duration: Duration::from_secs(p.step_duration.into()),
+			step_duration: step_duration_usize as u16,
 			validators: new_validator_set(p.validators),
 			start_step: p.start_step.map(Into::into),
 			validate_score_transition: p.validate_score_transition.map_or(0, Into::into),
@@ -92,26 +103,47 @@ impl From<ethjson::spec::AuthorityRoundParams> for AuthorityRoundParams {
 struct Step {
 	calibrate: bool, // whether calibration is enabled.
 	inner: AtomicUsize,
-	duration: Duration,
+	duration: u16,
 }
 
 impl Step {
 	fn load(&self) -> usize { self.inner.load(AtomicOrdering::SeqCst) }
 	fn duration_remaining(&self) -> Duration {
 		let now = unix_now();
-		let step_end = self.duration * (self.load() as u32 + 1);
-		if step_end > now {
-			step_end - now
-		} else {
-			Duration::from_secs(0)
+		let expected_seconds = (self.load() as u64)
+			.checked_add(1)
+			.and_then(|ctr| ctr.checked_mul(self.duration as u64));
+		match expected_seconds {
+			Some(secs) => {
+				let step_end = Duration::from_secs(secs);
+				if step_end > now {
+					step_end - now
+				} else {
+					Duration::from_secs(0)
+				}
+			},
+			None => {
+				let ctr = self.load();
+				error!(target: "engine", "Step counter is too high: {}, aborting", ctr);
+				panic!("step counter is too high: {}", ctr)
+			},
 		}
+
 	}
 	fn increment(&self) {
-		self.inner.fetch_add(1, AtomicOrdering::SeqCst);
+		use std::usize;
+		// fetch_add won't panic on overflow but will rather wrap
+		// around, leading to zero as the step counter, which might
+		// lead to unexpected situations, so it's better to shut down.
+		if self.inner.fetch_add(1, AtomicOrdering::SeqCst) == usize::MAX {
+			error!(target: "engine", "Step counter is too high: {}, aborting", usize::MAX);
+			panic!("step counter is too high: {}", usize::MAX);
+		}
+
 	}
 	fn calibrate(&self) {
 		if self.calibrate {
-			let new_step = unix_now().as_secs() / self.duration.as_secs();
+			let new_step = unix_now().as_secs() / (self.duration as u64);
 			self.inner.store(new_step as usize, AtomicOrdering::SeqCst);
 		}
 	}
@@ -359,8 +391,12 @@ impl AsMillis for Duration {
 impl AuthorityRound {
 	/// Create a new instance of AuthorityRound engine.
 	pub fn new(our_params: AuthorityRoundParams, machine: EthereumMachine) -> Result<Arc<Self>, Error> {
+		if our_params.step_duration == 0 {
+			error!(target: "engine", "Authority Round step duration can't be zero, aborting");
+			panic!("authority_round: step duration can't be zero")
+		}
 		let should_timeout = our_params.start_step.is_none();
-		let initial_step = our_params.start_step.unwrap_or_else(|| (unix_now().as_secs() / our_params.step_duration.as_secs())) as usize;
+		let initial_step = our_params.start_step.unwrap_or_else(|| (unix_now().as_secs() / (our_params.step_duration as u64))) as usize;
 		let engine = Arc::new(
 			AuthorityRound {
 				transition_service: IoService::<()>::start()?,
@@ -1011,7 +1047,7 @@ mod tests {
 	fn reports_skipped() {
 		let last_benign = Arc::new(AtomicUsize::new(0));
 		let params = AuthorityRoundParams {
-			step_duration: Default::default(),
+			step_duration: 1,
 			start_step: Some(1),
 			validators: Box::new(TestSet::new(Default::default(), last_benign.clone())),
 			validate_score_transition: 0,
@@ -1051,7 +1087,7 @@ mod tests {
 	fn test_uncles_transition() {
 		let last_benign = Arc::new(AtomicUsize::new(0));
 		let params = AuthorityRoundParams {
-			step_duration: Default::default(),
+			step_duration: 1,
 			start_step: Some(1),
 			validators: Box::new(TestSet::new(Default::default(), last_benign.clone())),
 			validate_score_transition: 0,
@@ -1073,4 +1109,50 @@ mod tests {
 		assert_eq!(aura.maximum_uncle_count(1), 0);
 		assert_eq!(aura.maximum_uncle_count(100), 0);
 	}
+
+    #[test]
+    #[should_panic(expected="counter is too high")]
+    fn test_counter_increment_too_high() {
+        use super::Step;
+        let step = Step {
+            calibrate: false,
+            inner: AtomicUsize::new(::std::usize::MAX)
+            duration: 1,
+        };
+        step.increment();
+	}
+
+	#[test]
+	#[should_panic(expected="counter is too high")]
+	fn test_counter_duration_remaining_too_high() {
+		use super::Step;
+		let step = Step {
+			calibrate: false,
+			inner: AtomicUsize::new(::std::usize::MAX),
+			duration: 1,
+		};
+		step.duration_remaining();
+	}
+
+	#[test]
+	#[should_panic(expected="authority_round: step duration can't be zero")]
+	fn test_step_duration_zero() {
+		let last_benign = Arc::new(AtomicUsize::new(0));
+		let params = AuthorityRoundParams {
+			step_duration: 0,
+			start_step: Some(1),
+			validators: Box::new(TestSet::new(Default::default(), last_benign.clone())),
+			validate_score_transition: 0,
+			validate_step_transition: 0,
+			immediate_transitions: true,
+			maximum_uncle_count_transition: 0,
+			maximum_uncle_count: 0,
+			block_reward: Default::default(),
+		};
+
+		let mut c_params = ::spec::CommonParams::default();
+		c_params.gas_limit_bound_divisor = 5.into();
+		let machine = ::machine::EthereumMachine::regular(c_params, Default::default());
+		AuthorityRound::new(params, machine).unwrap();
+	}
 }

json/src/spec/authority_round.rs

@@ -22,7 +22,7 @@ use super::ValidatorSet;
 /// Authority params deserialization.
 #[derive(Debug, PartialEq, Deserialize)]
 pub struct AuthorityRoundParams {
-	/// Block duration.
+	/// Block duration, in seconds.
 	#[serde(rename="stepDuration")]
 	pub step_duration: Uint,
 	/// Valid authorities