babe: fix report_equivocation weight (#6936)

* babe: fix report_equivocation weight

* node: bump spec_version

* babe: fix floor in report_equivocation weight calculation

Co-authored-by: Gavin Wood <gavin@parity.io>

* grandpa: fix floor in report_equivocation weight calculation

* babe, grandpa: add test for weight_for::report_equivocation

Co-authored-by: Gavin Wood <gavin@parity.io>

substrate/frame/grandpa/src/lib.rs

@@ -376,7 +376,7 @@ mod weight_for {
 	pub fn report_equivocation<T: super::Trait>(validator_count: u32) -> Weight {
 		// we take the validator set count from the membership proof to
 		// calculate the weight but we set a floor of 100 validators.
-		let validator_count = validator_count.min(100) as u64;
+		let validator_count = validator_count.max(100) as u64;
 
 		// worst case we are considering is that the given offender
 		// is backed by 200 nominators

substrate/frame/grandpa/src/tests.rs

@@ -842,3 +842,26 @@ fn always_schedules_a_change_on_new_session_when_stalled() {
 		assert_eq!(Grandpa::current_set_id(), 2);
 	});
 }
+
+#[test]
+fn report_equivocation_has_valid_weight() {
+	// the weight depends on the size of the validator set,
+	// but there's a lower bound of 100 validators.
+	assert!(
+		(1..=100)
+			.map(weight_for::report_equivocation::<Test>)
+			.collect::<Vec<_>>()
+			.windows(2)
+			.all(|w| w[0] == w[1])
+	);
+
+	// after 100 validators the weight should keep increasing
+	// with every extra validator.
+	assert!(
+		(100..=1000)
+			.map(weight_for::report_equivocation::<Test>)
+			.collect::<Vec<_>>()
+			.windows(2)
+			.all(|w| w[0] < w[1])
+	);
+}