babe: account for skipped epochs when handling equivocations (#13335)

* babe: account for skipped epochs when handling equivocations

* typos

* babe: enforce epoch index >= session index

substrate/frame/babe/src/tests.rs

@@ -826,6 +826,56 @@ fn report_equivocation_has_valid_weight() {
 		.all(|w| w[0].ref_time() < w[1].ref_time()));
 }
 
+#[test]
+fn report_equivocation_after_skipped_epochs_works() {
+	let (pairs, mut ext) = new_test_ext_with_pairs(3);
+
+	ext.execute_with(|| {
+		let epoch_duration: u64 = <Test as Config>::EpochDuration::get();
+
+		// this sets the genesis slot to 100;
+		let genesis_slot = 100;
+		go_to_block(1, genesis_slot);
+		assert_eq!(EpochIndex::<Test>::get(), 0);
+
+		// skip from epoch #0 to epoch #10
+		go_to_block(System::block_number() + 1, genesis_slot + epoch_duration * 10);
+
+		assert_eq!(EpochIndex::<Test>::get(), 10);
+		assert_eq!(SkippedEpochs::<Test>::get(), vec![(10, 1)]);
+
+		// generate an equivocation proof for validator at index 1
+		let authorities = Babe::authorities();
+		let offending_validator_index = 1;
+		let offending_authority_pair = pairs
+			.into_iter()
+			.find(|p| p.public() == authorities[offending_validator_index].0)
+			.unwrap();
+
+		let equivocation_proof = generate_equivocation_proof(
+			offending_validator_index as u32,
+			&offending_authority_pair,
+			CurrentSlot::<Test>::get(),
+		);
+
+		// create the key ownership proof
+		let key = (sp_consensus_babe::KEY_TYPE, &offending_authority_pair.public());
+		let key_owner_proof = Historical::prove(key).unwrap();
+
+		// which is for session index 1 (while current epoch index is 10)
+		assert_eq!(key_owner_proof.session, 1);
+
+		// report the equivocation, in order for the validation to pass the mapping
+		// between epoch index and session index must be checked.
+		assert!(Babe::report_equivocation_unsigned(
+			RuntimeOrigin::none(),
+			Box::new(equivocation_proof),
+			key_owner_proof
+		)
+		.is_ok());
+	})
+}
+
 #[test]
 fn valid_equivocation_reports_dont_pay_fees() {
 	let (pairs, mut ext) = new_test_ext_with_pairs(3);
@@ -977,5 +1027,17 @@ fn skipping_over_epochs_works() {
 
 		assert_eq!(EpochIndex::<Test>::get(), 4);
 		assert_eq!(Randomness::<Test>::get(), randomness_for_epoch_2);
+
+		// after skipping epochs the information is registered on-chain so that
+		// we can map epochs to sessions
+		assert_eq!(SkippedEpochs::<Test>::get(), vec![(4, 2)]);
+
+		// before epochs are skipped the mapping should be one to one
+		assert_eq!(Babe::session_index_for_epoch(0), 0);
+		assert_eq!(Babe::session_index_for_epoch(1), 1);
+
+		// otherwise the session index is offset by the number of skipped epochs
+		assert_eq!(Babe::session_index_for_epoch(4), 2);
+		assert_eq!(Babe::session_index_for_epoch(5), 3);
 	});
 }