babe: expose next epoch data (#7829)

* babe: expose next epoch data

* babe: add runtime api for next_epoch

* babe: avoid reading next authorities from storage unnecessarily

* babe: add notes about epoch duration constraints

* babe: guard against overflow

* babe: add test for fetching current and next epoch data

substrate/frame/babe/src/tests.rs

@@ -66,7 +66,7 @@ fn first_block_epoch_zero_start() {
 		let (vrf_output, vrf_proof, vrf_randomness) = make_vrf_output(genesis_slot, &pairs[0]);
 
 		let first_vrf = vrf_output;
-		let pre_digest = make_pre_digest(
+		let pre_digest = make_primary_pre_digest(
 			0,
 			genesis_slot,
 			first_vrf.clone(),
@@ -122,7 +122,7 @@ fn author_vrf_output_for_primary() {
 	ext.execute_with(|| {
 		let genesis_slot = 10;
 		let (vrf_output, vrf_proof, vrf_randomness) = make_vrf_output(genesis_slot, &pairs[0]);
-		let primary_pre_digest = make_pre_digest(0, genesis_slot, vrf_output, vrf_proof);
+		let primary_pre_digest = make_primary_pre_digest(0, genesis_slot, vrf_output, vrf_proof);
 
 		System::initialize(
 			&1,
@@ -252,6 +252,33 @@ fn can_enact_next_config() {
 	});
 }
 
+#[test]
+fn can_fetch_current_and_next_epoch_data() {
+	new_test_ext(5).execute_with(|| {
+		// 1 era = 3 epochs
+		// 1 epoch = 3 slots
+		// Eras start from 0.
+		// Therefore at era 1 we should be starting epoch 3 with slot 10.
+		start_era(1);
+
+		let current_epoch = Babe::current_epoch();
+		assert_eq!(current_epoch.epoch_index, 3);
+		assert_eq!(current_epoch.start_slot, 10);
+		assert_eq!(current_epoch.authorities.len(), 5);
+
+		let next_epoch = Babe::next_epoch();
+		assert_eq!(next_epoch.epoch_index, 4);
+		assert_eq!(next_epoch.start_slot, 13);
+		assert_eq!(next_epoch.authorities.len(), 5);
+
+		// the on-chain randomness should always change across epochs
+		assert!(current_epoch.randomness != next_epoch.randomness);
+
+		// but in this case the authorities stay the same
+		assert!(current_epoch.authorities == next_epoch.authorities);
+	});
+}
+
 #[test]
 fn report_equivocation_current_session_works() {
 	let (pairs, mut ext) = new_test_ext_with_pairs(3);