A0-0000: Choose a producer for every block in session (#1929)

Co-authored-by: Michal Swietek <[email protected]>

pallets/committee-management/src/impls.rs

@@ -4,8 +4,6 @@ use primitives::{
     AbftScoresProvider, BanHandler, BanInfo, BanReason, BannedValidators, CommitteeSeats,
     EraValidators, SessionCommittee, SessionValidatorError, SessionValidators, ValidatorProvider,
 };
-use rand::{seq::SliceRandom, SeedableRng};
-use rand_pcg::Pcg32;
 use sp_runtime::{traits::Get, Perbill, Perquintill};
 use sp_staking::{EraIndex, SessionIndex};
 use sp_std::{
@@ -57,17 +55,6 @@ fn choose_for_session<T: Clone>(validators: &[T], count: usize, session: usize)
     Some(chosen)
 }
 
-fn shuffle_order_for_session<T>(
-    producers: &mut Vec<T>,
-    validators: &mut Vec<T>,
-    session: SessionIndex,
-) {
-    let mut rng = Pcg32::seed_from_u64(session as u64);
-
-    producers.shuffle(&mut rng);
-    validators.shuffle(&mut rng);
-}
-
 /// Choose all items from `reserved` if present and extend it by #`non_reserved_seats` from
 /// `non_reserved` if present.
 fn choose_finality_committee<T: Clone>(
@@ -105,23 +92,20 @@ fn select_committee_inner<AccountId: Clone + PartialEq>(
     let non_reserved_committee =
         choose_for_session(non_reserved, non_reserved_seats, current_session as usize);
 
-    let mut finalizers = choose_finality_committee(
+    let finalizers = choose_finality_committee(
         &reserved_committee,
         &non_reserved_committee,
         non_reserved_finality_seats,
         current_session as usize,
     );
 
-    let mut producers = match (reserved_committee, non_reserved_committee) {
+    let producers = match (reserved_committee, non_reserved_committee) {
         (Some(rc), Some(nrc)) => Some(rc.into_iter().chain(nrc.into_iter()).collect()),
         (Some(rc), _) => Some(rc),
         (_, Some(nrc)) => Some(nrc),
         _ => None,
     }?;
 
-    // randomize order of the producers and committee
-    shuffle_order_for_session(&mut producers, &mut finalizers, current_session);
-
     Some(SessionCommittee {
         producers,
         finalizers,

pallets/committee-management/src/manager.rs

@@ -2,6 +2,9 @@ use frame_system::pallet_prelude::BlockNumberFor;
 use log::debug;
 use pallet_session::SessionManager;
 use primitives::{AbftScoresProvider, EraManager, FinalityCommitteeManager, SessionCommittee};
+use rand::{prelude::SliceRandom, SeedableRng};
+use rand_pcg::Pcg32;
+use sp_runtime::traits::Get;
 use sp_staking::{EraIndex, SessionIndex};
 use sp_std::{marker::PhantomData, vec::Vec};
 
@@ -127,7 +130,14 @@ where
         // Notify about elected next session finality committee
         C::FinalityCommitteeManager::on_next_session_finality_committee(finalizers);
 
-        Some(producers)
+        // Prepare a list of all block authors for the next session. We shuffle to minimize
+        // the impact of slow producer on his followers.
+        let full_size = C::SessionPeriod::get() as usize;
+        let mut full_aura: Vec<_> = producers.into_iter().cycle().take(full_size).collect();
+        let mut rng = Pcg32::seed_from_u64(new_index as u64);
+        full_aura.shuffle(&mut rng);
+
+        Some(full_aura)
     }
 
     fn end_session(end_index: SessionIndex) {