feat(data_structures): sample V2_0 superblock committee from validators

data_structures/src/superblock.rs

@@ -424,16 +424,20 @@ impl SuperBlockState {
         self.ars_current_identities = new_identities;
     }
 
-    /// Produces a `SuperBlock` that includes the blocks in `block_headers` if there is at least one of them.
-    /// `ars_identities` will be used to validate all the superblock votes received for the
-    /// next superblock. The votes for the current superblock must be validated using them
-    /// to calculate the superblock_signing_committee.
+    /// Produces a `SuperBlock` that includes the blocks in `block_headers` if there is at least
+    /// one of them.
+    ///
+    /// `census` will be used to validate all the superblock votes received for the next superblock.
+    ///
+    /// The votes for the current superblock must be validated using them to calculate
+    /// `superblock_signing_committee`.
+    ///
     /// The ordered bn256 keys will be merkelized and appended to the superblock
     #[allow(clippy::cast_possible_truncation, clippy::too_many_arguments)]
     pub fn build_superblock(
         &mut self,
         block_headers: &[BlockHeader],
-        ars_identities: Census,
+        census: Census,
         signing_committee_size: u32,
         superblock_index: u32,
         last_block_in_previous_superblock: Hash,
@@ -442,28 +446,23 @@ impl SuperBlockState {
         block_epoch: Epoch,
     ) -> SuperBlock {
         let protocol_version = ProtocolVersion::from_epoch(block_epoch);
-        let key_leaves = hash_key_leaves(&ars_identities.get_rep_ordered_bn256_list(alt_keys));
+        let key_leaves = hash_key_leaves(&census.get_rep_ordered_bn256_list(alt_keys));
 
-        self.update_ars_identities(ars_identities);
+        self.update_ars_identities(census);
 
         // Before updating the superblock_beacon, calculate the signing committee
-        let signing_committee = if let Some(ref sb) = sync_superblock {
-            calculate_superblock_signing_committee(
-                self.ars_previous_identities.clone(),
-                sb.signing_committee_length,
-                superblock_index,
-                self.current_superblock_beacon.hash_prev_block,
-                block_epoch,
-            )
+        let signing_committee_size = if let Some(ref sb) = sync_superblock {
+            sb.signing_committee_length
         } else {
-            calculate_superblock_signing_committee(
-                self.ars_previous_identities.clone(),
-                signing_committee_size,
-                superblock_index,
-                self.current_superblock_beacon.hash_prev_block,
-                block_epoch,
-            )
+            signing_committee_size
         };
+        let signing_committee = calculate_superblock_signing_committee(
+            self.ars_previous_identities.clone(),
+            signing_committee_size,
+            superblock_index,
+            self.current_superblock_beacon.hash_prev_block,
+            block_epoch,
+        );
 
         // Override superblock signing committee during each of the different emergency periods
         let emergency_committee =

node/src/actors/chain_manager/mod.rs

@@ -72,7 +72,7 @@ use witnet_data_structures::{
     radon_report::{RadonReport, ReportContext},
     register_protocol_version,
     staking::prelude::*,
-    superblock::{Census, AddSuperBlockVote, SuperBlockConsensus},
+    superblock::{AddSuperBlockVote, Census, SuperBlockConsensus},
     transaction::{RevealTransaction, TallyTransaction, Transaction},
     types::{
         visitor::{StatefulVisitor, Visitor},
@@ -2022,27 +2022,41 @@ impl ChainManager {
                 let reputation_engine = act.chain_state.reputation_engine.as_ref().unwrap();
                 let last_superblock_signed_by_bootstrap = last_superblock_signed_by_bootstrap(&chain_info.consensus_constants);
 
-                let ars_members =
-                    // Before reaching the epoch activity_period + collateral_age the bootstrap committee signs the superblock
-                    // collateral_age is measured in blocks instead of epochs, but this only means that the period in which
-                    // the bootstrap committee signs is at least epoch activity_period + collateral_age
-                    if let Some(ars_members) = in_emergency_period(superblock_index, get_environment()) {
-                        // Bootstrap committee
-                        ars_members
-                    } else if superblock_index >= last_superblock_signed_by_bootstrap {
+                // The base census is used during bootstrapping of the network and during emergency
+                // periods.
+                let base_census = if let Some(ars_members) = in_emergency_period(superblock_index, get_environment()) {
+                    // Bootstrap committee
+                    Some(ars_members)
+                } else if superblock_index < last_superblock_signed_by_bootstrap {
+                    Some(chain_info
+                        .consensus_constants
+                        .bootstrapping_committee
+                        .iter()
+                        .map(|add| add.parse().expect("Malformed bootstrapping committee"))
+                        .collect())
+                } else {
+                    None
+                };
+
+                // Different protocol versions sample the superblock signing committees from census
+                // that are built off different chain data:
+                // - In V1_X protocols, the census was sourced from the ARS. Namely, it contains
+                //   all identities with non-zero reputation, ordered by decreasing reputation.
+                // - In V2_x protocols, the census is instead sourced from the stakes tracker.
+                //   Namely, it contains the two top quartiles of the most powerful validators,
+                //   ordered by power.
+                // Exceptionally, if a base census has been constructed above, all of this logic is
+                // skipped, and the base census is used.
+                let census = Census::new(base_census.unwrap_or(
+                    if ProtocolVersion::from_epoch(block_epoch) < ProtocolVersion::V2_0 {
                         reputation_engine.get_rep_ordered_ars_list()
                     } else {
-                        chain_info
-                            .consensus_constants
-                            .bootstrapping_committee
-                            .iter()
-                            .map(|add| add.parse().expect("Malformed bootstrapping committee"))
-                            .collect()
-                    };
+                        let all_validators = act.chain_state.stakes.by_rank(Capability::Mining, block_epoch);
+                        let half_count = act.chain_state.stakes.validators_count() / 2;
+                        let top_validators = all_validators.map(|(StakeKey { validator, .. }, _)| validator).take(half_count).collect();
 
-                // Get the list of members of the ARS with reputation greater than 0
-                // the list itself is ordered by decreasing reputation
-                let ars_identities = Census::new(ars_members);
+                        top_validators
+                    }));
 
                 // After the second hard fork, the superblock committee size must be at least 50
                 let min_committee_size = if after_second_hard_fork(block_epoch, get_environment()) {
@@ -2066,7 +2080,7 @@ impl ChainManager {
 
                 let superblock = act.chain_state.superblock_state.build_superblock(
                     &block_headers,
-                    ars_identities,
+                    census,
                     committee_size,
                     superblock_index,
                     last_hash,