[leader election] Improve leader election for first 20 rounds (aptos-…

…labs#5683)

exclude_rounds is 20 by default, which means we don't consider last
20 rounds in leader reputation history - but we do so from the same epoch -
we consider everyone is caught up with the previous epoch.

genesis is epoch=0, round=0
first block is epoch=1 round=1, and that is a reconfig, that triggers epoch 2.
because votes in block metadata are for previous round, they are empty in that round.
So in that round - only "active" node is proposer.

For first 20 rounds of epoch 2, that is the only history we have - and so proposer of
epoch=1 round=1 will always be elected for the first 20 rounds.

That can cause issues in tests, as they might be executed over very few rounds.
So removing epoch=1,round=1 from being considered for history in epoch 2.

We can make this change without backward compatible gating, as this only affects
first window rounds (10 * num validators), and chain will continue successfully
after that (and all active chains are pass the epoch 2)

consensus/src/epoch_manager.rs

@@ -254,9 +254,12 @@ impl EpochManager {
                     vec![1; proposers.len()]
                 };
 
-                // First block (after genesis) is epoch=1, so that is the first epoch we consider. (Genesis is epoch=0)
+                // Genesis is epoch=0
+                // First block (after genesis) is epoch=1, and is the only block in that epoch.
+                // It has no votes, so we skip it unless we are in epoch 1, as otherwise it will
+                // skew leader elections for exclude_round number of rounds.
                 let first_epoch_to_consider = std::cmp::max(
-                    1,
+                    if epoch_state.epoch == 1 { 1 } else { 2 },
                     epoch_state
                         .epoch
                         .saturating_sub(use_history_from_previous_epoch_max_count as u64),
@@ -300,6 +303,7 @@ impl EpochManager {
                 ));
                 // LeaderReputation is not cheap, so we can cache the amount of rounds round_manager needs.
                 Box::new(CachedProposerElection::new(
+                    epoch_state.epoch,
                     proposer_election,
                     onchain_config.max_failed_authors_to_store()
                         + PROPSER_ELECTION_CACHING_WINDOW_ADDITION,

consensus/src/liveness/cached_proposer_election.rs

@@ -4,6 +4,7 @@
 use std::collections::BTreeMap;
 
 use aptos_infallible::Mutex;
+use aptos_logger::prelude::info;
 use consensus_types::common::{Author, Round};
 
 use crate::counters::PROPOSER_ELECTION_DURATION;
@@ -15,6 +16,7 @@ use super::proposer_election::ProposerElection;
 // Function get_valid_proposer can be expensive, and we want to make sure
 // it is computed only once for a given round.
 pub struct CachedProposerElection {
+    epoch: u64,
     proposer_election: Box<dyn ProposerElection + Send + Sync>,
     // We use BTreeMap since we want a fixed window of cached elements
     // to look back (and caller knows how big of a window it needs).
@@ -25,8 +27,13 @@ pub struct CachedProposerElection {
 }
 
 impl CachedProposerElection {
-    pub fn new(proposer_election: Box<dyn ProposerElection + Send + Sync>, window: usize) -> Self {
+    pub fn new(
+        epoch: u64,
+        proposer_election: Box<dyn ProposerElection + Send + Sync>,
+        window: usize,
+    ) -> Self {
         Self {
+            epoch,
             proposer_election,
             recent_elections: Mutex::new(BTreeMap::new()),
             window,
@@ -42,8 +49,14 @@ impl CachedProposerElection {
 
         *recent_elections.entry(round).or_insert_with(|| {
             let _timer = PROPOSER_ELECTION_DURATION.start_timer();
-            self.proposer_election
-                .get_valid_proposer_and_voting_power_participation_ratio(round)
+            let result = self
+                .proposer_election
+                .get_valid_proposer_and_voting_power_participation_ratio(round);
+            info!(
+                "ProposerElection for epoch {} and round {}: {:?}",
+                self.epoch, round, result
+            );
+            result
         })
     }
 }

consensus/src/liveness/cached_proposer_election_test.rs

@@ -35,6 +35,7 @@ fn test_get_valid_proposer_caching() {
     let asked = Arc::new(Mutex::new(Cell::new(0)));
     let authors: Vec<Author> = (0..4).map(|_| Author::random()).collect();
     let cpe = CachedProposerElection::new(
+        1,
         Box::new(MockProposerElection::new(authors.clone(), asked.clone())),
         10,
     );