Respect MAX_LEADER_SCHEDULE_STAKES constant

runtime/src/bank.rs

@@ -2157,7 +2157,9 @@ impl Bank {
         //  crossed a boundary
         if !self.epoch_stakes.contains_key(&leader_schedule_epoch) {
             self.epoch_stakes.retain(|&epoch, _| {
-                epoch >= leader_schedule_epoch.saturating_sub(MAX_LEADER_SCHEDULE_STAKES)
+                // Note the greater-than-or-equal (and the `- 1`) is needed here
+                // to ensure we retain the oldest epoch, if that epoch is 0.
+                epoch >= leader_schedule_epoch.saturating_sub(MAX_LEADER_SCHEDULE_STAKES - 1)
             });
             let stakes = self.stakes_cache.stakes().clone();
             let stakes = SerdeStakesToStakeFormat::from(stakes);

runtime/src/bank/tests.rs

@@ -371,37 +371,37 @@ fn test_bank_update_epoch_stakes() {
         assert_eq!(bank.epoch_stake_keys(), initial_epochs);
     }
 
-    for epoch in (initial_epochs.len() as Epoch)..MAX_LEADER_SCHEDULE_STAKES {
-        bank.update_epoch_stakes(epoch);
+    for epoch in (initial_epochs.len() as Epoch)..(MAX_LEADER_SCHEDULE_STAKES - 1) {
+        bank.update_epoch_stakes(dbg!(epoch));
         assert_eq!(bank.epoch_stakes.len() as Epoch, epoch + 1);
     }
 
     assert_eq!(
         bank.epoch_stake_key_info(),
         (
             0,
-            MAX_LEADER_SCHEDULE_STAKES - 1,
-            MAX_LEADER_SCHEDULE_STAKES as usize
+            MAX_LEADER_SCHEDULE_STAKES - 2,
+            MAX_LEADER_SCHEDULE_STAKES as usize - 1,
         )
     );
 
-    bank.update_epoch_stakes(MAX_LEADER_SCHEDULE_STAKES);
+    bank.update_epoch_stakes(MAX_LEADER_SCHEDULE_STAKES - 1);
     assert_eq!(
         bank.epoch_stake_key_info(),
         (
             0,
-            MAX_LEADER_SCHEDULE_STAKES,
-            MAX_LEADER_SCHEDULE_STAKES as usize + 1
+            MAX_LEADER_SCHEDULE_STAKES - 1,
+            MAX_LEADER_SCHEDULE_STAKES as usize,
         )
     );
 
-    bank.update_epoch_stakes(MAX_LEADER_SCHEDULE_STAKES + 1);
+    bank.update_epoch_stakes(MAX_LEADER_SCHEDULE_STAKES);
     assert_eq!(
         bank.epoch_stake_key_info(),
         (
             1,
-            MAX_LEADER_SCHEDULE_STAKES + 1,
-            MAX_LEADER_SCHEDULE_STAKES as usize + 1
+            MAX_LEADER_SCHEDULE_STAKES,
+            MAX_LEADER_SCHEDULE_STAKES as usize,
         )
     );
 }