rpc: add getSlotLeaders method (#16057) (#16078)

(cherry picked from commit e7fd7d46cf)

Co-authored-by: Justin Starry <justin@solana.com>

ledger/src/leader_schedule_cache.rs

@@ -204,6 +204,10 @@ impl LeaderScheduleCache {
         }
     }
 
+    pub fn get_epoch_leader_schedule(&self, epoch: Epoch) -> Option<Arc<LeaderSchedule>> {
+        self.cached_schedules.read().unwrap().0.get(&epoch).cloned()
+    }
+
     fn get_epoch_schedule_else_compute(
         &self,
         epoch: Epoch,
@@ -214,8 +218,7 @@ impl LeaderScheduleCache {
                 return Some(fixed_schedule.leader_schedule.clone());
             }
         }
-        let epoch_schedule = self.cached_schedules.read().unwrap().0.get(&epoch).cloned();
-
+        let epoch_schedule = self.get_epoch_leader_schedule(epoch);
         if epoch_schedule.is_some() {
             epoch_schedule
         } else if let Some(epoch_schedule) = self.compute_epoch_schedule(epoch, bank) {

ledger/src/leader_schedule_utils.rs

@@ -1,3 +1,5 @@
+use std::collections::HashMap;
+
 use crate::leader_schedule::LeaderSchedule;
 use solana_runtime::bank::Bank;
 use solana_sdk::{
@@ -21,6 +23,27 @@ pub fn leader_schedule(epoch: Epoch, bank: &Bank) -> Option<LeaderSchedule> {
     })
 }
 
+/// Map of leader base58 identity pubkeys to the slot indices relative to the first epoch slot
+pub type LeaderScheduleByIdentity = HashMap<String, Vec<usize>>;
+
+pub fn leader_schedule_by_identity<'a>(
+    upcoming_leaders: impl Iterator<Item = (usize, &'a Pubkey)>,
+) -> LeaderScheduleByIdentity {
+    let mut leader_schedule_by_identity = HashMap::new();
+
+    for (slot_index, identity_pubkey) in upcoming_leaders {
+        leader_schedule_by_identity
+            .entry(identity_pubkey)
+            .or_insert_with(Vec::new)
+            .push(slot_index);
+    }
+
+    leader_schedule_by_identity
+        .into_iter()
+        .map(|(identity_pubkey, slot_indices)| (identity_pubkey.to_string(), slot_indices))
+        .collect()
+}
+
 /// Return the leader for the given slot.
 pub fn slot_leader_at(slot: Slot, bank: &Bank) -> Option<Pubkey> {
     let (epoch, slot_index) = bank.get_epoch_and_slot_index(slot);