implements cluster-nodes cache

Cluster nodes are cached keyed by the respective epoch from which stakes
are obtained, and so if epoch changes cluster-nodes will be recomputed.

A time-to-live eviction policy is enforced to refresh entries in case
gossip contact-infos are updated.

core/src/cluster_nodes.rs

@@ -1,14 +1,26 @@
 use {
     crate::{broadcast_stage::BroadcastStage, retransmit_stage::RetransmitStage},
     itertools::Itertools,
+    lru::LruCache,
     solana_gossip::{
         cluster_info::{compute_retransmit_peers, ClusterInfo},
         contact_info::ContactInfo,
         crds_gossip_pull::CRDS_GOSSIP_PULL_CRDS_TIMEOUT_MS,
         weighted_shuffle::{weighted_best, weighted_shuffle},
     },
-    solana_sdk::pubkey::Pubkey,
-    std::{any::TypeId, cmp::Reverse, collections::HashMap, marker::PhantomData},
+    solana_runtime::bank::Bank,
+    solana_sdk::{
+        clock::{Epoch, Slot},
+        pubkey::Pubkey,
+    },
+    std::{
+        any::TypeId,
+        cmp::Reverse,
+        collections::HashMap,
+        marker::PhantomData,
+        sync::{Arc, Mutex},
+        time::{Duration, Instant},
+    },
 };
 
 enum NodeId {
@@ -35,6 +47,12 @@ pub struct ClusterNodes<T> {
     _phantom: PhantomData<T>,
 }
 
+pub struct ClusterNodesCache<T> {
+    #[allow(clippy::type_complexity)]
+    cache: Mutex<LruCache<Epoch, (Instant, Arc<ClusterNodes<T>>)>>,
+    ttl: Duration, // Time to live.
+}
+
 impl Node {
     #[inline]
     fn pubkey(&self) -> Pubkey {
@@ -211,6 +229,77 @@ fn get_nodes(cluster_info: &ClusterInfo, stakes: &HashMap<Pubkey, u64>) -> Vec<N
     .collect()
 }
 
+impl<T> ClusterNodesCache<T> {
+    pub fn new(
+        // Capacity of underlying LRU-cache in terms of number of epochs.
+        cap: usize,
+        // A time-to-live eviction policy is enforced to refresh entries in
+        // case gossip contact-infos are updated.
+        ttl: Duration,
+    ) -> Self {
+        Self {
+            cache: Mutex::new(LruCache::new(cap)),
+            ttl,
+        }
+    }
+}
+
+impl ClusterNodesCache<RetransmitStage> {
+    pub fn get(
+        &self,
+        cluster_info: &ClusterInfo,
+        working_bank: &Bank,
+    ) -> Arc<ClusterNodes<RetransmitStage>> {
+        let slot = working_bank.slot();
+        let epoch = working_bank.get_leader_schedule_epoch(slot);
+        {
+            let mut cache = self.cache.lock().unwrap();
+            if let Some((asof, nodes)) = cache.get(&epoch) {
+                if asof.elapsed() < self.ttl {
+                    return Arc::clone(nodes);
+                }
+                cache.pop(&epoch);
+            }
+        }
+        let epoch_staked_nodes = working_bank.epoch_staked_nodes(epoch).unwrap_or_default();
+        let nodes = ClusterNodes::<RetransmitStage>::new(cluster_info, &epoch_staked_nodes);
+        let nodes = Arc::new(nodes);
+        {
+            let mut cache = self.cache.lock().unwrap();
+            cache.put(epoch, (Instant::now(), Arc::clone(&nodes)));
+        }
+        nodes
+    }
+}
+
+impl ClusterNodesCache<BroadcastStage> {
+    pub fn get(
+        &self,
+        shred_slot: Slot,
+        root_bank: &Bank,
+        cluster_info: &ClusterInfo,
+    ) -> Arc<ClusterNodes<BroadcastStage>> {
+        let epoch = root_bank.get_leader_schedule_epoch(shred_slot);
+        {
+            let mut cache = self.cache.lock().unwrap();
+            if let Some((asof, nodes)) = cache.get(&epoch) {
+                if asof.elapsed() < self.ttl {
+                    return Arc::clone(nodes);
+                }
+                cache.pop(&epoch);
+            }
+        }
+        let epoch_staked_nodes = root_bank.epoch_staked_nodes(epoch).unwrap_or_default();
+        let nodes = ClusterNodes::<BroadcastStage>::new(cluster_info, &epoch_staked_nodes);
+        let nodes = Arc::new(nodes);
+        {
+            let mut cache = self.cache.lock().unwrap();
+            cache.put(epoch, (Instant::now(), Arc::clone(&nodes)));
+        }
+        nodes
+    }
+}
+
 impl From<ContactInfo> for NodeId {
     fn from(node: ContactInfo) -> Self {
         NodeId::ContactInfo(node)