implements copy-on-write for staked-nodes (#19090)

Bank::staked_nodes and Bank::epoch_staked_nodes redundantly clone
staked-nodes HashMap even though an immutable reference will suffice:
https://github.com/solana-labs/solana/blob/a9014cece/runtime/src/vote_account.rs#L77

This commit implements copy-on-write semantics for staked-nodes by
wrapping the underlying HashMap in Arc<...>.

gossip/src/cluster_info.rs

@@ -1690,7 +1690,7 @@ impl ClusterInfo {
                                 Some(root_bank.feature_set.clone()),
                             )
                         }
-                        None => (HashMap::new(), None),
+                        None => (Arc::default(), None),
                     };
                     let require_stake_for_gossip =
                         self.require_stake_for_gossip(feature_set.as_deref(), &stakes);
@@ -2485,7 +2485,7 @@ impl ClusterInfo {
         // feature does not roll back (if the feature happens to get enabled in
         // a minority fork).
         let (feature_set, stakes) = match bank_forks {
-            None => (None, HashMap::default()),
+            None => (None, Arc::default()),
             Some(bank_forks) => {
                 let bank = bank_forks.read().unwrap().root_bank();
                 let feature_set = bank.feature_set.clone();

ledger/src/leader_schedule_utils.rs

@@ -12,7 +12,10 @@ pub fn leader_schedule(epoch: Epoch, bank: &Bank) -> Option<LeaderSchedule> {
     bank.epoch_staked_nodes(epoch).map(|stakes| {
         let mut seed = [0u8; 32];
         seed[0..8].copy_from_slice(&epoch.to_le_bytes());
-        let mut stakes: Vec<_> = stakes.into_iter().collect();
+        let mut stakes: Vec<_> = stakes
+            .iter()
+            .map(|(pubkey, stake)| (*pubkey, *stake))
+            .collect();
         sort_stakes(&mut stakes);
         LeaderSchedule::new(
             &stakes,
@@ -88,7 +91,11 @@ mod tests {
                 .genesis_config;
         let bank = Bank::new_for_tests(&genesis_config);
 
-        let pubkeys_and_stakes: Vec<_> = bank.staked_nodes().into_iter().collect();
+        let pubkeys_and_stakes: Vec<_> = bank
+            .staked_nodes()
+            .iter()
+            .map(|(pubkey, stake)| (*pubkey, *stake))
+            .collect();
         let seed = [0u8; 32];
         let leader_schedule = LeaderSchedule::new(
             &pubkeys_and_stakes,

runtime/src/bank.rs

@@ -5137,7 +5137,7 @@ impl Bank {
         self.stakes.read().unwrap().stake_delegations().clone()
     }
 
-    pub fn staked_nodes(&self) -> HashMap<Pubkey, u64> {
+    pub fn staked_nodes(&self) -> Arc<HashMap<Pubkey, u64>> {
         self.stakes.read().unwrap().staked_nodes()
     }
 
@@ -5177,7 +5177,7 @@ impl Bank {
         &self.epoch_stakes
     }
 
-    pub fn epoch_staked_nodes(&self, epoch: Epoch) -> Option<HashMap<Pubkey, u64>> {
+    pub fn epoch_staked_nodes(&self, epoch: Epoch) -> Option<Arc<HashMap<Pubkey, u64>>> {
         Some(self.epoch_stakes.get(&epoch)?.stakes().staked_nodes())
     }
 

runtime/src/stakes.rs

@@ -1,7 +1,8 @@
 //! Stakes serve as a cache of stake and vote accounts to derive
 //! node stakes
-use crate::vote_account::{ArcVoteAccount, VoteAccounts};
+use {
-use solana_sdk::{
+    crate::vote_account::{ArcVoteAccount, VoteAccounts},
+    solana_sdk::{
         account::{AccountSharedData, ReadableAccount},
         clock::Epoch,
         pubkey::Pubkey,
@@ -10,10 +11,11 @@ use solana_sdk::{
             state::{Delegation, StakeState},
         },
         sysvar::stake_history::StakeHistory,
+    },
+    solana_stake_program::stake_state,
+    solana_vote_program::vote_state::VoteState,
+    std::{borrow::Borrow, collections::HashMap, sync::Arc},
 };
-use solana_stake_program::stake_state;
-use solana_vote_program::vote_state::VoteState;
-use std::{borrow::Borrow, collections::HashMap};
 
 #[derive(Default, Clone, PartialEq, Debug, Deserialize, Serialize, AbiExample)]
 pub struct Stakes {
@@ -217,7 +219,7 @@ impl Stakes {
         &self.stake_delegations
     }
 
-    pub fn staked_nodes(&self) -> HashMap<Pubkey, u64> {
+    pub fn staked_nodes(&self) -> Arc<HashMap<Pubkey, u64>> {
         self.vote_accounts.staked_nodes()
     }
 

runtime/src/vote_account.rs

@@ -1,16 +1,19 @@
-use serde::de::{Deserialize, Deserializer};
+use {
-use serde::ser::{Serialize, Serializer};
+    itertools::Itertools,
-use solana_sdk::{
+    serde::de::{Deserialize, Deserializer},
+    serde::ser::{Serialize, Serializer},
+    solana_sdk::{
         account::Account, account::AccountSharedData, instruction::InstructionError, pubkey::Pubkey,
-};
+    },
-use solana_vote_program::vote_state::VoteState;
+    solana_vote_program::vote_state::VoteState,
-use std::{
+    std::{
         borrow::Borrow,
         cmp::Ordering,
         collections::{hash_map::Entry, HashMap},
         iter::FromIterator,
         ops::Deref,
         sync::{Arc, Once, RwLock, RwLockReadGuard},
+    },
 };
 
 // The value here does not matter. It will be overwritten
@@ -31,12 +34,16 @@ pub struct VoteAccount {
 #[derive(Debug, AbiExample)]
 pub struct VoteAccounts {
     vote_accounts: HashMap<Pubkey, (u64 /*stake*/, ArcVoteAccount)>,
+    // Inner Arc is meant to implement copy-on-write semantics as opposed to
+    // sharing mutations (hence RwLock<Arc<...>> instead of Arc<RwLock<...>>).
     staked_nodes: RwLock<
+        Arc<
             HashMap<
                 Pubkey, // VoteAccount.vote_state.node_pubkey.
                 u64,    // Total stake across all vote-accounts.
             >,
         >,
+    >,
     staked_nodes_once: Once,
 }
 
@@ -59,20 +66,19 @@ impl VoteAccount {
 }
 
 impl VoteAccounts {
-    pub fn staked_nodes(&self) -> HashMap<Pubkey, u64> {
+    pub fn staked_nodes(&self) -> Arc<HashMap<Pubkey, u64>> {
         self.staked_nodes_once.call_once(|| {
-            let mut staked_nodes = HashMap::new();
+            let staked_nodes = self
-            for (stake, vote_account) in
+                .vote_accounts
-                self.vote_accounts.values().filter(|(stake, _)| *stake != 0)
+                .values()
-            {
+                .filter(|(stake, _)| *stake != 0)
-                if let Some(node_pubkey) = vote_account.node_pubkey() {
+                .filter_map(|(stake, vote_account)| {
-                    staked_nodes
+                    let node_pubkey = vote_account.node_pubkey()?;
-                        .entry(node_pubkey)
+                    Some((node_pubkey, stake))
-                        .and_modify(|s| *s += *stake)
+                })
-                        .or_insert(*stake);
+                .into_grouping_map()
-                }
+                .aggregate(|acc, _node_pubkey, stake| Some(acc.unwrap_or_default() + stake));
-            }
+            *self.staked_nodes.write().unwrap() = Arc::new(staked_nodes)
-            *self.staked_nodes.write().unwrap() = staked_nodes
         });
         self.staked_nodes.read().unwrap().clone()
     }
@@ -119,9 +125,9 @@ impl VoteAccounts {
     fn add_node_stake(&mut self, stake: u64, vote_account: &ArcVoteAccount) {
         if stake != 0 && self.staked_nodes_once.is_completed() {
             if let Some(node_pubkey) = vote_account.node_pubkey() {
-                self.staked_nodes
+                let mut staked_nodes = self.staked_nodes.write().unwrap();
-                    .write()
+                let staked_nodes = Arc::make_mut(&mut staked_nodes);
-                    .unwrap()
+                staked_nodes
                     .entry(node_pubkey)
                     .and_modify(|s| *s += stake)
                     .or_insert(stake);
@@ -132,7 +138,9 @@ impl VoteAccounts {
     fn sub_node_stake(&mut self, stake: u64, vote_account: &ArcVoteAccount) {
         if stake != 0 && self.staked_nodes_once.is_completed() {
             if let Some(node_pubkey) = vote_account.node_pubkey() {
-                match self.staked_nodes.write().unwrap().entry(node_pubkey) {
+                let mut staked_nodes = self.staked_nodes.write().unwrap();
+                let staked_nodes = Arc::make_mut(&mut staked_nodes);
+                match staked_nodes.entry(node_pubkey) {
                     Entry::Vacant(_) => panic!("this should not happen!"),
                     Entry::Occupied(mut entry) => match entry.get().cmp(&stake) {
                         Ordering::Less => panic!("subtraction value exceeds node's stake"),
@@ -474,7 +482,7 @@ mod tests {
             if (k + 1) % 128 == 0 {
                 assert_eq!(
                     staked_nodes(&accounts[..k + 1]),
-                    vote_accounts.staked_nodes()
+                    *vote_accounts.staked_nodes()
                 );
             }
         }
@@ -484,7 +492,7 @@ mod tests {
             let (pubkey, (_, _)) = accounts.swap_remove(index);
             vote_accounts.remove(&pubkey);
             if (k + 1) % 32 == 0 {
-                assert_eq!(staked_nodes(&accounts), vote_accounts.staked_nodes());
+                assert_eq!(staked_nodes(&accounts), *vote_accounts.staked_nodes());
             }
         }
         // Modify the stakes for some of the accounts.
@@ -499,7 +507,7 @@ mod tests {
             }
             *stake = new_stake;
             if (k + 1) % 128 == 0 {
-                assert_eq!(staked_nodes(&accounts), vote_accounts.staked_nodes());
+                assert_eq!(staked_nodes(&accounts), *vote_accounts.staked_nodes());
             }
         }
         // Remove everything.
@@ -508,9 +516,41 @@ mod tests {
             let (pubkey, (_, _)) = accounts.swap_remove(index);
             vote_accounts.remove(&pubkey);
             if accounts.len() % 32 == 0 {
-                assert_eq!(staked_nodes(&accounts), vote_accounts.staked_nodes());
+                assert_eq!(staked_nodes(&accounts), *vote_accounts.staked_nodes());
             }
         }
         assert!(vote_accounts.staked_nodes.read().unwrap().is_empty());
     }
+
+    // Asserts that returned staked-nodes are copy-on-write references.
+    #[test]
+    fn test_staked_nodes_cow() {
+        let mut rng = rand::thread_rng();
+        let mut accounts = new_rand_vote_accounts(&mut rng, 64);
+        // Add vote accounts.
+        let mut vote_accounts = VoteAccounts::default();
+        for (pubkey, (stake, vote_account)) in (&mut accounts).take(1024) {
+            vote_accounts.insert(pubkey, (stake, vote_account));
+        }
+        let staked_nodes = vote_accounts.staked_nodes();
+        let (pubkey, (more_stake, vote_account)) =
+            accounts.find(|(_, (stake, _))| *stake != 0).unwrap();
+        let node_pubkey = vote_account.node_pubkey().unwrap();
+        vote_accounts.insert(pubkey, (more_stake, vote_account));
+        assert_ne!(staked_nodes, vote_accounts.staked_nodes());
+        assert_eq!(
+            vote_accounts.staked_nodes()[&node_pubkey],
+            more_stake + staked_nodes.get(&node_pubkey).copied().unwrap_or_default()
+        );
+        for (pubkey, stake) in vote_accounts.staked_nodes().iter() {
+            if *pubkey != node_pubkey {
+                assert_eq!(*stake, staked_nodes[pubkey]);
+            } else {
+                assert_eq!(
+                    *stake,
+                    more_stake + staked_nodes.get(pubkey).copied().unwrap_or_default()
+                );
+            }
+        }
+    }
 }