From a6d737d773b85c8bd252948d2123394f474af055 Mon Sep 17 00:00:00 2001
From: Brooks Prumo <brooks@solana.com>
Date: Thu, 1 Sep 2022 12:11:07 -0400
Subject: [PATCH] Add EpochAccountsHash to AccountsDb (#27538)

---
 runtime/src/accounts_db.rs         |  6 ++++
 runtime/src/epoch_accounts_hash.rs | 58 ++++++++++++++++++++++++++++++
 runtime/src/lib.rs                 |  1 +
 3 files changed, 65 insertions(+)
 create mode 100644 runtime/src/epoch_accounts_hash.rs

diff --git a/runtime/src/accounts_db.rs b/runtime/src/accounts_db.rs
index 7ac56c1272..0cd4b1392e 100644
--- a/runtime/src/accounts_db.rs
+++ b/runtime/src/accounts_db.rs
@@ -45,6 +45,7 @@ use {
         bank::Rewrites,
         cache_hash_data::CacheHashData,
         contains::Contains,
+        epoch_accounts_hash::EpochAccountsHash,
         expected_rent_collection::{ExpectedRentCollection, SlotInfoInEpoch},
         pubkey_bins::PubkeyBinCalculator24,
         read_only_accounts_cache::ReadOnlyAccountsCache,
@@ -1182,6 +1183,10 @@ pub struct AccountsDb {
     /// Used to disable logging dead slots during removal.
     /// allow disabling noisy log
     pub(crate) log_dead_slots: AtomicBool,
+
+    /// A special accounts hash that occurs once per epoch
+    #[allow(dead_code)]
+    pub(crate) epoch_accounts_hash: Mutex<Option<EpochAccountsHash>>,
 }
 
 #[derive(Debug, Default)]
@@ -1979,6 +1984,7 @@ impl AccountsDb {
             filler_account_suffix: None,
             num_hash_scan_passes,
             log_dead_slots: AtomicBool::new(true),
+            epoch_accounts_hash: Mutex::new(None),
         }
     }
 
diff --git a/runtime/src/epoch_accounts_hash.rs b/runtime/src/epoch_accounts_hash.rs
new file mode 100644
index 0000000000..48c55c4301
--- /dev/null
+++ b/runtime/src/epoch_accounts_hash.rs
@@ -0,0 +1,58 @@
+//! The Epoch Accounts Hash (EAH) is a special hash of the whole accounts state that occurs once
+//! per epoch.
+//!
+//! This hash is special because all nodes in the cluster will calculate the accounts hash at a
+//! predetermined slot in the epoch and then save that result into a later Bank at a predetermined
+//! slot.
+//!
+//! This results in all nodes effectively voting on the accounts state (at least) once per epoch.
+
+use {
+    crate::bank::Bank,
+    serde::{Deserialize, Serialize},
+    solana_sdk::{clock::Slot, hash::Hash},
+};
+
+/// The EpochAccountsHash holds the result after calculating the accounts hash once per epoch
+#[derive(Debug, Serialize, Deserialize, Hash, PartialEq, Eq, Clone, Copy)]
+pub struct EpochAccountsHash(Hash);
+
+impl AsRef<Hash> for EpochAccountsHash {
+    fn as_ref(&self) -> &Hash {
+        &self.0
+    }
+}
+
+/// Calculation of the EAH occurs once per epoch.  All nodes in the cluster must agree on which
+/// slot the EAH is based on.  This slot will be at an offset into the epoch, and referred to as
+/// the "start" slot for the EAH calculation.
+#[must_use]
+#[allow(dead_code)]
+pub fn calculation_offset_start(bank: &Bank) -> Slot {
+    let slots_per_epoch = bank.epoch_schedule().slots_per_epoch;
+    slots_per_epoch / 4
+}
+
+/// Calculation of the EAH occurs once per epoch.  All nodes in the cluster must agree on which
+/// bank will hash the EAH into its `Bank::hash`.  This slot will be at an offset into the epoch,
+/// and referred to as the "stop" slot for the EAH calculation.  All nodes must complete the EAH
+/// calculation before this slot!
+#[must_use]
+#[allow(dead_code)]
+pub fn calculation_offset_stop(bank: &Bank) -> Slot {
+    let slots_per_epoch = bank.epoch_schedule().slots_per_epoch;
+    slots_per_epoch / 4 * 3
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_calculation_offset_bounds() {
+        let bank = Bank::default_for_tests();
+        let start = calculation_offset_start(&bank);
+        let stop = calculation_offset_stop(&bank);
+        assert!(start < stop);
+    }
+}
diff --git a/runtime/src/lib.rs b/runtime/src/lib.rs
index 15cc059f9a..247df17fe1 100644
--- a/runtime/src/lib.rs
+++ b/runtime/src/lib.rs
@@ -34,6 +34,7 @@ pub mod commitment;
 pub mod contains;
 pub mod cost_model;
 pub mod cost_tracker;
+mod epoch_accounts_hash;
 pub mod epoch_stakes;
 pub mod execute_cost_table;
 mod expected_rent_collection;