[TieredStorage] Avoid AccountHash copy in AccountMetaOptionalFields (s…

…olana-labs#34969)

#### Problem
Using non-reference type of AccountHash in 
AccountMetaOptionalFields causes an unnecessary copy
as mentioned in solana-labs#34948.

#### Summary of Changes
Uses &AccountHash in AccountMetaOptionalFields to
avoid copying.

#### Test Plan
Existing unit tests.

Fixes solana-labs#34948

accounts-db/src/tiered_storage/byte_block.rs

@@ -96,7 +96,7 @@ impl ByteBlockWriter {
             size += self.write_pod(&rent_epoch)?;
         }
         if let Some(hash) = opt_fields.account_hash {
-            size += self.write_pod(&hash)?;
+            size += self.write_pod(hash)?;
         }
 
         debug_assert_eq!(size, opt_fields.size());
@@ -352,11 +352,12 @@ mod tests {
         let mut writer = ByteBlockWriter::new(format);
         let mut opt_fields_vec = vec![];
         let mut some_count = 0;
+        let acc_hash = AccountHash(Hash::new_unique());
 
         // prepare a vector of optional fields that contains all combinations
         // of Some and None.
         for rent_epoch in [None, Some(test_epoch)] {
-            for account_hash in [None, Some(AccountHash(Hash::new_unique()))] {
+            for account_hash in [None, Some(&acc_hash)] {
                 some_count += rent_epoch.iter().count() + account_hash.iter().count();
 
                 opt_fields_vec.push(AccountMetaOptionalFields {
@@ -397,7 +398,7 @@ mod tests {
             }
             if let Some(expected_hash) = opt_fields.account_hash {
                 let hash = read_pod::<AccountHash>(&decoded_buffer, offset).unwrap();
-                assert_eq!(hash, &expected_hash);
+                assert_eq!(hash, expected_hash);
                 verified_count += 1;
                 offset += std::mem::size_of::<AccountHash>();
             }

accounts-db/src/tiered_storage/hot.rs

@@ -469,7 +469,7 @@ fn write_optional_fields(
         size += file.write_pod(&rent_epoch)?;
     }
     if let Some(hash) = opt_fields.account_hash {
-        size += file.write_pod(&hash)?;
+        size += file.write_pod(hash)?;
     }
 
     debug_assert_eq!(size, opt_fields.size());
@@ -500,7 +500,7 @@ impl HotStorageWriter {
         account_data: &[u8],
         executable: bool,
         rent_epoch: Option<Epoch>,
-        account_hash: Option<AccountHash>,
+        account_hash: Option<&AccountHash>,
     ) -> TieredStorageResult<usize> {
         let optional_fields = AccountMetaOptionalFields {
             rent_epoch,
@@ -567,9 +567,9 @@ impl HotStorageWriter {
                         acc.owner(),
                         acc.data(),
                         acc.executable(),
-                        // only persist rent_epoch for those non-rent-exempt accounts
+                        // only persist rent_epoch for those rent-paying accounts
                         (acc.rent_epoch() != RENT_EXEMPT_RENT_EPOCH).then_some(acc.rent_epoch()),
-                        Some(*account_hash),
+                        Some(account_hash),
                     )
                 })
                 .unwrap_or((0, &OWNER_NO_OWNER, &[], false, None, None));
@@ -722,10 +722,11 @@ pub mod tests {
         const TEST_PADDING: u8 = 5;
         const TEST_OWNER_OFFSET: OwnerOffset = OwnerOffset(0x1fef_1234);
         const TEST_RENT_EPOCH: Epoch = 7;
+        let acc_hash = AccountHash(Hash::new_unique());
 
         let optional_fields = AccountMetaOptionalFields {
             rent_epoch: Some(TEST_RENT_EPOCH),
-            account_hash: Some(AccountHash(Hash::new_unique())),
+            account_hash: Some(&acc_hash),
         };
 
         let flags = AccountMetaFlags::new_from(&optional_fields);
@@ -745,14 +746,15 @@ pub mod tests {
     fn test_hot_account_meta_full() {
         let account_data = [11u8; 83];
         let padding = [0u8; 5];
+        let acc_hash = AccountHash(Hash::new_unique());
 
         const TEST_LAMPORT: u64 = 2314232137;
         const OWNER_OFFSET: u32 = 0x1fef_1234;
         const TEST_RENT_EPOCH: Epoch = 7;
 
         let optional_fields = AccountMetaOptionalFields {
             rent_epoch: Some(TEST_RENT_EPOCH),
-            account_hash: Some(AccountHash(Hash::new_unique())),
+            account_hash: Some(&acc_hash),
         };
 
         let flags = AccountMetaFlags::new_from(&optional_fields);
@@ -789,7 +791,7 @@ pub mod tests {
         assert_eq!(account_data, meta.account_data(account_block));
         assert_eq!(meta.rent_epoch(account_block), optional_fields.rent_epoch);
         assert_eq!(
-            *(meta.account_hash(account_block).unwrap()),
+            (meta.account_hash(account_block).unwrap()),
             optional_fields.account_hash.unwrap()
         );
     }
@@ -1339,7 +1341,7 @@ pub mod tests {
                         acc.owner(),
                         acc.data(),
                         acc.executable(),
-                        // only persist rent_epoch for those non-rent-exempt accounts
+                        // only persist rent_epoch for those rent-paying accounts
                         Some(*account_hash),
                     )
                 })

accounts-db/src/tiered_storage/meta.rs

@@ -102,14 +102,14 @@ impl AccountMetaFlags {
 /// Note that the storage representation of the optional fields might be
 /// different from its in-memory representation.
 #[derive(Debug, PartialEq, Eq, Clone)]
-pub struct AccountMetaOptionalFields {
+pub struct AccountMetaOptionalFields<'a> {
     /// the epoch at which its associated account will next owe rent
     pub rent_epoch: Option<Epoch>,
     /// the hash of its associated account
-    pub account_hash: Option<AccountHash>,
+    pub account_hash: Option<&'a AccountHash>,
 }
 
-impl AccountMetaOptionalFields {
+impl<'a> AccountMetaOptionalFields<'a> {
     /// The size of the optional fields in bytes (excluding the boolean flags).
     pub fn size(&self) -> usize {
         self.rent_epoch.map_or(0, |_| std::mem::size_of::<Epoch>())
@@ -210,9 +210,10 @@ pub mod tests {
     #[test]
     fn test_optional_fields_update_flags() {
         let test_epoch = 5432312;
+        let acc_hash = AccountHash(Hash::new_unique());
 
         for rent_epoch in [None, Some(test_epoch)] {
-            for account_hash in [None, Some(AccountHash(Hash::new_unique()))] {
+            for account_hash in [None, Some(&acc_hash)] {
                 update_and_verify_flags(&AccountMetaOptionalFields {
                     rent_epoch,
                     account_hash,
@@ -224,9 +225,10 @@ pub mod tests {
     #[test]
     fn test_optional_fields_size() {
         let test_epoch = 5432312;
+        let acc_hash = AccountHash(Hash::new_unique());
 
         for rent_epoch in [None, Some(test_epoch)] {
-            for account_hash in [None, Some(AccountHash(Hash::new_unique()))] {
+            for account_hash in [None, Some(&acc_hash)] {
                 let opt_fields = AccountMetaOptionalFields {
                     rent_epoch,
                     account_hash,
@@ -249,16 +251,17 @@ pub mod tests {
     #[test]
     fn test_optional_fields_offset() {
         let test_epoch = 5432312;
+        let acc_hash = AccountHash(Hash::new_unique());
 
         for rent_epoch in [None, Some(test_epoch)] {
-            for account_hash in [None, Some(AccountHash(Hash::new_unique()))] {
+            for account_hash in [None, Some(&acc_hash)] {
                 let rent_epoch_offset = 0;
                 let account_hash_offset =
                     rent_epoch_offset + rent_epoch.as_ref().map(std::mem::size_of_val).unwrap_or(0);
                 let derived_size = account_hash_offset
                     + account_hash
                         .as_ref()
-                        .map(std::mem::size_of_val)
+                        .map(|acc_hash| std::mem::size_of_val(*acc_hash))
                         .unwrap_or(0);
                 let opt_fields = AccountMetaOptionalFields {
                     rent_epoch,