[TieredStorage] Repurpose TieredReadableAccount to HotAccount (#218)

#### Problem
As we further optimize the HotStorageMeta in #146, there is a need
for a HotAccount struct that contains all the hot account information.
Meanwhile, we currently don't have plans to develop a cold account
format at this moment.  As a result, this makes it desirable to repurpose
TieredReadableAccount to HotAccount.

#### Summary of Changes
Repurpose TieredReadableAccount to HotAccount.

#### Test Plan
Existing tiered-storage tests.

accounts-db/src/account_storage/meta.rs

@@ -3,7 +3,7 @@ use {
         accounts_hash::AccountHash,
         append_vec::AppendVecStoredAccountMeta,
         storable_accounts::StorableAccounts,
-        tiered_storage::{hot::HotAccountMeta, readable::TieredReadableAccount},
+        tiered_storage::hot::{HotAccount, HotAccountMeta},
     },
     solana_sdk::{account::ReadableAccount, hash::Hash, pubkey::Pubkey, stake_history::Epoch},
     std::{borrow::Borrow, marker::PhantomData},
@@ -114,7 +114,7 @@ impl<
 #[derive(PartialEq, Eq, Debug)]
 pub enum StoredAccountMeta<'storage> {
     AppendVec(AppendVecStoredAccountMeta<'storage>),
-    Hot(TieredReadableAccount<'storage, HotAccountMeta>),
+    Hot(HotAccount<'storage, HotAccountMeta>),
 }
 
 impl<'storage> StoredAccountMeta<'storage> {

accounts-db/src/tiered_storage/hot.rs

@@ -13,7 +13,6 @@ use {
             meta::{AccountMetaFlags, AccountMetaOptionalFields, TieredAccountMeta},
             mmap_utils::{get_pod, get_slice},
             owners::{OwnerOffset, OwnersBlockFormat, OwnersTable, OWNER_NO_OWNER},
-            readable::TieredReadableAccount,
             StorableAccounts, StorableAccountsWithHashesAndWriteVersions, TieredStorageError,
             TieredStorageFormat, TieredStorageResult,
         },
@@ -264,6 +263,81 @@ impl TieredAccountMeta for HotAccountMeta {
     }
 }
 
+/// The struct that offers read APIs for accessing a hot account.
+#[derive(PartialEq, Eq, Debug)]
+pub struct HotAccount<'accounts_file, M: TieredAccountMeta> {
+    /// TieredAccountMeta
+    pub meta: &'accounts_file M,
+    /// The address of the account
+    pub address: &'accounts_file Pubkey,
+    /// The address of the account owner
+    pub owner: &'accounts_file Pubkey,
+    /// The index for accessing the account inside its belonging AccountsFile
+    pub index: IndexOffset,
+    /// The account block that contains this account.  Note that this account
+    /// block may be shared with other accounts.
+    pub account_block: &'accounts_file [u8],
+}
+
+impl<'accounts_file, M: TieredAccountMeta> HotAccount<'accounts_file, M> {
+    /// Returns the address of this account.
+    pub fn address(&self) -> &'accounts_file Pubkey {
+        self.address
+    }
+
+    /// Returns the index to this account in its AccountsFile.
+    pub fn index(&self) -> IndexOffset {
+        self.index
+    }
+
+    /// Returns the data associated to this account.
+    pub fn data(&self) -> &'accounts_file [u8] {
+        self.meta.account_data(self.account_block)
+    }
+}
+
+impl<'accounts_file, M: TieredAccountMeta> ReadableAccount for HotAccount<'accounts_file, M> {
+    /// Returns the balance of the lamports of this account.
+    fn lamports(&self) -> u64 {
+        self.meta.lamports()
+    }
+
+    /// Returns the address of the owner of this account.
+    fn owner(&self) -> &'accounts_file Pubkey {
+        self.owner
+    }
+
+    /// Returns true if the data associated to this account is executable.
+    fn executable(&self) -> bool {
+        self.meta.flags().executable()
+    }
+
+    /// Returns the epoch that this account will next owe rent by parsing
+    /// the specified account block.  RENT_EXEMPT_RENT_EPOCH will be returned
+    /// if the account is rent-exempt.
+    ///
+    /// For a zero-lamport account, Epoch::default() will be returned to
+    /// default states of an AccountSharedData.
+    fn rent_epoch(&self) -> Epoch {
+        self.meta
+            .rent_epoch(self.account_block)
+            .unwrap_or(if self.lamports() != 0 {
+                RENT_EXEMPT_RENT_EPOCH
+            } else {
+                // While there is no valid-values for any fields of a zero
+                // lamport account, here we return Epoch::default() to
+                // match the default states of AccountSharedData.  Otherwise,
+                // a hash mismatch will occur.
+                Epoch::default()
+            })
+    }
+
+    /// Returns the data associated to this account.
+    fn data(&self) -> &'accounts_file [u8] {
+        self.data()
+    }
+}
+
 /// The reader to a hot accounts file.
 #[derive(Debug)]
 pub struct HotStorageReader {
@@ -437,7 +511,7 @@ impl HotStorageReader {
         let account_block = self.get_account_block(account_offset, index_offset)?;
 
         Ok(Some((
-            StoredAccountMeta::Hot(TieredReadableAccount {
+            StoredAccountMeta::Hot(HotAccount {
                 meta,
                 address,
                 owner,

accounts-db/src/tiered_storage/readable.rs

@@ -6,94 +6,13 @@ use {
             footer::{AccountMetaFormat, TieredStorageFooter},
             hot::HotStorageReader,
             index::IndexOffset,
-            meta::TieredAccountMeta,
             TieredStorageResult,
         },
     },
-    solana_sdk::{
-        account::ReadableAccount, pubkey::Pubkey, rent_collector::RENT_EXEMPT_RENT_EPOCH,
-        stake_history::Epoch,
-    },
+    solana_sdk::pubkey::Pubkey,
     std::path::Path,
 };
 
-/// The struct that offers read APIs for accessing a TieredAccount.
-#[derive(PartialEq, Eq, Debug)]
-pub struct TieredReadableAccount<'accounts_file, M: TieredAccountMeta> {
-    /// TieredAccountMeta
-    pub meta: &'accounts_file M,
-    /// The address of the account
-    pub address: &'accounts_file Pubkey,
-    /// The address of the account owner
-    pub owner: &'accounts_file Pubkey,
-    /// The index for accessing the account inside its belonging AccountsFile
-    pub index: IndexOffset,
-    /// The account block that contains this account.  Note that this account
-    /// block may be shared with other accounts.
-    pub account_block: &'accounts_file [u8],
-}
-
-impl<'accounts_file, M: TieredAccountMeta> TieredReadableAccount<'accounts_file, M> {
-    /// Returns the address of this account.
-    pub fn address(&self) -> &'accounts_file Pubkey {
-        self.address
-    }
-
-    /// Returns the index to this account in its AccountsFile.
-    pub fn index(&self) -> IndexOffset {
-        self.index
-    }
-
-    /// Returns the data associated to this account.
-    pub fn data(&self) -> &'accounts_file [u8] {
-        self.meta.account_data(self.account_block)
-    }
-}
-
-impl<'accounts_file, M: TieredAccountMeta> ReadableAccount
-    for TieredReadableAccount<'accounts_file, M>
-{
-    /// Returns the balance of the lamports of this account.
-    fn lamports(&self) -> u64 {
-        self.meta.lamports()
-    }
-
-    /// Returns the address of the owner of this account.
-    fn owner(&self) -> &'accounts_file Pubkey {
-        self.owner
-    }
-
-    /// Returns true if the data associated to this account is executable.
-    fn executable(&self) -> bool {
-        self.meta.flags().executable()
-    }
-
-    /// Returns the epoch that this account will next owe rent by parsing
-    /// the specified account block.  RENT_EXEMPT_RENT_EPOCH will be returned
-    /// if the account is rent-exempt.
-    ///
-    /// For a zero-lamport account, Epoch::default() will be returned to
-    /// default states of an AccountSharedData.
-    fn rent_epoch(&self) -> Epoch {
-        self.meta
-            .rent_epoch(self.account_block)
-            .unwrap_or(if self.lamports() != 0 {
-                RENT_EXEMPT_RENT_EPOCH
-            } else {
-                // While there is no valid-values for any fields of a zero
-                // lamport account, here we return Epoch::default() to
-                // match the default states of AccountSharedData.  Otherwise,
-                // a hash mismatch will occur.
-                Epoch::default()
-            })
-    }
-
-    /// Returns the data associated to this account.
-    fn data(&self) -> &'accounts_file [u8] {
-        self.data()
-    }
-}
-
 /// The reader of a tiered storage instance.
 #[derive(Debug)]
 pub enum TieredStorageReader {