[TieredStorage] HotStorageReader::get_account_meta_from_offset() (sol…

…ana-labs#33724)

#### Problem
HotStorageReader currently only implements get_footer().  It does not
have a function to obtain the account meta.

#### Summary of Changes
This PR implements HotStorageReader::get_account_meta_from_offset().
A function that returns the account meta located at the specified offset.
This will be the helper function that will be later used to obtain the account
meta when the offset is available from the index block of a hot storage file. 

#### Test Plan
A new test is included in this PR.

accounts-db/src/tiered_storage/hot.rs

@@ -7,6 +7,7 @@ use {
         footer::{AccountBlockFormat, AccountMetaFormat, OwnersBlockFormat, TieredStorageFooter},
         index::AccountIndexFormat,
         meta::{AccountMetaFlags, AccountMetaOptionalFields, TieredAccountMeta},
+        mmap_utils::get_type,
         TieredStorageFormat, TieredStorageResult,
     },
     memmap2::{Mmap, MmapOptions},
@@ -215,6 +216,12 @@ impl HotStorageReader {
     pub fn num_accounts(&self) -> usize {
         self.footer.account_entry_count as usize
     }
+
+    /// Returns the account meta located at the specified offset.
+    fn get_account_meta_from_offset(&self, offset: usize) -> TieredStorageResult<&HotAccountMeta> {
+        let (meta, _) = get_type::<HotAccountMeta>(&self.mmap, offset)?;
+        Ok(meta)
+    }
 }
 
 #[cfg(test)]
@@ -234,6 +241,7 @@ pub mod tests {
         },
         ::solana_sdk::{hash::Hash, pubkey::Pubkey, stake_history::Epoch},
         memoffset::offset_of,
+        rand::Rng,
         tempfile::TempDir,
     };
 
@@ -398,4 +406,53 @@ pub mod tests {
             assert_eq!(expected_footer, *hot_storage.footer());
         }
     }
+
+    #[test]
+    fn test_hot_storage_get_account_meta_from_offset() {
+        // Generate a new temp path that is guaranteed to NOT already have a file.
+        let temp_dir = TempDir::new().unwrap();
+        let path = temp_dir.path().join("test_hot_storage_footer");
+
+        const NUM_ACCOUNTS: u32 = 10;
+        let mut rng = rand::thread_rng();
+
+        let hot_account_metas: Vec<_> = (0..NUM_ACCOUNTS)
+            .map(|_| {
+                HotAccountMeta::new()
+                    .with_lamports(rng.gen_range(0..u64::MAX))
+                    .with_owner_index(rng.gen_range(0..NUM_ACCOUNTS))
+            })
+            .collect();
+
+        let account_offsets: Vec<_>;
+        let footer = TieredStorageFooter {
+            account_meta_format: AccountMetaFormat::Hot,
+            account_entry_count: NUM_ACCOUNTS,
+            ..TieredStorageFooter::default()
+        };
+        {
+            let file = TieredStorageFile::new_writable(&path).unwrap();
+            let mut current_offset = 0;
+
+            account_offsets = hot_account_metas
+                .iter()
+                .map(|meta| {
+                    let prev_offset = current_offset;
+                    current_offset += file.write_type(meta).unwrap();
+                    prev_offset
+                })
+                .collect();
+            // while the test only focuses on account metas, writing a footer
+            // here is necessary to make it a valid tiered-storage file.
+            footer.write_footer_block(&file).unwrap();
+        }
+
+        let hot_storage = HotStorageReader::new_from_path(&path).unwrap();
+
+        for (offset, expected_meta) in account_offsets.iter().zip(hot_account_metas.iter()) {
+            let meta = hot_storage.get_account_meta_from_offset(*offset).unwrap();
+            assert_eq!(meta, expected_meta);
+        }
+        assert_eq!(&footer, hot_storage.footer());
+    }
 }