From 2082f8b3cab9a5ffc07f2a4e6efaadb532180320 Mon Sep 17 00:00:00 2001
From: Yueh-Hsuan Chiang <93241502+yhchiang-sol@users.noreply.github.com>
Date: Mon, 8 Jan 2024 14:14:30 -0800
Subject: [PATCH] [TieredStorage] HotStorageWriter::new() (#34659)

#### Problem
The implementation of HotAccountWriter is missing.

#### Summary of Changes
This PR kicks off the implementation of HotStorageWriter by
adding HotStorageWriter::new().

#### Test Plan
Add a new unit-test that verifies the correctness of HotStorageWriter
writing zero accounts using HotStorageReader.
---
 accounts-db/src/tiered_storage/hot.rs | 43 +++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/accounts-db/src/tiered_storage/hot.rs b/accounts-db/src/tiered_storage/hot.rs
index 9adfe08541..4d42cbb4bc 100644
--- a/accounts-db/src/tiered_storage/hot.rs
+++ b/accounts-db/src/tiered_storage/hot.rs
@@ -7,6 +7,7 @@ use {
         accounts_hash::AccountHash,
         tiered_storage::{
             byte_block,
+            file::TieredStorageFile,
             footer::{
                 AccountBlockFormat, AccountMetaFormat, OwnersBlockFormat, TieredStorageFooter,
             },
@@ -33,6 +34,19 @@ pub const HOT_FORMAT: TieredStorageFormat = TieredStorageFormat {
     account_block_format: AccountBlockFormat::AlignedRaw,
 };
 
+/// An helper function that creates a new default footer for hot
+/// accounts storage.
+fn new_hot_footer() -> TieredStorageFooter {
+    TieredStorageFooter {
+        account_meta_format: HOT_FORMAT.account_meta_format,
+        account_meta_entry_size: HOT_FORMAT.meta_entry_size as u32,
+        account_block_format: HOT_FORMAT.account_block_format,
+        index_block_format: HOT_FORMAT.index_block_format,
+        owners_block_format: HOT_FORMAT.owners_block_format,
+        ..TieredStorageFooter::default()
+    }
+}
+
 /// The maximum number of padding bytes used in a hot account entry.
 const MAX_HOT_PADDING: u8 = 7;
 
@@ -430,6 +444,21 @@ impl HotStorageReader {
     }
 }
 
+/// The writer that creates a hot accounts file.
+#[derive(Debug)]
+pub struct HotStorageWriter {
+    storage: TieredStorageFile,
+}
+
+impl HotStorageWriter {
+    /// Create a new HotStorageWriter with the specified path.
+    pub fn new(file_path: impl AsRef<Path>) -> TieredStorageResult<Self> {
+        Ok(Self {
+            storage: TieredStorageFile::new_writable(file_path)?,
+        })
+    }
+}
+
 #[cfg(test)]
 pub mod tests {
     use {
@@ -1026,4 +1055,18 @@ pub mod tests {
             Ok(None)
         );
     }
+
+    #[test]
+    fn test_hot_storage_writer_twice_on_same_path() {
+        let temp_dir = TempDir::new().unwrap();
+        let path = temp_dir
+            .path()
+            .join("test_hot_storage_writer_twice_on_same_path");
+
+        // Expect the first returns Ok
+        assert_matches!(HotStorageWriter::new(&path), Ok(_));
+        // Expect the second call on the same path returns Err, as the
+        // HotStorageWriter only writes once.
+        assert_matches!(HotStorageWriter::new(&path), Err(_));
+    }
 }