BlobDB: use char array instead of string as buffer (#4662)

Summary:
As pointed out in #4059, we miss use string as buffer for file read. Changing to use char array instead.

Closing #4059
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4662

Differential Revision: D13012998

Pulled By: yiwu-arbug

fbshipit-source-id: 41234ba17c0bccea65bd647e362a0e979152bd1e

utilities/blob_db/blob_log_format.h

@@ -10,7 +10,9 @@
 #ifndef ROCKSDB_LITE
 
 #include <limits>
+#include <memory>
 #include <utility>
+
 #include "rocksdb/options.h"
 #include "rocksdb/slice.h"
 #include "rocksdb/status.h"
@@ -106,8 +108,8 @@ struct BlobLogRecord {
   uint32_t blob_crc = 0;
   Slice key;
   Slice value;
-  std::string key_buf;
-  std::string value_buf;
+  std::unique_ptr<char[]> key_buf;
+  std::unique_ptr<char[]> value_buf;
 
   uint64_t record_size() const { return kHeaderSize + key_size + value_size; }
 

utilities/blob_db/blob_log_reader.cc

@@ -24,10 +24,9 @@ Reader::Reader(unique_ptr<RandomAccessFileReader>&& file_reader, Env* env,
       buffer_(),
       next_byte_(0) {}
 
-Status Reader::ReadSlice(uint64_t size, Slice* slice, std::string* buf) {
+Status Reader::ReadSlice(uint64_t size, Slice* slice, char* buf) {
   StopWatch read_sw(env_, statistics_, BLOB_DB_BLOB_FILE_READ_MICROS);
-  buf->reserve(static_cast<size_t>(size));
-  Status s = file_->Read(next_byte_, static_cast<size_t>(size), slice, &(*buf)[0]);
+  Status s = file_->Read(next_byte_, static_cast<size_t>(size), slice, buf);
   next_byte_ += size;
   if (!s.ok()) {
     return s;
@@ -42,7 +41,7 @@ Status Reader::ReadSlice(uint64_t size, Slice* slice, std::string* buf) {
 Status Reader::ReadHeader(BlobLogHeader* header) {
   assert(file_.get() != nullptr);
   assert(next_byte_ == 0);
-  Status s = ReadSlice(BlobLogHeader::kSize, &buffer_, &backing_store_);
+  Status s = ReadSlice(BlobLogHeader::kSize, &buffer_, header_buf_);
   if (!s.ok()) {
     return s;
   }
@@ -56,7 +55,7 @@ Status Reader::ReadHeader(BlobLogHeader* header) {
 
 Status Reader::ReadRecord(BlobLogRecord* record, ReadLevel level,
                           uint64_t* blob_offset) {
-  Status s = ReadSlice(BlobLogRecord::kHeaderSize, &buffer_, &backing_store_);
+  Status s = ReadSlice(BlobLogRecord::kHeaderSize, &buffer_, header_buf_);
   if (!s.ok()) {
     return s;
   }
@@ -80,14 +79,18 @@ Status Reader::ReadRecord(BlobLogRecord* record, ReadLevel level,
       break;
 
     case kReadHeaderKey:
-      s = ReadSlice(record->key_size, &record->key, &record->key_buf);
+      record->key_buf.reset(new char[record->key_size]);
+      s = ReadSlice(record->key_size, &record->key, record->key_buf.get());
       next_byte_ += record->value_size;
       break;
 
     case kReadHeaderKeyBlob:
-      s = ReadSlice(record->key_size, &record->key, &record->key_buf);
+      record->key_buf.reset(new char[record->key_size]);
+      s = ReadSlice(record->key_size, &record->key, record->key_buf.get());
       if (s.ok()) {
-        s = ReadSlice(record->value_size, &record->value, &record->value_buf);
+        record->value_buf.reset(new char[record->value_size]);
+        s = ReadSlice(record->value_size, &record->value,
+                      record->value_buf.get());
       }
       if (s.ok()) {
         s = record->CheckBlobCRC();

utilities/blob_db/blob_log_reader.h

@@ -60,19 +60,19 @@ class Reader {
   Status ReadRecord(BlobLogRecord* record, ReadLevel level = kReadHeader,
                     uint64_t* blob_offset = nullptr);
 
-  Status ReadSlice(uint64_t size, Slice* slice, std::string* buf);
-
   void ResetNextByte() { next_byte_ = 0; }
 
   uint64_t GetNextByte() const { return next_byte_; }
 
  private:
+  Status ReadSlice(uint64_t size, Slice* slice, char* buf);
+
   const std::unique_ptr<RandomAccessFileReader> file_;
   Env* env_;
   Statistics* statistics_;
 
-  std::string backing_store_;
   Slice buffer_;
+  char header_buf_[BlobLogRecord::kHeaderSize];
 
   // which byte to read next. For asserting proper usage
   uint64_t next_byte_;