SimpleCache: open with fewer seeks.

net/disk_cache/simple/simple_entry_format.h

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NET_DISK_CACHE_SIMPLE_SIMPLE_ENTRY_FORMAT_H_
 #define NET_DISK_CACHE_SIMPLE_SIMPLE_ENTRY_FORMAT_H_
 
 #include <stdint.h>
 
 #include "net/base/net_export.h"
 
 namespace base {
 class Time;
 }
 
 namespace disk_cache {
 
 const uint64_t kSimpleInitialMagicNumber = UINT64_C(0xfcfb6d1ba7725c30);
 const uint64_t kSimpleFinalMagicNumber = UINT64_C(0xf4fa6f45970d41d8);
 const uint64_t kSimpleSparseRangeMagicNumber = UINT64_C(0xeb97bf016553676b);
 
 // A file containing stream 0 and stream 1 in the Simple cache consists of:
 //   - a SimpleFileHeader.
 //   - the key.
 //   - the data from stream 1.
 //   - a SimpleFileEOF record for stream 1.
 //   - the data from stream 0.
+//   - (optionally) the SHA256 of the key.
 //   - a SimpleFileEOF record for stream 0.
+//
+// Because stream 0 data (typically HTTP headers) is on the critical path of
+// requests, on open, the cache reads the end of the record and does not
+// read the SimpleFileHeader. If the key can be validated with a SHA256, then
+// the stream 0 data can be returned to the caller without reading the
+// SimpleFileHeader. If the key SHA256 is not present, then the cache must
+// read the SimpleFileHeader to confirm key equality.
 
 // A file containing stream 2 in the Simple cache consists of:
 //   - a SimpleFileHeader.
 //   - the key.
 //   - the data.
 //   - at the end, a SimpleFileEOF record.
 static const int kSimpleEntryFileCount = 2;
 static const int kSimpleEntryStreamCount = 3;
 
 struct NET_EXPORT_PRIVATE SimpleFileHeader {
   SimpleFileHeader();
 
   uint64_t initial_magic_number;
   uint32_t version;
   uint32_t key_length;
   uint32_t key_hash;
 };
 
 struct NET_EXPORT_PRIVATE SimpleFileEOF {
   enum Flags {
     FLAG_HAS_CRC32 = (1U << 0),
+    FLAG_HAS_KEY_SHA256 = (1U << 1),  // Preceding the record if present.
   };
 
   SimpleFileEOF();
 
   uint64_t final_magic_number;
   uint32_t flags;
   uint32_t data_crc32;
   // |stream_size| is only used in the EOF record for stream 0.
   uint32_t stream_size;
 };
 
 struct SimpleFileSparseRangeHeader {
   SimpleFileSparseRangeHeader();
 
   uint64_t sparse_range_magic_number;
   int64_t offset;
   int64_t length;
   uint32_t data_crc32;
 };
 
 }  // namespace disk_cache
 
 #endif  // NET_DISK_CACHE_SIMPLE_SIMPLE_ENTRY_FORMAT_H_