net/disk_cache/simple/simple_entry_format.h

// Copyright 2013 The Chromium Authors
// 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 <type_traits>

#include "net/base/net_export.h"

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.

// This is the number of files we can use for representing normal/dense streams.
static const int kSimpleEntryNormalFileCount = 2;
static const int kSimpleEntryStreamCount = 3;

// Total # of files name we can potentially use; this includes both normal
// API and sparse streams.
static const int kSimpleEntryTotalFileCount = kSimpleEntryNormalFileCount + 1;

// Note that stream 0/stream 1 files rely on the footer to verify the entry,
// so if the format changes, it's insufficient to change the version here;
// likely the EOF magic should be updated as well.
struct NET_EXPORT_PRIVATE SimpleFileHeader {
  SimpleFileHeader();

  uint64_t initial_magic_number = 0;
  uint32_t version = 0;
  uint32_t key_length = 0;
  uint32_t key_hash = 0;

  // Avoid implicit padding so `std::has_unique_object_representations_v<>` will
  // hold.
  uint32_t unused_padding = 0;
};
static_assert(std::has_unique_object_representations_v<SimpleFileHeader>);

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 = 0;
  uint32_t flags = 0;
  uint32_t data_crc32 = 0;
  // |stream_size| is only used in the EOF record for stream 0. The value must
  // smaller than int32 max.
  uint32_t stream_size = 0;

  // Avoid implicit padding so `std::has_unique_object_representations_v<>` will
  // hold.
  uint32_t unused_padding = 0;
};

struct SimpleFileSparseRangeHeader {
  SimpleFileSparseRangeHeader();

  uint64_t sparse_range_magic_number = 0;
  uint64_t offset = 0;

  // `length` must be size-fixed to avoid padding, so using uint64_t instead of
  // size_t.
  uint64_t length = 0;
  uint32_t data_crc32 = 0;

  // Avoid implicit padding so `std::has_unique_object_representations_v<>` will
  // hold.
  uint32_t unused_padding = 0;
};

}  // namespace disk_cache

#endif  // NET_DISK_CACHE_SIMPLE_SIMPLE_ENTRY_FORMAT_H_