xref: /aosp_15_r20/external/icing/icing/file/file-backed-proto-log.h (revision 8b6cd535a057e39b3b86660c4aa06c99747c2136)

// Copyright (C) 2019 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// File-backed log of protos with append-only writes and position based reads.
//
// The implementation in this file is deprecated and replaced by
// portable-file-backed-proto-log.h.
//
// This deprecated implementation has been made read-only for the purposes of
// migration; writing and erasing this format of log is no longer supported and
// the methods to accomplish this have been removed.
//
// The details of this format follow below:
// Each proto written to the file will have a metadata written just before it.
// The metadata consists of
//   {
//     1 bytes of kProtoMagic;
//     3 bytes of the proto size
//     n bytes of the proto itself
//   }
// TODO(b/136514769): Add versioning to the header and a UpgradeToVersion
// migration method.
#ifndef ICING_FILE_FILE_BACKED_PROTO_LOG_H_
#define ICING_FILE_FILE_BACKED_PROTO_LOG_H_

#include <cstdint>
#include <memory>
#include <string>
#include <string_view>

#include "icing/text_classifier/lib3/utils/base/statusor.h"
#include "icing/absl_ports/canonical_errors.h"
#include "icing/absl_ports/str_cat.h"
#include "icing/file/constants.h"
#include "icing/file/filesystem.h"
#include "icing/file/memory-mapped-file.h"
#include "icing/legacy/core/icing-string-util.h"
#include "icing/portable/gzip_stream.h"
#include "icing/portable/platform.h"
#include "icing/portable/zlib.h"
#include "icing/util/crc32.h"
#include "icing/util/data-loss.h"
#include "icing/util/logging.h"
#include "icing/util/status-macros.h"
#include <google/protobuf/io/zero_copy_stream_impl_lite.h>

namespace icing {
namespace lib {

template <typename ProtoT>
class FileBackedProtoLog {
 public:
  struct Options {
    // Whether to compress each proto before writing to the proto log.
    bool compress;

    // Byte-size limit for each proto written to the store. This does not
    // include the bytes needed for the metadata of each proto.
    //
    // NOTE: Currently, we only support protos up to 16MiB. We store the proto
    // size in 3 bytes within the metadata.
    //
    // NOTE: This limit is only enforced for future writes. If the store
    // previously had a higher limit, then reading older entries could return
    // larger protos.
    //
    // NOTE: The max_proto_size is the upper limit for input protos into the
    // ProtoLog. Even if the proto is larger than max_proto_size, but compresses
    // to a smaller size, ProtoLog will not accept it. Protos that result in a
    // compressed size larger than max_proto_size are also not accepted.
    const int32_t max_proto_size;

    // Must specify values for options.
    Options() = delete;
    explicit Options(bool compress_in,
                     const int32_t max_proto_size_in = constants::kMaxProtoSize)
        : compress(compress_in), max_proto_size(max_proto_size_in) {}
  };

  // Header stored at the beginning of the file before the rest of the log
  // contents. Stores metadata on the log.
  struct Header {
    static constexpr int32_t kMagic = 0xf4c6f67a;

    // Holds the magic as a quick sanity check against file corruption.
    int32_t magic = kMagic;

    // Whether to compress the protos before writing to the log.
    bool compress = true;

    // The maximum proto size that can be written to the log.
    int32_t max_proto_size = 0;

    // Checksum of the log elements, doesn't include the header fields.
    uint32_t log_checksum = 0;

    // Last known good offset at which the log and its checksum were updated.
    // If we crash between writing to the log and updating the checksum, we can
    // try to rewind the log to this offset and verify the checksum is still
    // valid instead of throwing away the entire log.
    int64_t rewind_offset = sizeof(Header);

    // Must be at the end. Contains the crc checksum of the preceding fields.
    uint32_t header_checksum = 0;

    uint32_t CalculateHeaderChecksum() const {
      Crc32 crc;
      std::string_view header_str(reinterpret_cast<const char*>(this),
                                  offsetof(Header, header_checksum));
      crc.Append(header_str);
      return crc.Get();
    }
  };

  struct CreateResult {
    // A successfully initialized log.
    std::unique_ptr<FileBackedProtoLog<ProtoT>> proto_log;

    // The data status after initializing from a previous state. Data loss can
    // happen if the file is corrupted or some previously added data was
    // unpersisted. This may be used to signal that any derived data off of the
    // proto log may need to be regenerated.
    DataLoss data_loss;

    bool has_data_loss() {
      return data_loss == DataLoss::PARTIAL || data_loss == DataLoss::COMPLETE;
    }
  };

  // Factory method to create, initialize, and return a FileBackedProtoLog. Will
  // create the file if it doesn't exist.
  //
  // If on re-initialization the log detects disk corruption or some previously
  // added data was unpersisted, the log will rewind to the last-good state. The
  // log saves these checkpointed "good" states when PersistToDisk() is called
  // or the log is safely destructed. If the log rewinds successfully to the
  // last-good state, then the returned CreateResult.data_loss indicates
  // whether it has a data loss and what kind of data loss it is (partial or
  // complete) so that any derived data may know that it needs to be updated. If
  // the log re-initializes successfully without any data loss,
  // CreateResult.data_loss will be NONE.
  //
  // Params:
  //   filesystem: Handles system level calls
  //   file_path: Path of the underlying file. Directory of the file should
  //   already exist
  //   options: Configuration options for the proto log
  //
  // Returns:
  //   FileBackedProtoLog::CreateResult on success
  //   INVALID_ARGUMENT on an invalid option
  //   INTERNAL_ERROR on IO error
  static libtextclassifier3::StatusOr<CreateResult> Create(
      const Filesystem* filesystem, const std::string& file_path,
      const Options& options);

  // Not copyable
  FileBackedProtoLog(const FileBackedProtoLog&) = delete;
  FileBackedProtoLog& operator=(const FileBackedProtoLog&) = delete;

  // Reads out a proto located at file_offset from the file.
  //
  // Returns:
  //   A proto on success
  //   NOT_FOUND if the proto at the given offset has been erased
  //   OUT_OF_RANGE_ERROR if file_offset exceeds file size
  //   INTERNAL_ERROR on IO error
  libtextclassifier3::StatusOr<ProtoT> ReadProto(int64_t file_offset) const;

  // An iterator helping to find offsets of all the protos in file.
  // Example usage:
  //
  // while (iterator.Advance().ok()) {
  //   int64_t offset = iterator.GetOffset();
  //   // Do something
  // }
  class Iterator {
   public:
    explicit Iterator(const Filesystem& filesystem,
                      const std::string& file_path, int64_t initial_offset,
                      MemoryMappedFile&& mmapped_file);

    // Advances to the position of next proto whether it has been erased or not.
    //
    // Returns:
    //   OK on success
    //   OUT_OF_RANGE_ERROR if it reaches the end
    //   INTERNAL_ERROR on IO error
    libtextclassifier3::Status Advance();

    // Returns the file offset of current proto.
    int64_t GetOffset();

   private:
    static constexpr int64_t kInvalidOffset = -1;
    // Used to read proto metadata
    MemoryMappedFile mmapped_file_;
    // Offset of first proto
    int64_t initial_offset_;
    int64_t current_offset_;
    int64_t file_size_;
  };

  // Returns an iterator of current proto log. The caller needs to keep the
  // proto log unchanged while using the iterator, otherwise unexpected
  // behaviors could happen.
  libtextclassifier3::StatusOr<Iterator> GetIterator();

 private:
  // Object can only be instantiated via the ::Create factory.
  FileBackedProtoLog(const Filesystem* filesystem, const std::string& file_path,
                     std::unique_ptr<Header> header);

  // Initializes a new proto log.
  //
  // Returns:
  //   std::unique_ptr<CreateResult> on success
  //   INTERNAL_ERROR on IO error
  static libtextclassifier3::StatusOr<CreateResult> InitializeNewFile(
      const Filesystem* filesystem, const std::string& file_path,
      const Options& options);

  // Verifies that the existing proto log is in a good state. If not in a good
  // state, then the proto log may be truncated to the last good state and
  // content will be lost.
  //
  // Returns:
  //   std::unique_ptr<CreateResult> on success
  //   INTERNAL_ERROR on IO error or internal inconsistencies in the file
  //   INVALID_ARGUMENT_ERROR if options aren't consistent with previous
  //     instances
  static libtextclassifier3::StatusOr<CreateResult> InitializeExistingFile(
      const Filesystem* filesystem, const std::string& file_path,
      const Options& options, int64_t file_size);

  // Takes an initial checksum and updates it with the content between `start`
  // and `end` offsets in the file.
  //
  // Returns:
  //   Crc of the content between `start`, inclusive, and `end`, exclusive.
  //   INTERNAL_ERROR on IO error
  //   INVALID_ARGUMENT_ERROR if start and end aren't within the file size
  static libtextclassifier3::StatusOr<Crc32> ComputeChecksum(
      const Filesystem* filesystem, const std::string& file_path,
      Crc32 initial_crc, int64_t start, int64_t end);

  static bool IsEmptyBuffer(const char* buffer, int size) {
    return std::all_of(buffer, buffer + size,
                       [](const char byte) { return byte == 0; });
  }

  // Helper function to get stored proto size from the metadata.
  // Metadata format: 8 bits magic + 24 bits size
  static int GetProtoSize(int metadata) { return metadata & 0x00FFFFFF; }

  // Helper function to get stored proto magic from the metadata.
  // Metadata format: 8 bits magic + 24 bits size
  static uint8_t GetProtoMagic(int metadata) { return metadata >> 24; }

  // Reads out the metadata of a proto located at file_offset from the file.
  //
  // Returns:
  //   Proto's metadata on success
  //   OUT_OF_RANGE_ERROR if file_offset exceeds file_size
  //   INTERNAL_ERROR if the metadata is invalid or any IO errors happen
  static libtextclassifier3::StatusOr<int> ReadProtoMetadata(
      MemoryMappedFile* mmapped_file, int64_t file_offset, int64_t file_size);

  // Magic number added in front of every proto. Used when reading out protos
  // as a first check for corruption in each entry in the file. Even if there is
  // a corruption, the best we can do is roll back to our last recovery point
  // and throw away un-flushed data. We can discard/reuse this byte if needed so
  // that we have 4 bytes to store the size of protos, and increase the size of
  // protos we support.
  static constexpr uint8_t kProtoMagic = 0x5C;

  // Chunks of the file to mmap at a time, so we don't mmap the entire file.
  // Only used on 32-bit devices
  static constexpr int kMmapChunkSize = 4 * 1024 * 1024;  // 4MiB

  ScopedFd fd_;
  const Filesystem* const filesystem_;
  const std::string file_path_;
  std::unique_ptr<Header> header_;
};

template <typename ProtoT>
FileBackedProtoLog<ProtoT>::FileBackedProtoLog(const Filesystem* filesystem,
                                               const std::string& file_path,
                                               std::unique_ptr<Header> header)
    : filesystem_(filesystem),
      file_path_(file_path),
      header_(std::move(header)) {
  fd_.reset(filesystem_->OpenForAppend(file_path.c_str()));
}

template <typename ProtoT>
libtextclassifier3::StatusOr<typename FileBackedProtoLog<ProtoT>::CreateResult>
FileBackedProtoLog<ProtoT>::Create(const Filesystem* filesystem,
                                   const std::string& file_path,
                                   const Options& options) {
  if (options.max_proto_size <= 0) {
    return absl_ports::InvalidArgumentError(IcingStringUtil::StringPrintf(
        "options.max_proto_size must be greater than 0, was %d",
        options.max_proto_size));
  }

  // Since we store the proto_size in 3 bytes, we can only support protos of up
  // to 16MiB.
  if (options.max_proto_size > constants::kMaxProtoSize) {
    return absl_ports::InvalidArgumentError(IcingStringUtil::StringPrintf(
        "options.max_proto_size must be under 16MiB, was %d",
        options.max_proto_size));
  }

  if (!filesystem->FileExists(file_path.c_str())) {
    return InitializeNewFile(filesystem, file_path, options);
  }

  int64_t file_size = filesystem->GetFileSize(file_path.c_str());
  if (file_size == Filesystem::kBadFileSize) {
    return absl_ports::InternalError(
        absl_ports::StrCat("Bad file size '", file_path, "'"));
  }

  if (file_size == 0) {
    return InitializeNewFile(filesystem, file_path, options);
  }

  return InitializeExistingFile(filesystem, file_path, options, file_size);
}

template <typename ProtoT>
libtextclassifier3::StatusOr<typename FileBackedProtoLog<ProtoT>::CreateResult>
FileBackedProtoLog<ProtoT>::InitializeNewFile(const Filesystem* filesystem,
                                              const std::string& file_path,
                                              const Options& options) {
  // Create the header
  std::unique_ptr<Header> header = std::make_unique<Header>();
  header->compress = options.compress;
  header->max_proto_size = options.max_proto_size;
  header->header_checksum = header->CalculateHeaderChecksum();

  if (!filesystem->Write(file_path.c_str(), header.get(), sizeof(Header))) {
    return absl_ports::InternalError(
        absl_ports::StrCat("Failed to write header for file: ", file_path));
  }

  CreateResult create_result = {
      std::unique_ptr<FileBackedProtoLog<ProtoT>>(
          new FileBackedProtoLog<ProtoT>(filesystem, file_path,
                                         std::move(header))),
      /*data_loss=*/DataLoss::NONE};

  return create_result;
}

template <typename ProtoT>
libtextclassifier3::StatusOr<typename FileBackedProtoLog<ProtoT>::CreateResult>
FileBackedProtoLog<ProtoT>::InitializeExistingFile(const Filesystem* filesystem,
                                                   const std::string& file_path,
                                                   const Options& options,
                                                   int64_t file_size) {
  if (file_size < sizeof(Header)) {
    return absl_ports::InternalError(
        absl_ports::StrCat("File header too short for: ", file_path));
  }

  std::unique_ptr<Header> header = std::make_unique<Header>();
  if (!filesystem->PRead(file_path.c_str(), header.get(), sizeof(Header),
                         /*offset=*/0)) {
    return absl_ports::InternalError(
        absl_ports::StrCat("Failed to read header for file: ", file_path));
  }

  // Make sure the header is still valid before we use any of its values. This
  // is covered by the header_checksum check below, but this is a quick check
  // that can save us from an extra crc computation.
  if (header->magic != Header::kMagic) {
    return absl_ports::InternalError(
        absl_ports::StrCat("Invalid header kMagic for file: ", file_path));
  }

  if (header->header_checksum != header->CalculateHeaderChecksum()) {
    return absl_ports::InternalError(
        absl_ports::StrCat("Invalid header checksum for: ", file_path));
  }

  if (header->compress != options.compress) {
    return absl_ports::InvalidArgumentError(IcingStringUtil::StringPrintf(
        "Inconsistent compress option, expected %d, actual %d",
        header->compress, options.compress));
  }

  if (header->max_proto_size > options.max_proto_size) {
    return absl_ports::InvalidArgumentError(IcingStringUtil::StringPrintf(
        "Max proto size cannot be smaller than previous "
        "instantiations, previous size %d, wanted size %d",
        header->max_proto_size, options.max_proto_size));
  }
  header->max_proto_size = options.max_proto_size;

  DataLoss data_loss = DataLoss::NONE;
  ICING_ASSIGN_OR_RETURN(Crc32 calculated_log_checksum,
                         ComputeChecksum(filesystem, file_path, Crc32(),
                                         sizeof(Header), file_size));

  // Double check that the log checksum is the same as the one that was
  // persisted last time. If not, we start recovery logic.
  if (header->log_checksum != calculated_log_checksum.Get()) {
    // Need to rewind the proto log since the checksums don't match.
    // Worst case, we have to rewind the entire log back to just the header
    int64_t last_known_good = sizeof(Header);

    // Calculate the checksum of the log contents just up to the last rewind
    // offset point. This will be valid if we just appended contents to the log
    // without updating the checksum, and we can rewind back to this point
    // safely.
    ICING_ASSIGN_OR_RETURN(
        calculated_log_checksum,
        ComputeChecksum(filesystem, file_path, Crc32(), sizeof(Header),
                        header->rewind_offset));
    if (header->log_checksum == calculated_log_checksum.Get()) {
      // Check if it matches our last rewind state. If so, this becomes our last
      // good state and we can safely truncate and recover from here.
      last_known_good = header->rewind_offset;
      data_loss = DataLoss::PARTIAL;
    } else {
      // Otherwise, we're going to truncate the entire log and this resets the
      // checksum to an empty log state.
      header->log_checksum = 0;
      data_loss = DataLoss::COMPLETE;
    }

    if (!filesystem->Truncate(file_path.c_str(), last_known_good)) {
      return absl_ports::InternalError(
          absl_ports::StrCat("Error truncating file: ", file_path));
    }

    ICING_LOG(WARNING) << "Truncated '" << file_path << "' to size "
                       << last_known_good;
  }

  CreateResult create_result = {
      std::unique_ptr<FileBackedProtoLog<ProtoT>>(
          new FileBackedProtoLog<ProtoT>(filesystem, file_path,
                                         std::move(header))),
      data_loss};

  return create_result;
}

template <typename ProtoT>
libtextclassifier3::StatusOr<Crc32> FileBackedProtoLog<ProtoT>::ComputeChecksum(
    const Filesystem* filesystem, const std::string& file_path,
    Crc32 initial_crc, int64_t start, int64_t end) {
  ICING_ASSIGN_OR_RETURN(
      MemoryMappedFile mmapped_file,
      MemoryMappedFile::Create(*filesystem, file_path,
                               MemoryMappedFile::Strategy::READ_ONLY));
  Crc32 new_crc(initial_crc.Get());

  if (start < 0) {
    return absl_ports::InvalidArgumentError(IcingStringUtil::StringPrintf(
        "Starting checksum offset of file '%s' must be greater than 0, was "
        "%lld",
        file_path.c_str(), static_cast<long long>(start)));
  }

  if (end < start) {
    return absl_ports::InvalidArgumentError(IcingStringUtil::StringPrintf(
        "Ending checksum offset of file '%s' must be greater than start "
        "'%lld', was '%lld'",
        file_path.c_str(), static_cast<long long>(start),
        static_cast<long long>(end)));
  }

  int64_t file_size = filesystem->GetFileSize(file_path.c_str());
  if (end > file_size) {
    return absl_ports::InvalidArgumentError(IcingStringUtil::StringPrintf(
        "Ending checksum offset of file '%s' must be within "
        "file size of %lld, was %lld",
        file_path.c_str(), static_cast<long long>(file_size),
        static_cast<long long>(end)));
  }

  Architecture architecture = GetArchitecture();
  switch (architecture) {
    case Architecture::BIT_64: {
      // Don't mmap in chunks here since mmapping can be harmful on 64-bit
      // devices where mmap/munmap calls need the mmap write semaphore, which
      // blocks mmap/munmap/mprotect and all page faults from executing while
      // they run. On 64-bit devices, this doesn't actually load into memory, it
      // just makes the file faultable. So the whole file should be ok.
      // b/185822878.
      ICING_RETURN_IF_ERROR(mmapped_file.Remap(start, end - start));
      auto mmap_str = std::string_view(mmapped_file.region(), end - start);
      new_crc.Append(mmap_str);
      break;
    }
    case Architecture::BIT_32:
      [[fallthrough]];
    case Architecture::UNKNOWN: {
      // 32-bit devices only have 4GB of RAM. Mmap in chunks to not use up too
      // much memory at once. If we're unknown, then also chunk it because we're
      // not sure what the device can handle.
      for (int i = start; i < end; i += kMmapChunkSize) {
        // Don't read past the file size.
        int next_chunk_size = kMmapChunkSize;
        if ((i + kMmapChunkSize) >= end) {
          next_chunk_size = end - i;
        }

        ICING_RETURN_IF_ERROR(mmapped_file.Remap(i, next_chunk_size));

        auto mmap_str =
            std::string_view(mmapped_file.region(), next_chunk_size);
        new_crc.Append(mmap_str);
      }
      break;
    }
  }

  return new_crc;
}

template <typename ProtoT>
libtextclassifier3::StatusOr<ProtoT> FileBackedProtoLog<ProtoT>::ReadProto(
    int64_t file_offset) const {
  int64_t file_size = filesystem_->GetFileSize(fd_.get());
  ICING_ASSIGN_OR_RETURN(
      MemoryMappedFile mmapped_file,
      MemoryMappedFile::Create(*filesystem_, file_path_,
                               MemoryMappedFile::Strategy::READ_ONLY));
  if (file_offset >= file_size) {
    // file_size points to the next byte to write at, so subtract one to get
    // the inclusive, actual size of file.
    return absl_ports::OutOfRangeError(
        IcingStringUtil::StringPrintf("Trying to read from a location, %lld, "
                                      "out of range of the file size, %lld",
                                      static_cast<long long>(file_offset),
                                      static_cast<long long>(file_size - 1)));
  }

  // Read out the metadata
  ICING_ASSIGN_OR_RETURN(
      int metadata, ReadProtoMetadata(&mmapped_file, file_offset, file_size));

  // Copy out however many bytes it says the proto is
  int stored_size = GetProtoSize(metadata);

  ICING_RETURN_IF_ERROR(
      mmapped_file.Remap(file_offset + sizeof(metadata), stored_size));

  if (IsEmptyBuffer(mmapped_file.region(), mmapped_file.region_size())) {
    return absl_ports::NotFoundError("The proto data has been erased.");
  }

  google::protobuf::io::ArrayInputStream proto_stream(mmapped_file.mutable_region(),
                                            stored_size);

  // Deserialize proto
  ProtoT proto;
  if (header_->compress) {
    protobuf_ports::GzipInputStream decompress_stream(&proto_stream);
    proto.ParseFromZeroCopyStream(&decompress_stream);
  } else {
    proto.ParseFromZeroCopyStream(&proto_stream);
  }

  return proto;
}

template <typename ProtoT>
FileBackedProtoLog<ProtoT>::Iterator::Iterator(const Filesystem& filesystem,
                                               const std::string& file_path,
                                               int64_t initial_offset,
                                               MemoryMappedFile&& mmapped_file)
    : mmapped_file_(std::move(mmapped_file)),
      initial_offset_(initial_offset),
      current_offset_(kInvalidOffset),
      file_size_(filesystem.GetFileSize(file_path.c_str())) {
  if (file_size_ == Filesystem::kBadFileSize) {
    // Fails all Advance() calls
    file_size_ = 0;
  }
}

template <typename ProtoT>
libtextclassifier3::Status FileBackedProtoLog<ProtoT>::Iterator::Advance() {
  if (current_offset_ == kInvalidOffset) {
    // First Advance() call
    current_offset_ = initial_offset_;
  } else {
    // Jumps to the next proto position
    ICING_ASSIGN_OR_RETURN(
        int metadata,
        ReadProtoMetadata(&mmapped_file_, current_offset_, file_size_));
    current_offset_ += sizeof(metadata) + GetProtoSize(metadata);
  }

  if (current_offset_ < file_size_) {
    return libtextclassifier3::Status::OK;
  } else {
    return absl_ports::OutOfRangeError(IcingStringUtil::StringPrintf(
        "The next proto offset, %lld, is out of file range [0, %lld)",
        static_cast<long long>(current_offset_),
        static_cast<long long>(file_size_)));
  }
}

template <typename ProtoT>
int64_t FileBackedProtoLog<ProtoT>::Iterator::GetOffset() {
  return current_offset_;
}

template <typename ProtoT>
libtextclassifier3::StatusOr<typename FileBackedProtoLog<ProtoT>::Iterator>
FileBackedProtoLog<ProtoT>::GetIterator() {
  ICING_ASSIGN_OR_RETURN(
      MemoryMappedFile mmapped_file,
      MemoryMappedFile::Create(*filesystem_, file_path_,
                               MemoryMappedFile::Strategy::READ_ONLY));
  return Iterator(*filesystem_, file_path_,
                  /*initial_offset=*/sizeof(Header), std::move(mmapped_file));
}

template <typename ProtoT>
libtextclassifier3::StatusOr<int> FileBackedProtoLog<ProtoT>::ReadProtoMetadata(
    MemoryMappedFile* mmapped_file, int64_t file_offset, int64_t file_size) {
  // Checks file_offset
  if (file_offset >= file_size) {
    return absl_ports::OutOfRangeError(IcingStringUtil::StringPrintf(
        "offset, %lld, is out of file range [0, %lld)",
        static_cast<long long>(file_offset),
        static_cast<long long>(file_size)));
  }
  int metadata;
  int metadata_size = sizeof(metadata);
  if (file_offset + metadata_size >= file_size) {
    return absl_ports::InternalError(IcingStringUtil::StringPrintf(
        "Wrong metadata offset %lld, metadata doesn't fit in "
        "with file range [0, %lld)",
        static_cast<long long>(file_offset),
        static_cast<long long>(file_size)));
  }
  // Reads metadata
  ICING_RETURN_IF_ERROR(mmapped_file->Remap(file_offset, metadata_size));
  memcpy(&metadata, mmapped_file->region(), metadata_size);
  // Checks magic number
  uint8_t stored_k_proto_magic = GetProtoMagic(metadata);
  if (stored_k_proto_magic != kProtoMagic) {
    return absl_ports::InternalError(IcingStringUtil::StringPrintf(
        "Failed to read kProtoMagic, expected %d, actual %d", kProtoMagic,
        stored_k_proto_magic));
  }
  return metadata;
}

}  // namespace lib
}  // namespace icing

#endif  // ICING_FILE_FILE_BACKED_PROTO_LOG_H_