std::unique_ptr<WritableFile>* result) override;
  Status NewReadOnlyMemoryRegionFromFile(
      const std::string& fname, TransactionToken* token,
      std::unique_ptr<ReadOnlyMemoryRegion>* result) override;
  Status FileExists(const std::string& fname, TransactionToken* token) override;
  bool FilesExist(const std::vector<std::string>& files,
                  TransactionToken* token,

  std::unique_ptr<ReadOnlyMemoryRegion> region;
  Status status = env_->NewReadOnlyMemoryRegionFromFile(filepath, &region);
  EXPECT_PRED2(UnimplementedOrReturnsCode, status, Code::NOT_FOUND);
}

TEST_P(ModularFileSystemTest, TestCreateMemoryRegionNonExisting) {
  const std::string filepath = GetURIForPath("dir_not_found/a_file");
  std::unique_ptr<ReadOnlyMemoryRegion> region;

  tf_writable_file::Close(file, status_);
  ASSERT_TF_OK(status_);
  tf_writable_file::Cleanup(file);
  delete file;
}

TEST_F(GCSFilesystemTest, ReadOnlyMemoryRegion) {
  tf_gcs_filesystem::Init(filesystem_, status_);
  ASSERT_TF_OK(status_) << "Could not initialize filesystem. "
                        << TF_Message(status_);
  std::string path = GetURIForPath("a_file");

  return StatusFromTF_Status(plugin_status.get());
}

Status ModularFileSystem::NewReadOnlyMemoryRegionFromFile(
    const std::string& fname, TransactionToken* token,
    std::unique_ptr<ReadOnlyMemoryRegion>* result) {
  if (ops_->new_read_only_memory_region_from_file == nullptr)
    return errors::Unimplemented(tensorflow::strings::StrCat(
        "Filesystem for ", fname,

/// operation pointers may change.
///
/// There are 4 function tables: one for each of the 3 file objects in
/// TensorFlow (i.e., `RandomAccessFile`, `WritableFile`,
/// `ReadOnlyMemoryRegion`) and one for all the operations a `Filesystem`
/// implements. Each of them is in a 1-to-1 correspondence with the wrapper
/// structures from the first section: these tables only contain function