components/language_detection/core/background_file.h

// Copyright 2024 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef COMPONENTS_LANGUAGE_DETECTION_CORE_BACKGROUND_FILE_H_
#define COMPONENTS_LANGUAGE_DETECTION_CORE_BACKGROUND_FILE_H_

#include "base/component_export.h"
#include "base/files/file.h"
#include "base/functional/callback_forward.h"
#include "base/task/sequenced_task_runner.h"

namespace language_detection {

// Holds a `base::File`, and provides and API that pushes the user to perform
// opens and closes on a background task runner.
class COMPONENT_EXPORT(LANGUAGE_DETECTION) BackgroundFile {
 public:
  using ReplacedCallback = base::OnceCallback<void()>;
  using FileOpener = base::OnceCallback<base::File()>;
  explicit BackgroundFile(
      scoped_refptr<base::SequencedTaskRunner> background_task_runner);
  ~BackgroundFile();

  // Replaces the current file with the invalid `base::File()`. It closes the
  // previous file on the background task runner.
  void InvalidateFile();
  // Replaces the current file with the return value of running `file_opener` on
  // the background thread then calls `callback` on the default task runner. It
  // closes the previous file on the background task runner.
  void ReplaceFile(FileOpener file_opener, ReplacedCallback replaced_callback);
  base::File& GetFile() { return file_.GetFile(); }

 private:
  // A wrapper of `base::File` that always closes the file on the provided
  // task runner. See https://crbug.com/366698727.
  class TaskRunnerBoundFile {
   public:
    explicit TaskRunnerBoundFile(
        base::File file,
        scoped_refptr<base::SequencedTaskRunner> background_task_runner);
    ~TaskRunnerBoundFile();
    TaskRunnerBoundFile(TaskRunnerBoundFile&& other);

    // Runs the `file_opener` callback and created a `TaskRunnerBoundFile` from
    // the result.
    static TaskRunnerBoundFile Create(
        BackgroundFile::FileOpener file_opener,
        scoped_refptr<base::SequencedTaskRunner> background_task_runner);

    base::File& GetFile() { return file_; }

    TaskRunnerBoundFile& operator=(TaskRunnerBoundFile&& other);

   private:
    void Invalidate();
    base::File file_;
    scoped_refptr<base::SequencedTaskRunner> background_task_runner_;
  };

  void SwapFile(ReplacedCallback callback, TaskRunnerBoundFile new_file);

  TaskRunnerBoundFile file_;

  // Used for blocking IO calls.
  scoped_refptr<base::SequencedTaskRunner> background_task_runner_;

  // All calls that mutate state in the object should be on the same sequence
  // that created it.
  SEQUENCE_CHECKER(sequence_checker_);

  base::WeakPtrFactory<BackgroundFile> weak_ptr_factory_{this};
};

}  //  namespace language_detection

#endif  // COMPONENTS_LANGUAGE_DETECTION_CORE_BACKGROUND_FILE_H_