1// Copyright (c) 2011 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5#ifndef CHROME_COMMON_IMPORTANT_FILE_WRITER_H_
6#define CHROME_COMMON_IMPORTANT_FILE_WRITER_H_
7#pragma once
8
9#include <string>
10
11#include "base/basictypes.h"
12#include "base/file_path.h"
13#include "base/memory/ref_counted.h"
14#include "base/threading/non_thread_safe.h"
15#include "base/time.h"
16#include "base/timer.h"
17
18namespace base {
19class MessageLoopProxy;
20class Thread;
21}
22
23// Helper to ensure that a file won't be corrupted by the write (for example on
24// application crash). Consider a naive way to save an important file F:
25//
26// 1. Open F for writing, truncating it.
27// 2. Write new data to F.
28//
29// It's good when it works, but it gets very bad if step 2. doesn't complete.
30// It can be caused by a crash, a computer hang, or a weird I/O error. And you
31// end up with a broken file.
32//
33// To be safe, we don't start with writing directly to F. Instead, we write to
34// to a temporary file. Only after that write is successful, we rename the
35// temporary file to target filename.
36//
37// If you want to know more about this approach and ext3/ext4 fsync issues, see
38// http://valhenson.livejournal.com/37921.html
39class ImportantFileWriter : public base::NonThreadSafe {
40 public:
41  // Used by ScheduleSave to lazily provide the data to be saved. Allows us
42  // to also batch data serializations.
43  class DataSerializer {
44   public:
45    virtual ~DataSerializer() {}
46
47    // Should put serialized string in |data| and return true on successful
48    // serialization. Will be called on the same thread on which
49    // ImportantFileWriter has been created.
50    virtual bool SerializeData(std::string* data) = 0;
51  };
52
53  // Initialize the writer.
54  // |path| is the name of file to write.
55  // |file_message_loop_proxy| is the MessageLoopProxy for a thread on which
56  // file I/O can be done.
57  // All non-const methods, ctor and dtor must be called on the same thread.
58  ImportantFileWriter(const FilePath& path,
59                      base::MessageLoopProxy* file_message_loop_proxy);
60
61  // You have to ensure that there are no pending writes at the moment
62  // of destruction.
63  ~ImportantFileWriter();
64
65  const FilePath& path() const { return path_; }
66
67  // Returns true if there is a scheduled write pending which has not yet
68  // been started.
69  bool HasPendingWrite() const;
70
71  // Save |data| to target filename. Does not block. If there is a pending write
72  // scheduled by ScheduleWrite, it is cancelled.
73  void WriteNow(const std::string& data);
74
75  // Schedule a save to target filename. Data will be serialized and saved
76  // to disk after the commit interval. If another ScheduleWrite is issued
77  // before that, only one serialization and write to disk will happen, and
78  // the most recent |serializer| will be used. This operation does not block.
79  // |serializer| should remain valid through the lifetime of
80  // ImportantFileWriter.
81  void ScheduleWrite(DataSerializer* serializer);
82
83  // Serialize data pending to be saved and execute write on backend thread.
84  void DoScheduledWrite();
85
86  base::TimeDelta commit_interval() const {
87    return commit_interval_;
88  }
89
90  void set_commit_interval(const base::TimeDelta& interval) {
91    commit_interval_ = interval;
92  }
93
94 private:
95  // Path being written to.
96  const FilePath path_;
97
98  // MessageLoopProxy for the thread on which file I/O can be done.
99  scoped_refptr<base::MessageLoopProxy> file_message_loop_proxy_;
100
101  // Timer used to schedule commit after ScheduleWrite.
102  base::OneShotTimer<ImportantFileWriter> timer_;
103
104  // Serializer which will provide the data to be saved.
105  DataSerializer* serializer_;
106
107  // Time delta after which scheduled data will be written to disk.
108  base::TimeDelta commit_interval_;
109
110  DISALLOW_COPY_AND_ASSIGN(ImportantFileWriter);
111};
112
113#endif  // CHROME_COMMON_IMPORTANT_FILE_WRITER_H_
114