1// Copyright (c) 2012 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 BASE_FILES_IMPORTANT_FILE_WRITER_H_
6#define BASE_FILES_IMPORTANT_FILE_WRITER_H_
7
8#include <string>
9
10#include "base/base_export.h"
11#include "base/basictypes.h"
12#include "base/callback.h"
13#include "base/files/file_path.h"
14#include "base/memory/ref_counted.h"
15#include "base/threading/non_thread_safe.h"
16#include "base/time/time.h"
17#include "base/timer/timer.h"
18
19namespace base {
20
21class SequencedTaskRunner;
22class Thread;
23
24// Helper to ensure that a file won't be corrupted by the write (for example on
25// application crash). Consider a naive way to save an important file F:
26//
27// 1. Open F for writing, truncating it.
28// 2. Write new data to F.
29//
30// It's good when it works, but it gets very bad if step 2. doesn't complete.
31// It can be caused by a crash, a computer hang, or a weird I/O error. And you
32// end up with a broken file.
33//
34// To be safe, we don't start with writing directly to F. Instead, we write to
35// to a temporary file. Only after that write is successful, we rename the
36// temporary file to target filename.
37//
38// If you want to know more about this approach and ext3/ext4 fsync issues, see
39// http://valhenson.livejournal.com/37921.html
40class BASE_EXPORT ImportantFileWriter : public NonThreadSafe {
41 public:
42  // Used by ScheduleSave to lazily provide the data to be saved. Allows us
43  // to also batch data serializations.
44  class BASE_EXPORT DataSerializer {
45   public:
46    // Should put serialized string in |data| and return true on successful
47    // serialization. Will be called on the same thread on which
48    // ImportantFileWriter has been created.
49    virtual bool SerializeData(std::string* data) = 0;
50
51   protected:
52    virtual ~DataSerializer() {}
53  };
54
55  // Save |data| to |path| in an atomic manner (see the class comment above).
56  // Blocks and writes data on the current thread.
57  static bool WriteFileAtomically(const FilePath& path,
58                                  const std::string& data);
59
60  // Initialize the writer.
61  // |path| is the name of file to write.
62  // |task_runner| is the SequencedTaskRunner instance where on which we will
63  // execute file I/O operations.
64  // All non-const methods, ctor and dtor must be called on the same thread.
65  ImportantFileWriter(
66      const FilePath& path,
67      const scoped_refptr<base::SequencedTaskRunner>& task_runner);
68
69  // You have to ensure that there are no pending writes at the moment
70  // of destruction.
71  ~ImportantFileWriter();
72
73  const FilePath& path() const { return path_; }
74
75  // Returns true if there is a scheduled write pending which has not yet
76  // been started.
77  bool HasPendingWrite() const;
78
79  // Save |data| to target filename. Does not block. If there is a pending write
80  // scheduled by ScheduleWrite, it is cancelled.
81  void WriteNow(const std::string& data);
82
83  // Schedule a save to target filename. Data will be serialized and saved
84  // to disk after the commit interval. If another ScheduleWrite is issued
85  // before that, only one serialization and write to disk will happen, and
86  // the most recent |serializer| will be used. This operation does not block.
87  // |serializer| should remain valid through the lifetime of
88  // ImportantFileWriter.
89  void ScheduleWrite(DataSerializer* serializer);
90
91  // Serialize data pending to be saved and execute write on backend thread.
92  void DoScheduledWrite();
93
94  // Registers |on_next_successful_write| to be called once, on the next
95  // successful write event. Only one callback can be set at once.
96  void RegisterOnNextSuccessfulWriteCallback(
97      const base::Closure& on_next_successful_write);
98
99  TimeDelta commit_interval() const {
100    return commit_interval_;
101  }
102
103  void set_commit_interval(const TimeDelta& interval) {
104    commit_interval_ = interval;
105  }
106
107 private:
108  // Helper method for WriteNow().
109  bool PostWriteTask(const std::string& data);
110
111  // If |result| is true and |on_next_successful_write_| is set, invokes
112  // |on_successful_write_| and then resets it; no-ops otherwise.
113  void ForwardSuccessfulWrite(bool result);
114
115  // Invoked once and then reset on the next successful write event.
116  base::Closure on_next_successful_write_;
117
118  // Path being written to.
119  const FilePath path_;
120
121  // TaskRunner for the thread on which file I/O can be done.
122  const scoped_refptr<base::SequencedTaskRunner> task_runner_;
123
124  // Timer used to schedule commit after ScheduleWrite.
125  OneShotTimer<ImportantFileWriter> timer_;
126
127  // Serializer which will provide the data to be saved.
128  DataSerializer* serializer_;
129
130  // Time delta after which scheduled data will be written to disk.
131  TimeDelta commit_interval_;
132
133  WeakPtrFactory<ImportantFileWriter> weak_factory_;
134
135  DISALLOW_COPY_AND_ASSIGN(ImportantFileWriter);
136};
137
138}  // namespace base
139
140#endif  // BASE_FILES_IMPORTANT_FILE_WRITER_H_
141