base/files/important_file_writer.h

   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
  19 namespace base {
  20
  21 class SequencedTaskRunner;
  22 class 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
  40 class 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(const FilePath& path,
  66                       const scoped_refptr<SequencedTaskRunner>& task_runner);
  67
  68   // Same as above, but with a custom commit interval.
  69   ImportantFileWriter(const FilePath& path,
  70                       const scoped_refptr<SequencedTaskRunner>& task_runner,
  71                       TimeDelta interval);
  72
  73   // You have to ensure that there are no pending writes at the moment
  74   // of destruction.
  75   ~ImportantFileWriter();
  76
  77   const FilePath& path() const { return path_; }
  78
  79   // Returns true if there is a scheduled write pending which has not yet
  80   // been started.
  81   bool HasPendingWrite() const;
  82
  83   // Save |data| to target filename. Does not block. If there is a pending write
  84   // scheduled by ScheduleWrite(), it is cancelled.
  85   void WriteNow(scoped_ptr<std::string> data);
  86
  87   // Schedule a save to target filename. Data will be serialized and saved
  88   // to disk after the commit interval. If another ScheduleWrite is issued
  89   // before that, only one serialization and write to disk will happen, and
  90   // the most recent |serializer| will be used. This operation does not block.
  91   // |serializer| should remain valid through the lifetime of
  92   // ImportantFileWriter.
  93   void ScheduleWrite(DataSerializer* serializer);
  94
  95   // Serialize data pending to be saved and execute write on backend thread.
  96   void DoScheduledWrite();
  97
  98   // Registers |on_next_successful_write| to be called once, on the next
  99   // successful write event. Only one callback can be set at once.
 100   void RegisterOnNextSuccessfulWriteCallback(
 101       const Closure& on_next_successful_write);
 102
 103   TimeDelta commit_interval() const {
 104     return commit_interval_;
 105   }
 106
 107  private:
 108   // Helper method for WriteNow().
 109   bool PostWriteTask(const Callback<bool()>& task);
 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   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<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   const 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_