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/files/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
  18 namespace base {
  19
  20 class SequencedTaskRunner;
  21 class Thread;
  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
  39 class BASE_EXPORT ImportantFileWriter : public 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 BASE_EXPORT DataSerializer {
  44    public:
  45     // Should put serialized string in |data| and return true on successful
  46     // serialization. Will be called on the same thread on which
  47     // ImportantFileWriter has been created.
  48     virtual bool SerializeData(std::string* data) = 0;
  49
  50    protected:
  51     virtual ~DataSerializer() {}
  52   };
  53
  54   // Save |data| to |path| in an atomic manner (see the class comment above).
  55   // Blocks and writes data on the current thread.
  56   static bool WriteFileAtomically(const FilePath& path,
  57                                   const std::string& data);
  58
  59   // Initialize the writer.
  60   // |path| is the name of file to write.
  61   // |task_runner| is the SequencedTaskRunner instance where on which we will
  62   // execute file I/O operations.
  63   // All non-const methods, ctor and dtor must be called on the same thread.
  64   ImportantFileWriter(const FilePath& path,
  65                       base::SequencedTaskRunner* task_runner);
  66
  67   // You have to ensure that there are no pending writes at the moment
  68   // of destruction.
  69   ~ImportantFileWriter();
  70
  71   const FilePath& path() const { return path_; }
  72
  73   // Returns true if there is a scheduled write pending which has not yet
  74   // been started.
  75   bool HasPendingWrite() const;
  76
  77   // Save |data| to target filename. Does not block. If there is a pending write
  78   // scheduled by ScheduleWrite, it is cancelled.
  79   void WriteNow(const std::string& data);
  80
  81   // Schedule a save to target filename. Data will be serialized and saved
  82   // to disk after the commit interval. If another ScheduleWrite is issued
  83   // before that, only one serialization and write to disk will happen, and
  84   // the most recent |serializer| will be used. This operation does not block.
  85   // |serializer| should remain valid through the lifetime of
  86   // ImportantFileWriter.
  87   void ScheduleWrite(DataSerializer* serializer);
  88
  89   // Serialize data pending to be saved and execute write on backend thread.
  90   void DoScheduledWrite();
  91
  92   TimeDelta commit_interval() const {
  93     return commit_interval_;
  94   }
  95
  96   void set_commit_interval(const TimeDelta& interval) {
  97     commit_interval_ = interval;
  98   }
  99
 100  private:
 101   // Path being written to.
 102   const FilePath path_;
 103
 104   // TaskRunner for the thread on which file I/O can be done.
 105   const scoped_refptr<base::SequencedTaskRunner> task_runner_;
 106
 107   // Timer used to schedule commit after ScheduleWrite.
 108   OneShotTimer<ImportantFileWriter> timer_;
 109
 110   // Serializer which will provide the data to be saved.
 111   DataSerializer* serializer_;
 112
 113   // Time delta after which scheduled data will be written to disk.
 114   TimeDelta commit_interval_;
 115
 116   DISALLOW_COPY_AND_ASSIGN(ImportantFileWriter);
 117 };
 118
 119 }  // namespace base
 120
 121 #endif  // BASE_FILES_IMPORTANT_FILE_WRITER_H_