// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_FILES_IMPORTANT_FILE_WRITER_H_
#define BASE_FILES_IMPORTANT_FILE_WRITER_H_
#include <string>
#include "base/base_export.h"
#include "base/basictypes.h"
#include "base/files/file_path.h"
#include "base/memory/ref_counted.h"
#include "base/threading/non_thread_safe.h"
#include "base/time/time.h"
#include "base/timer/timer.h"
namespace base {
class SequencedTaskRunner;
class Thread;
// Helper to ensure that a file won't be corrupted by the write (for example on
// application crash). Consider a naive way to save an important file F:
//
// 1. Open F for writing, truncating it.
// 2. Write new data to F.
//
// It's good when it works, but it gets very bad if step 2. doesn't complete.
// It can be caused by a crash, a computer hang, or a weird I/O error. And you
// end up with a broken file.
//
// To be safe, we don't start with writing directly to F. Instead, we write to
// to a temporary file. Only after that write is successful, we rename the
// temporary file to target filename.
//
// If you want to know more about this approach and ext3/ext4 fsync issues, see
// http://valhenson.livejournal.com/37921.html
class BASE_EXPORT ImportantFileWriter : public NonThreadSafe {
public:
// Used by ScheduleSave to lazily provide the data to be saved. Allows us
// to also batch data serializations.
class BASE_EXPORT DataSerializer {
public:
// Should put serialized string in |data| and return true on successful
// serialization. Will be called on the same thread on which
// ImportantFileWriter has been created.
virtual bool SerializeData(std::string* data) = 0;
protected:
virtual ~DataSerializer() {}
};
// Save |data| to |path| in an atomic manner (see the class comment above).
// Blocks and writes data on the current thread.
static bool WriteFileAtomically(const FilePath& path,
const std::string& data);
// Initialize the writer.
// |path| is the name of file to write.
// |task_runner| is the SequencedTaskRunner instance where on which we will
// execute file I/O operations.
// All non-const methods, ctor and dtor must be called on the same thread.
ImportantFileWriter(const FilePath& path,
base::SequencedTaskRunner* task_runner);
// You have to ensure that there are no pending writes at the moment
// of destruction.
~ImportantFileWriter();
const FilePath& path() const { return path_; }
// Returns true if there is a scheduled write pending which has not yet
// been started.
bool HasPendingWrite() const;
// Save |data| to target filename. Does not block. If there is a pending write
// scheduled by ScheduleWrite, it is cancelled.
void WriteNow(const std::string& data);
// Schedule a save to target filename. Data will be serialized and saved
// to disk after the commit interval. If another ScheduleWrite is issued
// before that, only one serialization and write to disk will happen, and
// the most recent |serializer| will be used. This operation does not block.
// |serializer| should remain valid through the lifetime of
// ImportantFileWriter.
void ScheduleWrite(DataSerializer* serializer);
// Serialize data pending to be saved and execute write on backend thread.
void DoScheduledWrite();
TimeDelta commit_interval() const {
return commit_interval_;
}
void set_commit_interval(const TimeDelta& interval) {
commit_interval_ = interval;
}
private:
// Path being written to.
const FilePath path_;
// TaskRunner for the thread on which file I/O can be done.
const scoped_refptr<base::SequencedTaskRunner> task_runner_;
// Timer used to schedule commit after ScheduleWrite.
OneShotTimer<ImportantFileWriter> timer_;
// Serializer which will provide the data to be saved.
DataSerializer* serializer_;
// Time delta after which scheduled data will be written to disk.
TimeDelta commit_interval_;
DISALLOW_COPY_AND_ASSIGN(ImportantFileWriter);
};
} // namespace base
#endif // BASE_FILES_IMPORTANT_FILE_WRITER_H_