ImportantFileWriter

chrome/common/important_file_writer.h

+// Copyright (c) 2009 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 CHROME_COMMON_IMPORTANT_FILE_WRITER_H_
+#define CHROME_COMMON_IMPORTANT_FILE_WRITER_H_
+
+#include <string>
+
+#include "base/basictypes.h"
+#include "base/file_path.h"
+#include "base/non_thread_safe.h"
+#include "base/time.h"
+#include "base/timer.h"
+
+namespace base {
+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 ImportantFileWriter : public NonThreadSafe {
+ public:
+  // Initialize the writer.
+  // |path| is the name of file to write. Disk operations will be executed on
+  // |backend_thread|, or current thread if |backend_thread| is NULL.
+  //
+  // All non-const methods, ctor and dtor must be called on the same thread.
+  ImportantFileWriter(const FilePath& path, const base::Thread* backend_thread);
+
+  // On destruction all pending writes are executed on |backend_thread|.
+  ~ImportantFileWriter();
+
+  FilePath path() const { return path_; }
+
+  // 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 saving |data| to target filename. Data will be saved to disk after
+  // the commit interval. If an other ScheduleWrite is issued before that, only
+  // one write to disk will happen - with the more recent data. This operation
+  // does not block.
+  void ScheduleWrite(const std::string& data);
+
+  base::TimeDelta commit_interval() const {
+    return commit_interval_;
+  }
+
+  void set_commit_interval(const base::TimeDelta& interval) {
+    commit_interval_ = interval;
+  }
+
+ private:
+  // If there is a data scheduled to write, issue a disk operation.
+  void CommitPendingData();
+
+  // Path being written to.
+  const FilePath path_;
+
+  // Thread on which disk operation run. NULL means no separate thread is used.
+  const base::Thread* backend_thread_;
+
+  // Timer used to schedule commit after ScheduleWrite.
+  base::OneShotTimer<ImportantFileWriter> timer_;
+
+  // Data scheduled to save.
+  std::string data_;
+
+  // Time delta after which scheduled data will be written to disk.
+  base::TimeDelta commit_interval_;
+
+  DISALLOW_COPY_AND_ASSIGN(ImportantFileWriter);
+};
+
+#endif  // CHROME_COMMON_IMPORTANT_FILE_WRITER_H_