Move ImportantFileWriter to base/. It now has two consumers, base/prefs and chrome/.

TBR=ben@chromium.org
BUG=155525


Review URL: https://chromiumcodereview.appspot.com/11227026

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@163840 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 116 insertions, 0 deletions

diff --git a/base/files/important_file_writer.h b/base/files/important_file_writer.h
new file mode 100644
index 0000000..4a6794c
--- /dev/null
+++ b/base/files/important_file_writer.h
@@ -0,0 +1,116 @@
+// 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/file_path.h"
+#include "base/memory/ref_counted.h"
+#include "base/threading/non_thread_safe.h"
+#include "base/time.h"
+#include "base/timer.h"
+
+namespace base {
+
+class MessageLoopProxy;
+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 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() {}
+  };
+
+  // Initialize the writer.
+  // |path| is the name of file to write.
+  // |file_message_loop_proxy| is the MessageLoopProxy for a thread on which
+  // file I/O can be done.
+  // All non-const methods, ctor and dtor must be called on the same thread.
+  ImportantFileWriter(const FilePath& path,
+                      MessageLoopProxy* file_message_loop_proxy);
+
+  // 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_;
+
+  // MessageLoopProxy for the thread on which file I/O can be done.
+  scoped_refptr<MessageLoopProxy> file_message_loop_proxy_;
+
+  // 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_