Introduce the NotificationDatabase class.

content/browser/notifications/notification_database.h

+// Copyright 2015 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 CONTENT_BROWSER_NOTIFICATIONS_NOTIFICATION_DATABASE_H_
+#define CONTENT_BROWSER_NOTIFICATIONS_NOTIFICATION_DATABASE_H_
+
+#include "base/callback.h"
+#include "base/files/file_path.h"
+#include "base/sequence_checker.h"
+#include "content/common/content_export.h"
+
+namespace leveldb {
+class DB;
+class Env;
+}
+
+namespace content {
+
+// Implementation of the persistent notification database.
+//
+// This class can be constructed on any thread, but method calls must only be
+// made on a thread or sequenced task runner that allows file I/O. The same
+// thread or task runner must be used for all method calls.
+class CONTENT_EXPORT NotificationDatabase {
+ public:
+  // Result status codes for interations with the database. Will be used for
+  // UMA, so the assigned ids must remain stable.
+  enum Status {
+    STATUS_OK = 0,
+
+    // The key associated with the operation could not be found.

[Bernhard Bauer, 2015/03/11 18:20:01]

This error code is also used if the file is not found on disk, right?

[Peter Beverloo, 2015/03/11 19:32:25]

Done.

+    STATUS_ERROR_NOT_FOUND = 1,
+
+    // General failure code. More specific failures should be used if available.
+    STATUS_ERROR_FAILED = 2,

[cmumford, 2015/03/11 18:24:33]

I would suggest adding STATUS_CORRUPTED as well. leveldb has a concept of this,
and the only solution is to either call DeleteDB or RepairDB. You will also have
other (more frequent) failures (file busy, insufficient disk space, etc.) and
you'll want to handle these differently.

[Peter Beverloo, 2015/03/11 19:32:25]

Done.

+  };
+
+  explicit NotificationDatabase(const base::FilePath& path);
+  ~NotificationDatabase();
+
+  // Opens the database. If |path| is non-empty, it will be created on the given
+  // directory on the filesystem. If |path| is empty, the database will be
+  // created in memory instead, and its lifetime will be tied to this instance.
+  // |create_if_missing| determines whether to create the database if necessary.
+  Status Open(bool create_if_missing);
+
+  // Completely destroys the contents of this database.
+  Status Destroy();
+
+ private:
+  friend class NotificationDatabaseTest;
+
+  // TODO(peter): Convert to an enum class when DCHECK_EQ supports this.
+  // See https://crbug.com/463869.
+  enum State {
+    STATE_UNINITIALIZED,
+    STATE_INITIALIZED,
+    STATE_DISABLED,
+  };
+
+  // Returns whether the database has been opened.
+  bool IsOpen() const { return db_ != nullptr; }
+
+  // Returns whether the database should only exist in memory.
+  bool IsInMemoryDatabase() const { return path_.empty(); }
+
+  base::FilePath path_;
+  scoped_ptr<leveldb::Env> env_;
+  scoped_ptr<leveldb::DB> db_;
+
+  State state_;
+
+  base::SequenceChecker sequence_checker_;
+
+  DISALLOW_COPY_AND_ASSIGN(NotificationDatabase);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_NOTIFICATIONS_NOTIFICATION_DATABASE_H_