Scoped recovery module for sql/

sql/recovery.h

+// Copyright (c) 2013 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 SQL_RECOVERY_H_
+#define SQL_RECOVERY_H_
+
+#include "base/basictypes.h"
+
+#include "sql/connection.h"
+#include "sql/scoped_attach.h"
+
+namespace base {
+class FilePath;
+}
+
+namespace sql {
+
+// Recovery module for sql/.  The basic idea is that a fresh database
+// is created, which is populated with the recovered contents of the
+// original database, and if recovery is successful the recovered
+// database is backed up over the original database.  If recovery is
+// not successful, the original database is razed.
+//
+// Usage is something like:
+// {
+//   sql::Recovery recover(orig_db);
+//   if (recover.Open(orig_db_path)) {
+//     if (recover.db()->Execute(kCreateSchemaSql) &&
+//         recover.db()->Execute(kCopyDataFromOrigSql)) {
+//       recover.Recovered();
+//     }
+//   }
+// }
+//
+// If Recovered() does not return successfully, then RazeAndClose() is
+// called on orig_db.
+
+
+// TODO(shess): Move the scoper to a separate file.
+class SQL_EXPORT Recovery {

[erikwright (departed), 2013/07/05 19:02:30]

An idiom I've been using from time to time is like this:

class Recovery {
 public:
  static scoped_ptr<Recovery> Begin(db, path);
  sql::Connection* db() { return db_; }
  static bool Recovered(scoped_ptr<Recovery>);
  static void Unrecoverable(scoped_ptr<Recovery>);
 private:
  Recovery(...);
};

In plain English, you can only construct one on the heap, and the
Recoverable/Unrecoverable transitions explicitly require you to give up your
instance (which makes sense, since the instance methods are no longer valid at
that point).

[Scott Hess - ex-Googler, 2013/07/08 21:41:51]

I'll prototype that and see how it looks.  My main goal is to make it easier to
get right than wrong, but realistically code like this can't really enforce
strong assertions - you can call the API in the right order but still do
everything completely wrong in between and end up with garbage results.

+ public:
+  // TODO(shess): Allow specifying the attachment point?
+  explicit Recovery(Connection* connection);
+  ~Recovery();
+
+  // TODO(shess): Later versions of SQLite allow querying for the
+  // existing db's path.
+  bool Open(const base::FilePath& db_path) WARN_UNUSED_RESULT;
+
+  // Mark recovery completed by replicating the recovery database over
+  // the original database, then closing the recovery database.  The
+  // original database is closed and the handle poisoned, causing
+  // future calls against it to fail.
+  //
+  // If this is not called before the object goes out of scope, the
+  // destructor will call Unrecoverable().
+  // TODO(shess): What does it mean for this to fail?  Should there be
+  // an option for "Bail out without razing the original database"?
+  bool Recovered() WARN_UNUSED_RESULT;
+
+  // Indicate that the database is unrecoverable.  The original
+  // database is razed, and the handle is closed and poisoned.  The
+  // recovery database is also closed.
+  void Unrecoverable();
+
+  // Handle to the temporary recovery database.
+  sql::Connection* db() { return &recover_db_; }
+
+ private:
+  Connection* db_;
+  Connection recover_db_;
+  ScopedAttach attach_;
+
+  DISALLOW_COPY_AND_ASSIGN(Recovery);
+};
+
+}  // namespace sql
+
+#endif  // SQL_RECOVERY_H_