[sql] Consider fresh databases suitable for memory-mapped I/O.

sql/meta_table.h

 // 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 SQL_META_TABLE_H_
 #define SQL_META_TABLE_H_
 
 #include <stdint.h>
 #include <string>
 
 #include "base/macros.h"
 #include "sql/sql_export.h"
 
 namespace sql {
 
 class Connection;
 class Statement;
 
 class SQL_EXPORT MetaTable {
  public:
   MetaTable();
   ~MetaTable();
 
+  // Values for Get/SetMmapStatus().  |kMmapFailure| indicates that there was at
+  // some point a read error and the database should not be memory-mapped, while
+  // |kMmapSuccess| indicates that the entire file was read at some point and
+  // can be memory-mapped without constraint.
+  static int64_t kMmapFailure;
+  static int64_t kMmapSuccess;
+
   // Returns true if the 'meta' table exists.
   static bool DoesTableExist(Connection* db);
 
   // If the current version of the database is less than or equal to
   // |deprecated_version|, raze the database.  Must be called outside
   // of a transaction.
   // TODO(shess): At this time the database is razed IFF meta exists
   // and contains a version row with value <= deprecated_version.  It
   // may make sense to also raze if meta exists but has no version
   // row, or if meta doesn't exist.  In those cases if the database is
   // not already empty, it probably resulted from a broken
   // initialization.
   // TODO(shess): Folding this into Init() would allow enforcing
   // |deprecated_version|<|version|.  But Init() is often called in a
   // transaction.
   static void RazeIfDeprecated(Connection* db, int deprecated_version);
 
+  // Used to tuck some data into the meta table about mmap status.  The value
+  // represents how much data in bytes has successfully been read from the
+  // database, or |kMmapFailure| or |kMmapSuccess|.
+  static bool GetMmapStatus(Connection* db, int64_t* status);
+  static bool SetMmapStatus(Connection* db, int64_t status);
+
   // Initializes the MetaTableHelper, creating the meta table if necessary. For
   // new tables, it will initialize the version number to |version| and the
   // compatible version number to |compatible_version|.  Versions must be
   // greater than 0 to distinguish missing versions (see GetVersionNumber()).
+  // If there was no meta table (proxy for a fresh database), mmap status is set
+  // to |kMmapSuccess|.
   bool Init(Connection* db, int version, int compatible_version);
 
   // Resets this MetaTable object, making another call to Init() possible.
   void Reset();
 
   // The version number of the database. This should be the version number of
   // the creator of the file. The version number will be 0 if there is no
   // previously set version number.
   //
   // See also Get/SetCompatibleVersionNumber().
@@ skipping 38 matching lines @@
   bool PrepareGetStatement(Statement* statement, const char* key);
 
   Connection* db_;
 
   DISALLOW_COPY_AND_ASSIGN(MetaTable);
 };
 
 }  // namespace sql
 
 #endif  // SQL_META_TABLE_H_