[sql] Use memory-mapped I/O for sql::Connection.

sql/connection.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_CONNECTION_H_
 #define SQL_CONNECTION_H_
 
 #include <stdint.h>
 #include <map>
 #include <set>
@@ skipping 244 matching lines @@
   // decision.  Preloading should almost always improve later performance
   // numbers for this database simply because it pulls operations forward, but
   // if the data isn't actually used soon then preloading just slows down
   // everything else.
   void Preload();
 
   // Try to trim the cache memory used by the database.  If |aggressively| is
   // true, this function will try to free all of the cache memory it can. If
   // |aggressively| is false, this function will try to cut cache memory
   // usage by half.
+  // NOTE(shess): See also ReleaseCacheMemory().
   void TrimMemory(bool aggressively);
 
   // Raze the database to the ground.  This approximates creating a
   // fresh database from scratch, within the constraints of SQLite's
   // locking protocol (locks and open handles can make doing this with
   // filesystem operations problematic).  Returns true if the database
   // was razed.
   //
   // false is returned if the database is locked by some other
   // process.  RazeWithTimeout() may be used if appropriate.
@@ skipping 356 matching lines @@
   // true, autocommit time if the database is not in a transaction, or update
   // time if the database is in a transaction.  Also records change count to
   // EVENT_CHANGES_AUTOCOMMIT or EVENT_CHANGES_COMMIT.
   void RecordTimeAndChanges(const base::TimeDelta& delta, bool read_only);
 
   // Helper to return the current time from the time source.
   base::TimeTicks Now() {
     return clock_->Now();
   }
 
+  // Release page-cache memory if memory-mapped I/O is enabled and the database
+  // was changed.  Passing true for |assume_changed| allows overriding the
+  // change detection for cases like DDL (CREATE, DROP, etc), which do not
+  // participate in the total-rows-changed tracking.
+  void ReleaseCacheMemoryIfNeeded(bool assume_changed);
+
   // The actual sqlite database. Will be NULL before Init has been called or if
   // Init resulted in an error.
   sqlite3* db_;
 
   // Parameters we'll configure in sqlite before doing anything else. Zero means
   // use the default value.
   int page_size_;
   int cache_size_;
   bool exclusive_locking_;
   bool restrict_to_user_;
@@ skipping 21 matching lines @@
   // True if database is open with OpenInMemory(), False if database is open
   // with Open().
   bool in_memory_;
 
   // |true| if the connection was closed using RazeAndClose().  Used
   // to enable diagnostics to distinguish calls to never-opened
   // databases (incorrect use of the API) from calls to once-valid
   // databases.
   bool poisoned_;
 
+  // |true| if SQLite memory-mapped I/O is enabled for this connection.
+  // Used by ReleaseCacheMemoryIfNeeded().
+  bool mmap_enabled_;
+
+  // Used by ReleaseCacheMemoryIfNeeded() to track if new changes have happened
+  // since memory was last released.
+  int total_changes_;
+
   ErrorCallback error_callback_;
 
   // Tag for auxiliary histograms.
   std::string histogram_tag_;
 
   // Linear histogram for RecordEvent().
   base::HistogramBase* stats_histogram_;
 
   // Histogram for tracking time taken in commit.
   base::HistogramBase* commit_time_histogram_;
@@ skipping 11 matching lines @@
   // Source for timing information, provided to allow tests to inject time
   // changes.
   scoped_ptr<TimeSource> clock_;
 
   DISALLOW_COPY_AND_ASSIGN(Connection);
 };
 
 }  // namespace sql
 
 #endif  // SQL_CONNECTION_H_