Adds a MemoryPressureListener to sql::Connection.

This will allow SQLite database to reduce their page cache memory on systems
which provide a MemoryPressureListener signal.

BUG=243769

Review URL: https://chromiumcodereview.appspot.com/17071007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@212374 0039d316-1c4b-4281-b951-d872f2087c98

2 files changed, 35 insertions, 0 deletions

diff --git a/sql/connection.cc b/sql/connection.cc
index d11b40c..e7b6fe1 100644
--- a/sql/connection.cc
+++ b/sql/connection.cc
@@ -279,6 +279,35 @@ void Connection::Preload() {
 #endif
 }
 
+void Connection::TrimMemory(bool aggressively) {
+  if (!db_)
+    return;
+
+  // TODO(shess): investigate using sqlite3_db_release_memory() when possible.
+  int original_cache_size;
+  {
+    Statement sql_get_original(GetUniqueStatement("PRAGMA cache_size"));
+    if (!sql_get_original.Step()) {
+      DLOG(WARNING) << "Could not get cache size " << GetErrorMessage();
+      return;
+    }
+    original_cache_size = sql_get_original.ColumnInt(0);
+  }
+  int shrink_cache_size = aggressively ? 1 : (original_cache_size / 2);
+
+  // Force sqlite to try to reduce page cache usage.
+  const std::string sql_shrink =
+      base::StringPrintf("PRAGMA cache_size=%d", shrink_cache_size);
+  if (!Execute(sql_shrink.c_str()))
+    DLOG(WARNING) << "Could not shrink cache size: " << GetErrorMessage();
+
+  // Restore cache size.
+  const std::string sql_restore =
+      base::StringPrintf("PRAGMA cache_size=%d", original_cache_size);
+  if (!Execute(sql_restore.c_str()))
+    DLOG(WARNING) << "Could not restore cache size: " << GetErrorMessage();
+}
+
 // Create an in-memory database with the existing database's page
 // size, then backup that database over the existing database.
 bool Connection::Raze() {
diff --git a/sql/connection.h b/sql/connection.h
index ab80a6c..3ee91ef 100644
--- a/sql/connection.h
+++ b/sql/connection.h
@@ -190,6 +190,12 @@ class SQL_EXPORT Connection {
   // generally exist either.
   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.
+  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