Are custom file engines in use?

If no custom file engine handlers have been registered, we can avoid
using the Read/Write lock and, most of the time, we also avoid the
creating the handler list at all.

Since the custom handler for QResource's engine has been folded into the
default handler, there should be no other internal use of this API, thus
triggering this optimization by default for all users.

The new flag may also in the future be exported to other modules to
trigger bypassing of file engines completely.

Reviewed-by: Marius Storm-Olsen

1 files changed, 10 insertions, 3 deletions

diff --git a/src/corelib/io/qabstractfileengine.cpp b/src/corelib/io/qabstractfileengine.cpp
index 6646e4e7d7..37a093c513 100644
--- a/src/corelib/io/qabstractfileengine.cpp
+++ b/src/corelib/io/qabstractfileengine.cpp
@@ -98,6 +98,8 @@ QT_BEGIN_NAMESPACE
     \sa QAbstractFileEngine, QAbstractFileEngine::create()
 */
 
+static bool qt_file_engine_handlers_in_use = false;
+
 /*
     All application-wide handlers are stored in this list. The mutex must be
     acquired to ensure thread safety.
@@ -127,6 +129,7 @@ Q_GLOBAL_STATIC(QAbstractFileEngineHandlerList, fileEngineHandlers)
 QAbstractFileEngineHandler::QAbstractFileEngineHandler()
 {
     QWriteLocker locker(fileEngineHandlerMutex());
+    qt_file_engine_handlers_in_use = true;
     fileEngineHandlers()->prepend(this);
 }
 
@@ -138,8 +141,12 @@ QAbstractFileEngineHandler::~QAbstractFileEngineHandler()
 {
     QWriteLocker locker(fileEngineHandlerMutex());
     // Remove this handler from the handler list only if the list is valid.
-    if (!qt_abstractfileenginehandlerlist_shutDown)
-        fileEngineHandlers()->removeOne(this);
+    if (!qt_abstractfileenginehandlerlist_shutDown) {
+        QAbstractFileEngineHandlerList *handlers = fileEngineHandlers();
+        handlers->removeOne(this);
+        if (handlers->isEmpty())
+            qt_file_engine_handlers_in_use = false;
+    }
 }
 
 /*!
@@ -170,7 +177,7 @@ QAbstractFileEngineHandler::~QAbstractFileEngineHandler()
 */
 QAbstractFileEngine *QAbstractFileEngine::create(const QString &fileName)
 {
-    {
+    if (qt_file_engine_handlers_in_use) {
         QReadLocker locker(fileEngineHandlerMutex());
 
         // check for registered handlers that can load the file