QPluginLoader: rework the loading and the caching of instance

There was a race condition in accessing the cached instance factory
member, so rework loadPlugin() to return the cached or newly discovered
instance, with proper, atomic caching. Because I had to change that, I
took the opportunity to fix the QFactoryLoader code that calls
loadPlugin().

Note that QLibraryPrivate::loadPlugin() returns non-nullptr now if the
instance is known, which means the last return in QPluginLoader::load()
will convert to true, not false, if the instance got cached between the
earlier check and the call to loadPlugin(). That's probably what was
intended.

Task-number: QTBUG-39642
Change-Id: I46bf1f65e8db46afbde5fffd15e1a42d2b6cbf2c
Reviewed-by: David Faure <david.faure@kdab.com>

1 files changed, 5 insertions, 10 deletions

diff --git a/src/corelib/plugin/qfactoryloader.cpp b/src/corelib/plugin/qfactoryloader.cpp
index 18f10c9b43..9a6a69676d 100644
--- a/src/corelib/plugin/qfactoryloader.cpp
+++ b/src/corelib/plugin/qfactoryloader.cpp
@@ -389,17 +389,12 @@ QObject *QFactoryLoader::instance(int index) const
     QMutexLocker lock(&d->mutex);
     if (index < d->libraryList.size()) {
         QLibraryPrivate *library = d->libraryList.at(index);
-        if (library->instance || library->loadPlugin()) {
-            if (!library->inst)
-                library->inst = library->instance();
-            QObject *obj = library->inst.data();
-            if (obj) {
-                if (!obj->parent())
-                    obj->moveToThread(QCoreApplicationPrivate::mainThread());
-                return obj;
-            }
+        if (QObject *obj = library->pluginInstance()) {
+            if (!obj->parent())
+                obj->moveToThread(QCoreApplicationPrivate::mainThread());
+            return obj;
         }
-        return 0;
+        return nullptr;
     }
     index -= d->libraryList.size();
     lock.unlock();