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, 3 insertions, 3 deletions

diff --git a/src/corelib/plugin/qlibrary_p.h b/src/corelib/plugin/qlibrary_p.h
index db5afac98e..29724655a3 100644
--- a/src/corelib/plugin/qlibrary_p.h
+++ b/src/corelib/plugin/qlibrary_p.h
@@ -86,7 +86,7 @@ public:
     QString fullVersion;
 
     bool load();
-    bool loadPlugin(); // loads and resolves instance
+    QtPluginInstanceFunction loadPlugin(); // loads and resolves instance
     bool unload(UnloadFlag flag = UnloadSys);
     void release();
     QFunctionPointer resolve(const char *);
@@ -100,8 +100,8 @@ public:
     static QStringList suffixes_sys(const QString &fullVersion);
     static QStringList prefixes_sys();
 
-    QPointer<QObject> inst;
-    QtPluginInstanceFunction instance;
+    QPointer<QObject> inst;         // used by QFactoryLoader
+    QAtomicPointer<std::remove_pointer<QtPluginInstanceFunction>::type> instanceFactory;
     QJsonObject metaData;
 
     QString errorString;