Fix plugin implicit loading when calling QPluginLoader::instance().

QPluginLoader::instance() didn't increase loading refcount if another
QPluginLoader had already loaded the plugin. This meant that if the
another QPluginLoader subsequently unloaded the plugin, the instance
would be destroyed even if the second loader still wanted to use it.

Also improved the tst_QPluginLoader::deleteinstanceOnUnload() test case
to test more combinations of deletion order and explicit/implicit
loading.

Task-number: QT-5259
Reviewed-by: Sami Merila

2 files changed, 21 insertions, 10 deletions

diff --git a/src/corelib/plugin/qpluginloader.cpp b/src/corelib/plugin/qpluginloader.cpp
index bbb64e4..9f9ea1a 100644
--- a/src/corelib/plugin/qpluginloader.cpp
+++ b/src/corelib/plugin/qpluginloader.cpp
@@ -198,7 +198,7 @@ QPluginLoader::~QPluginLoader()
 */
 QObject *QPluginLoader::instance()
 {
-    if (!isLoaded() && !load())
+    if (!load())
         return 0;
     if (!d->inst && d->instance)
         d->inst = d->instance();
diff --git a/tests/auto/qpluginloader/tst_qpluginloader.cpp b/tests/auto/qpluginloader/tst_qpluginloader.cpp
index d2d92a5..152d1f4 100644
--- a/tests/auto/qpluginloader/tst_qpluginloader.cpp
+++ b/tests/auto/qpluginloader/tst_qpluginloader.cpp
@@ -272,10 +272,10 @@ void tst_QPluginLoader::loadHints()
 
 void tst_QPluginLoader::deleteinstanceOnUnload()
 {
-    for (int pass = 0; pass < 2; ++pass) {
+    for (int pass = 0; pass < 4; ++pass) {
         QPluginLoader loader1;
         loader1.setFileName( sys_qualifiedLibraryName("theplugin"));     //a plugin
-        if (pass == 0)
+        if (pass < 2)
             loader1.load(); // not recommended, instance() should do the job.
         PluginInterface *instance1 = qobject_cast<PluginInterface*>(loader1.instance());
         QVERIFY(instance1);
@@ -283,21 +283,32 @@ void tst_QPluginLoader::deleteinstanceOnUnload()
 
         QPluginLoader loader2;
         loader2.setFileName( sys_qualifiedLibraryName("theplugin"));     //a plugin
-        if (pass == 0)
+        if (pass < 2)
             loader2.load(); // not recommended, instance() should do the job.
         PluginInterface *instance2 = qobject_cast<PluginInterface*>(loader2.instance());
         QCOMPARE(instance2->pluginName(), QLatin1String("Plugin ok"));
 
         QSignalSpy spy1(loader1.instance(), SIGNAL(destroyed()));
         QSignalSpy spy2(loader2.instance(), SIGNAL(destroyed()));
-        if (pass == 0) {
-            QCOMPARE(loader2.unload(), false);  // refcount not reached 0, not really unloaded
-            QCOMPARE(spy1.count(), 0);
-            QCOMPARE(spy2.count(), 0);
-        }
+
+        // refcount not reached 0, not really unloaded
+        if (pass % 2)
+            QCOMPARE(loader1.unload(), false);
+        else
+            QCOMPARE(loader2.unload(), false);
+
+        QCOMPARE(spy1.count(), 0);
+        QCOMPARE(spy2.count(), 0);
+
         QCOMPARE(instance1->pluginName(), QLatin1String("Plugin ok"));
         QCOMPARE(instance2->pluginName(), QLatin1String("Plugin ok"));
-        QVERIFY(loader1.unload());   // refcount reached 0, did really unload
+
+        // refcount reached 0, did really unload
+        if (pass % 2)
+            QVERIFY(loader2.unload());
+        else
+            QVERIFY(loader1.unload());
+
         QCOMPARE(spy1.count(), 1);
         QCOMPARE(spy2.count(), 1);
     }