QPluginLoader: report the right load hints

A default-constructed QPluginLoader erroneously reports that the
load hints are empty. However, setting a filename would then
automatically set the PreventUnload hint, surprising the user.

Return the correct flags instead.

Amends 494376f980e96339b6f1eff7c41336ca4d853065

Change-Id: I7a95964cb680afd3adf2f71ed73d2f93023238f2
Fixes: QTBUG-100416
Pick-to: 5.15 6.2 6.3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 20 insertions, 4 deletions

diff --git a/src/corelib/plugin/qpluginloader.cpp b/src/corelib/plugin/qpluginloader.cpp
index fb69142149..eaca077c90 100644
--- a/src/corelib/plugin/qpluginloader.cpp
+++ b/src/corelib/plugin/qpluginloader.cpp
@@ -108,6 +108,8 @@ QT_BEGIN_NAMESPACE
     \sa QLibrary, {Plug & Paint Example}
 */
 
+static constexpr QLibrary::LoadHints defaultLoadHints = QLibrary::PreventUnloadHint;
+
 /*!
     Constructs a plugin loader with the given \a parent.
 */
@@ -131,7 +133,7 @@ QPluginLoader::QPluginLoader(const QString &fileName, QObject *parent)
     : QObject(parent), d(nullptr), did_load(false)
 {
     setFileName(fileName);
-    setLoadHints(QLibrary::PreventUnloadHint);
+    setLoadHints(defaultLoadHints);
 }
 
 /*!
@@ -327,7 +329,7 @@ static QString locatePlugin(const QString& fileName)
 void QPluginLoader::setFileName(const QString &fileName)
 {
 #if defined(QT_SHARED)
-    QLibrary::LoadHints lh = QLibrary::PreventUnloadHint;
+    QLibrary::LoadHints lh = defaultLoadHints;
     if (d) {
         lh = d->loadHints();
         d->release();
@@ -389,7 +391,12 @@ void QPluginLoader::setLoadHints(QLibrary::LoadHints loadHints)
 
 QLibrary::LoadHints QPluginLoader::loadHints() const
 {
-    return d ? d->loadHints() : QLibrary::LoadHints();
+    // Not having a d-pointer means that the user hasn't called
+    // setLoadHints() / setFileName() yet. In setFileName() we will
+    // then force defaultLoadHints on loading, so we must return them
+    // from here as well.
+
+    return d ? d->loadHints() : defaultLoadHints;
 }
 
 #endif // QT_CONFIG(library)
diff --git a/tests/auto/corelib/plugin/qpluginloader/tst_qpluginloader.cpp b/tests/auto/corelib/plugin/qpluginloader/tst_qpluginloader.cpp
index fcce21433e..1ea48e7ded 100644
--- a/tests/auto/corelib/plugin/qpluginloader/tst_qpluginloader.cpp
+++ b/tests/auto/corelib/plugin/qpluginloader/tst_qpluginloader.cpp
@@ -349,10 +349,19 @@ void tst_QPluginLoader::loadHints()
     QSKIP("This test requires Qt to create shared libraries.");
 #endif
     QPluginLoader loader;
-    QCOMPARE(loader.loadHints(), QLibrary::LoadHints{});   //Do not crash
+    QCOMPARE(loader.loadHints(), QLibrary::PreventUnloadHint);   //Do not crash
     loader.setLoadHints(QLibrary::ResolveAllSymbolsHint);
+    QCOMPARE(loader.loadHints(), QLibrary::ResolveAllSymbolsHint);
     loader.setFileName( sys_qualifiedLibraryName("theplugin"));     //a plugin
     QCOMPARE(loader.loadHints(), QLibrary::ResolveAllSymbolsHint);
+
+    QPluginLoader loader2;
+    QCOMPARE(loader2.loadHints(), QLibrary::PreventUnloadHint);
+    loader2.setFileName(sys_qualifiedLibraryName("theplugin"));
+    QCOMPARE(loader2.loadHints(), QLibrary::PreventUnloadHint);
+
+    QPluginLoader loader3(sys_qualifiedLibraryName("theplugin"));
+    QCOMPARE(loader3.loadHints(), QLibrary::PreventUnloadHint);
 }
 
 void tst_QPluginLoader::deleteinstanceOnUnload()