Make QOpenGLContext::globalShareContext() lazy creation thread-safe

We move the context to the main thread (or the thread of qGuiApp),
as that's where it will be deleted.

Amends 4a7ccb65f0065e878c5762db05eca9c5cd6731e5.

Caught-by: Eskil Abrahamsen Blomfeldt <[email protected]>
Change-Id: I8e5dc512dce02d22980730a46474b2262684a713
Reviewed-by: Allan Sandfeld Jensen <[email protected]>

2 files changed, 23 insertions, 0 deletions

diff --git a/src/gui/kernel/qopenglcontext.cpp b/src/gui/kernel/qopenglcontext.cpp
index fa6572300b2..d49886087c0 100644
--- a/src/gui/kernel/qopenglcontext.cpp
+++ b/src/gui/kernel/qopenglcontext.cpp
@@ -968,11 +968,16 @@ bool QOpenGLContext::supportsThreadedOpenGL()
 QOpenGLContext *QOpenGLContext::globalShareContext()
 {
     Q_ASSERT(qGuiApp);
+
+    static QMutex mutex;
+    QMutexLocker locker(&mutex);
+
     // Lazily create a global share context when enabled unless there is already one
     if (!qt_gl_global_share_context() && qGuiApp->testAttribute(Qt::AA_ShareOpenGLContexts)) {
         QOpenGLContext *ctx = new QOpenGLContext;
         ctx->setFormat(QSurfaceFormat::defaultFormat());
         ctx->create();
+        ctx->moveToThread(qGuiApp->thread());
         qt_gl_set_global_share_context(ctx);
         QGuiApplicationPrivate::instance()->ownGlobalShareContext = true;
     }
diff --git a/tests/auto/gui/kernel/qguiapplication/tst_qguiapplication.cpp b/tests/auto/gui/kernel/qguiapplication/tst_qguiapplication.cpp
index c6bf6c650c3..b903dc3fc1d 100644
--- a/tests/auto/gui/kernel/qguiapplication/tst_qguiapplication.cpp
+++ b/tests/auto/gui/kernel/qguiapplication/tst_qguiapplication.cpp
@@ -1292,6 +1292,24 @@ void tst_QGuiApplication::globalShareContext()
     QCoreApplication::setAttribute(Qt::AA_ShareOpenGLContexts, true);
     ctx = QOpenGLContext::globalShareContext();
     QVERIFY(ctx);
+
+    // Test that the global share context is lazily created,
+    // even from secondary threads.
+    app.reset();
+    app.reset(new QGuiApplication(argc, argv));
+    QCoreApplication::setAttribute(Qt::AA_ShareOpenGLContexts, true);
+    class Thread : public QThread
+    {
+        void run() override
+        {
+            auto *ctx = QOpenGLContext::globalShareContext();
+            QVERIFY(ctx);
+            QCOMPARE(ctx->thread(), qGuiApp->thread());
+        }
+    };
+    Thread thread;
+    thread.start();
+    thread.wait();
 #else
     QSKIP("No OpenGL support");
 #endif