QOpenGLContext: add a way to disable the thread affinity check

The problem of the check is that it makes Qt non compliant with OpenGL
thread affinity semantics. One is allowed to make a GL context
current on any thread, without the Qt-specific idea of moving
the QOpenGLContext to that thread first. Moreover, the move is
plain impossible if the user needs to handle the context
to 3rd party code which assumes the context to have GL semantics.

Add an application flag to disable that.

Change-Id: I55ca02ee62f8cc171a9a1bddef5331ad0949c061
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

1 files changed, 12 insertions, 3 deletions

diff --git a/src/gui/kernel/qopenglcontext.cpp b/src/gui/kernel/qopenglcontext.cpp
index 59bf2bb065..19464eeca3 100644
--- a/src/gui/kernel/qopenglcontext.cpp
+++ b/src/gui/kernel/qopenglcontext.cpp
@@ -940,13 +940,20 @@ GLuint QOpenGLContext::defaultFramebufferObject() const
 
     If \a surface is 0 this is equivalent to calling doneCurrent().
 
-    Do not call this function from a different thread than the one the
+    Avoid calling this function from a different thread than the one the
     QOpenGLContext instance lives in. If you wish to use QOpenGLContext from a
     different thread you should first call make sure it's not current in the
     current thread, by calling doneCurrent() if necessary. Then call
     moveToThread(otherThread) before using it in the other thread.
 
-    \sa functions(), doneCurrent()
+    By default Qt employs a check that enforces the above condition on the
+    thread affinity. It is still possible to disable this check by setting the
+    \c{Qt::AA_DontCheckOpenGLContextThreadAffinity} application attribute. Be
+    sure to understand the consequences of using QObjects from outside
+    the thread they live in, as explained in the
+    \l{QObject#Thread Affinity}{QObject thread affinity} documentation.
+
+    \sa functions(), doneCurrent(), Qt::AA_DontCheckOpenGLContextThreadAffinity
 */
 bool QOpenGLContext::makeCurrent(QSurface *surface)
 {
@@ -954,8 +961,10 @@ bool QOpenGLContext::makeCurrent(QSurface *surface)
     if (!isValid())
         return false;
 
-    if (Q_UNLIKELY(thread() != QThread::currentThread()))
+    if (Q_UNLIKELY(!qApp->testAttribute(Qt::AA_DontCheckOpenGLContextThreadAffinity)
+                   && thread() != QThread::currentThread())) {
         qFatal("Cannot make QOpenGLContext current in a different thread");
+    }
 
     if (!surface) {
         doneCurrent();