Prevent current context from becoming inconsistent upon create()

Platform plugins have a tendency to make the newly created native
context current with a temporary surface. This is usually needed to
query some information related to the new context. Afterwards most of
them just reset to having nothing current.

This has two issues:

It unexpectedly changes the current context/surface. A call into
QOpenGLContext::create() does not imply that the current context will
get changed. This is the minor issue and we could probably live with
it (at least if it had been documented).

However, the real issue is that QOpenGLContext::currentContext() will
become inconsistent: it will still report whatever was current before
the create() even though on the EGL/WGL/GLX level that's not the case
anymore.

To prevent all this confusion the platform plugins can easily be
changed to restore whatever context/surface was current before they
altered it.

Change-Id: I6a5b4597c86571327524ddb13e0d02538593cc7b
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

1 files changed, 10 insertions, 1 deletions

diff --git a/src/platformsupport/eglconvenience/qeglplatformcontext.cpp b/src/platformsupport/eglconvenience/qeglplatformcontext.cpp
index 148c5d9b2f..dce8bd888c 100644
--- a/src/platformsupport/eglconvenience/qeglplatformcontext.cpp
+++ b/src/platformsupport/eglconvenience/qeglplatformcontext.cpp
@@ -249,6 +249,15 @@ void QEGLPlatformContext::adopt(const QVariant &nativeHandle, QPlatformOpenGLCon
 void QEGLPlatformContext::updateFormatFromGL()
 {
 #ifndef QT_NO_OPENGL
+    // Have to save & restore to prevent QOpenGLContext::currentContext() from becoming
+    // inconsistent after QOpenGLContext::create().
+    EGLDisplay prevDisplay = eglGetCurrentDisplay();
+    if (prevDisplay == EGL_NO_DISPLAY) // when no context is current
+        prevDisplay = m_eglDisplay;
+    EGLContext prevContext = eglGetCurrentContext();
+    EGLSurface prevSurfaceDraw = eglGetCurrentSurface(EGL_DRAW);
+    EGLSurface prevSurfaceRead = eglGetCurrentSurface(EGL_READ);
+
     // Make the context current to ensure the GL version query works. This needs a surface too.
     const EGLint pbufferAttributes[] = {
         EGL_WIDTH, 1,
@@ -300,7 +309,7 @@ void QEGLPlatformContext::updateFormatFromGL()
                 }
             }
         }
-        eglMakeCurrent(m_eglDisplay, EGL_NO_SURFACE, EGL_NO_SURFACE, EGL_NO_CONTEXT);
+        eglMakeCurrent(prevDisplay, prevSurfaceDraw, prevSurfaceRead, prevContext);
     }
     eglDestroySurface(m_eglDisplay, pbuffer);
 #endif // QT_NO_OPENGL