X11GLXContext.getPlatformExtensionsStringImpl(): Add comment from pull request regarding GLX extensions

https://github.com/sgothel/jogl/pull/107

Return either glXQueryClientString or glXQueryExtensionsString when getting the GLX extensions

ndjojo:
glXQueryExtensionsString will make a request for glXQueryServerString if needed and will append the necessary client-side extensions.
This doc, under the section "Using GLS Extensions", also suggests checking the glXQueryExtensionsString before using GLX extensions.

+++

aschleck:
For some more context this came up with the release of Mesa 20.3.0,
which has client support for GLX_EXT_swap_control but no server support.
The current JOGL behavior of appending the client extensions to the server extensions is incorrect.
They should instead be intersected (with client-only extensions then appended) as the doc Nicole linked above says,
which is precisely what glXQueryExtensionsString does.
With the current extension querying behavior JOGL thinks glXSwapIntervalEXT is available under Mesa/llvmpipe
even though it is not, causing a segfault at JOGL initialization time.

I originally filed this as a Mesa bug (https://gitlab.freedesktop.org/mesa/mesa/-/issues/4128)
along with some code that repro'd JOGL's checking behavior but it became apparent that Mesa is fine and the checking behavior is incorrect.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/jogl/classes/jogamp/opengl/x11/glx/X11GLXContext.java b/src/jogl/classes/jogamp/opengl/x11/glx/X11GLXContext.java
index 1204432b4..ec1230894 100644
--- a/src/jogl/classes/jogamp/opengl/x11/glx/X11GLXContext.java
+++ b/src/jogl/classes/jogamp/opengl/x11/glx/X11GLXContext.java
@@ -551,6 +551,10 @@ public class X11GLXContext extends GLContextImpl {
                              ", server: "+ GLXUtil.getGLXServerVersionNumber(x11Device));
         }
         if(((X11GLXDrawableFactory)drawable.getFactoryImpl()).isGLXVersionGreaterEqualOneOne(x11Device)) {
+            /**
+             * Return either glXQueryClientString or glXQueryExtensionsString when getting the GLX extensions
+             * https://github.com/sgothel/jogl/pull/107
+             */
             if (ns.getScreenIndex() < 0) {
                 final String ret = GLX.glXGetClientString(x11Device.getHandle(), GLX.GLX_EXTENSIONS);
                 if (DEBUG) {