More fixups for GL_CONTEXT_LOST

Fixes the rest of the places we use the pattern of emptying the OpenGL
error stack to be able to handle GL_CONTEXT_LOST, and adds a note about
it in the documentation.

Change-Id: I7eb97dbca45f39295b832d44937023b538b19947
Reviewed-by: David Edmundson <davidedmundson@kde.org>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/gui/opengl/qopenglbuffer.cpp

@@ -43,6 +43,10 @@
 #include "qopenglbuffer.h"
 #include <private/qopenglextensions_p.h>
 
+#ifndef GL_CONTEXT_LOST
+#define GL_CONTEXT_LOST                   0x0507
+#endif
+
 QT_BEGIN_NAMESPACE
 
 /*!
@@ -346,7 +350,14 @@ bool QOpenGLBuffer::read(int offset, void *data, int count)
     Q_D(QOpenGLBuffer);
     if (!d->funcs->hasOpenGLFeature(QOpenGLFunctions::Buffers) || !d->guard->id())
         return false;
-    while (d->funcs->glGetError() != GL_NO_ERROR) ; // Clear error state.
+
+    while (true) { // Clear error state.
+        GLenum error = d->funcs->glGetError();
+        if (error == GL_NO_ERROR)
+            break;
+        if (error == GL_CONTEXT_LOST)
+            return false;
+    };
     d->funcs->glGetBufferSubData(d->type, offset, count, data);
     return d->funcs->glGetError() == GL_NO_ERROR;
 #else

src/gui/opengl/qopengldebug.cpp

@@ -100,6 +100,10 @@ QT_BEGIN_NAMESPACE
 
     \endcode
 
+    If you try to clear the error stack, make sure not just keep going until
+    GL_NO_ERROR is returned but also break on GL_CONTEXT_LOST as that error
+    value will keep repeating.
+
     There are also many other information we are interested in (as application
     developers), for instance performance issues, or warnings about using
     deprecated APIs. Those kind of messages are not reported through the

src/gui/opengl/qopenglprogrambinarycache.cpp

@@ -54,6 +54,10 @@ QT_BEGIN_NAMESPACE
 
 Q_DECLARE_LOGGING_CATEGORY(DBG_SHADER_CACHE)
 
+#ifndef GL_CONTEXT_LOST
+#define GL_CONTEXT_LOST                   0x0507
+#endif
+
 #ifndef GL_PROGRAM_BINARY_LENGTH
 #define GL_PROGRAM_BINARY_LENGTH          0x8741
 #endif
@@ -161,7 +165,11 @@ bool QOpenGLProgramBinaryCache::verifyHeader(const QByteArray &buf) const
 bool QOpenGLProgramBinaryCache::setProgramBinary(uint programId, uint blobFormat, const void *p, uint blobSize)
 {
     QOpenGLExtraFunctions *funcs = QOpenGLContext::currentContext()->extraFunctions();
-    while (funcs->glGetError() != GL_NO_ERROR) { }
+    while (true) {
+        GLenum error = funcs->glGetError();
+        if (error == GL_NO_ERROR || error == GL_CONTEXT_LOST)
+            break;
+    }
     funcs->glProgramBinary(programId, blobFormat, p, blobSize);
 
     GLenum err = funcs->glGetError();
@@ -337,7 +345,11 @@ void QOpenGLProgramBinaryCache::save(const QByteArray &cacheKey, uint programId)
 
     QOpenGLExtraFunctions *funcs = QOpenGLContext::currentContext()->extraFunctions();
     GLint blobSize = 0;
-    while (funcs->glGetError() != GL_NO_ERROR) { }
+    while (true) {
+        GLenum error = funcs->glGetError();
+        if (error == GL_NO_ERROR || error == GL_CONTEXT_LOST)
+            break;
+    }
     funcs->glGetProgramiv(programId, GL_PROGRAM_BINARY_LENGTH, &blobSize);
 
     const int headerSize = FULL_HEADER_SIZE(info.glvendor.size() + info.glrenderer.size() + info.glversion.size());