opengl: Bail if cached shader fails to load

QOpenGLProgramBinaryCache::setProgramBinary() should check
GL_LINK_STATUS after glProgramBinary(), but doesn't.

In practice, this means that SDDM is a white screen, and KDE is just
a gray task bar.

So far, Qt tries to check this using its internal ::link() function.
But in case the cached binary fails to load, Qt currently attempts to
link the inexistent program, resulting in a zero-length, fixed
pipeline shader.

Checking this already in ::setProgramBinary() makes the call to
::link() superfluous, so we remove that as well.

Done-with: Max Staudt <mstaudt@suse.com>
Done-with: Michal Srb <msrb@suse.com>
Done-with: Fabian Vogt <fvogt@suse.de>
Task-number: QTBUG-66420
Change-Id: Iabb51d0eb2c0c16bde696efff623e57d15f28d82
Reviewed-by: Jesus Fernandez <Jesus.Fernandez@qt.io>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
(cherry picked from commit fa091640134b3ff99a9eb92df8286d15203122bf)

src/gui/opengl/qopenglprogrambinarycache.cpp

@@ -161,10 +161,26 @@ bool QOpenGLProgramBinaryCache::setProgramBinary(uint programId, uint blobFormat
     QOpenGLExtraFunctions *funcs = QOpenGLContext::currentContext()->extraFunctions();
     while (funcs->glGetError() != GL_NO_ERROR) { }
     funcs->glProgramBinary(programId, blobFormat, p, blobSize);
-    int err = funcs->glGetError();
+
+    GLenum err = funcs->glGetError();
+    if (err != GL_NO_ERROR) {
+        qCDebug(DBG_SHADER_CACHE, "Program binary failed to load for program %u, size %d, "
+                                  "format 0x%x, err = 0x%x",
+                programId, blobSize, blobFormat, err);
+        return false;
+    }
+    GLint linkStatus = 0;
+    funcs->glGetProgramiv(programId, GL_LINK_STATUS, &linkStatus);
+    if (linkStatus != GL_TRUE) {
+        qCDebug(DBG_SHADER_CACHE, "Program binary failed to load for program %u, size %d, "
+                                  "format 0x%x, linkStatus = 0x%x, err = 0x%x",
+                programId, blobSize, blobFormat, linkStatus, err);
+        return false;
+    }
+
     qCDebug(DBG_SHADER_CACHE, "Program binary set for program %u, size %d, format 0x%x, err = 0x%x",
             programId, blobSize, blobFormat, err);
-    return err == 0;
+    return true;
 }
 
 #ifdef Q_OS_UNIX

src/gui/opengl/qopenglshaderprogram.cpp

@@ -3824,13 +3824,7 @@ bool QOpenGLShaderProgramPrivate::linkBinary()
     bool needsCompile = true;
     if (binCache.load(cacheKey, q->programId())) {
         qCDebug(DBG_SHADER_CACHE, "Program binary received from cache");
-        linkBinaryRecursion = true;
-        bool ok = q->link();
-        linkBinaryRecursion = false;
-        if (ok)
-            needsCompile = false;
-        else
-            qCDebug(DBG_SHADER_CACHE, "Link failed after glProgramBinary");
+        needsCompile = false;
     }
 
     bool needsSave = false;