st/mesa: fix frontbuffer glReadPixels regressions

The change "mesa/readpix: Don't clip in _mesa_readpixels()" caused a
few piglit regressions.  The failing tests use glReadPixels to read
from the front color buffer.  The problem is we were trying to read
from a non-existant front color buffer.  The front color buffer is
created on demand in st/mesa.  Since the missing buffer bounds were
effectively 0 x 0 the glReadPixels was totally clipped and returned
early.

The fix involves creating the real front color buffer when we're about
to try reading from it.

Tested with llvmpipe and VMware driver on Linux, Windows.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94253
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94254
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94257
Cc: [email protected]
Reviewed-by: Roland Scheidegger <[email protected]>
(cherry picked from commit 83b589301f4a150f4b1b13fd3ffd9f6d98ee6546)

1 files changed, 11 insertions, 2 deletions

diff --git a/src/mesa/state_tracker/st_cb_fbo.c b/src/mesa/state_tracker/st_cb_fbo.c
index 2a2eb0992c8..82ab914503b 100644
--- a/src/mesa/state_tracker/st_cb_fbo.c
+++ b/src/mesa/state_tracker/st_cb_fbo.c
@@ -44,6 +44,7 @@
 #include "pipe/p_context.h"
 #include "pipe/p_defines.h"
 #include "pipe/p_screen.h"
+#include "st_atom.h"
 #include "st_context.h"
 #include "st_cb_fbo.h"
 #include "st_cb_flush.h"
@@ -711,9 +712,17 @@ st_ReadBuffer(struct gl_context *ctx, GLenum buffer)
 
    (void) buffer;
 
-   /* add the renderbuffer on demand */
-   if (fb->_ColorReadBufferIndex >= 0)
+   /* Check if we need to allocate a front color buffer.
+    * Front buffers are often allocated on demand (other color buffers are
+    * always allocated in advance).
+    */
+   if ((fb->_ColorReadBufferIndex == BUFFER_FRONT_LEFT ||
+        fb->_ColorReadBufferIndex == BUFFER_FRONT_RIGHT) &&
+       fb->Attachment[fb->_ColorReadBufferIndex].Type == GL_NONE) {
+      /* add the buffer */
       st_manager_add_color_renderbuffer(st, fb, fb->_ColorReadBufferIndex);
+      st_validate_state(st, ST_PIPELINE_RENDER);
+   }
 }