[PATCH 2/2] DRI2SwapBuffers: Fix uninitialized target SBC.

[Jamey Sharp]

Fixes Piglit test "swapbuffersmsc-return swap_interval 0".

Ensure that *swap_target gets initialized on any 'return Success' path,
even if the swap request can't be completed by the driver and the server
falls back to a simple blit. That path can also be triggered by setting
swap_interval to 0, which disables sync to vertical retrace.

We originally found this bug because for some reason SDL2 automatically
sets swap_interval to 0, when we were trying to test OML_sync_control in
an SDL2 test application. We then discovered that the above-mentioned
Piglit test has been failing for the same reason since it was
introduced.

Signed-off-by: Jamey Sharp <jamey at minilop.net>
Signed-off-by: Theo Hill <Theo0x48 at gmail.com>
---
 hw/xfree86/dri2/dri2.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/hw/xfree86/dri2/dri2.c b/hw/xfree86/dri2/dri2.c
index 962f40c..76708ca 100644
--- a/hw/xfree86/dri2/dri2.c
+++ b/hw/xfree86/dri2/dri2.c
@@ -1092,6 +1092,14 @@ DRI2SwapBuffers(ClientPtr client, DrawablePtr pDraw, CARD64 target_msc,
         return BadDrawable;
     }
 
+    /* According to spec, return expected swapbuffers count SBC after this swap
+     * will complete. This is ignored unless we return Success, but it must be
+     * initialized on every path where we return Success or the caller will send
+     * an uninitialized value off the stack to the client. So let's initialize
+     * it as early as possible, just to be sure.
+     */
+    *swap_target = pPriv->swap_count + pPriv->swapsPending + 1;
+
     for (i = 0; i < pPriv->bufferCount; i++) {
         if (pPriv->buffers[i]->attachment == DRI2BufferFrontLeft)
             pDestBuffer = (DRI2BufferPtr) pPriv->buffers[i];
@@ -1165,11 +1173,6 @@ DRI2SwapBuffers(ClientPtr client, DrawablePtr pDraw, CARD64 target_msc,
 
     pPriv->last_swap_target = target_msc;
 
-    /* According to spec, return expected swapbuffers count SBC after this swap
-     * will complete.
-     */
-    *swap_target = pPriv->swap_count + pPriv->swapsPending;
-
     DRI2InvalidateDrawableAll(pDraw);
 
     return Success;
-- 
1.8.5.3