glx/dri3: Don't fail on glXSwapBuffersMscOML(dpy, window, 0, 0, 0) (v2)

glXSwapBuffersMscOML() with target_msc=divisor=remainder=0 gets
translated into target_msc=divisor=0 but remainder=1 by the mesa
api. This is done for server DRI2 where there needs to be a way
to tell the server-side DRI2ScheduleSwap implementation if a call
to glXSwapBuffers() or glXSwapBuffersMscOML(dpy,window,0,0,0) was
done. remainder = 1 was (ab)used as a flag to tell the server to
select proper semantic. The DRI3/Present backend ignored this
signalling, treated any target_msc=0 as glXSwapBuffers() request,
and called xcb_present_pixmap with invalid divisor=0, remainder=1
combo. The present extension responded kindly to this with a
BadValue error and dropped the request, but mesa's DRI3/Present
backend doesn't check for error codes. From there on stuff went
downhill quickly for the calling OpenGL client...

This patch fixes the problem.

v2: Change comments to be more clear, with reference to
relevant spec, as suggested by Eric Anholt.

Cc: "10.3 10.4" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Axel Davy <axel.davy@ens.fr>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 0d7f4c8658)

src/glx/dri3_glx.c

@@ -1560,11 +1560,24 @@ dri3_swap_buffers(__GLXDRIdrawable *pdraw, int64_t target_msc, int64_t divisor,
       dri3_fence_reset(c, back);
 
       /* Compute when we want the frame shown by taking the last known successful
-       * MSC and adding in a swap interval for each outstanding swap request
+       * MSC and adding in a swap interval for each outstanding swap request.
+       * target_msc=divisor=remainder=0 means "Use glXSwapBuffers() semantic"
        */
       ++priv->send_sbc;
-      if (target_msc == 0)
+      if (target_msc == 0 && divisor == 0 && remainder == 0)
          target_msc = priv->msc + priv->swap_interval * (priv->send_sbc - priv->recv_sbc);
+      else if (divisor == 0 && remainder > 0) {
+         /* From the GLX_OML_sync_control spec:
+          *
+          *     "If <divisor> = 0, the swap will occur when MSC becomes
+          *      greater than or equal to <target_msc>."
+          *
+          * Note that there's no mention of the remainder.  The Present extension
+          * throws BadValue for remainder != 0 with divisor == 0, so just drop
+          * the passed in value.
+          */
+         remainder = 0;
+      }
 
       /* From the GLX_EXT_swap_control spec:
        *