clutter/stage-cogl: Don't skip over the next frame

The `last_presentation_time` is usually a little in the past (although
sometimes in the future depending on the driver). When it's over 2ms
(`sync_delay`) in the past that would trigger the while loop to count up so
that the next `update_time` is in the future.

The problem with that is for common values of `last_presentation_time`
which are only a few milliseconds ago, incrementing `update_time` by
`refresh_interval` also means counting past the next physical frame that
we haven't rendered yet. And so mutter would skip that frame.

**Example**

Given:
```
  last_presentation_time = now - 3ms
  sync_delay = 2ms
  refresh_interval = 16ms
  next_presentation_time = last_presentation_time + refresh_interval
                         = now + 13ms

          -3ms now        +13ms           +29ms           +45ms
        ----|--+------------|---------------|---------------|----
            :               :
  last_presentation_time  next_presentation_time
```

Old algorithm:
```
  update_time = last_presentation_time + sync_delay
              = now - 1ms
  while (update_time < now)
        (now - 1ms   < now)
    update_time = now - 1ms + 16ms
  update_time = now + 15ms
  next_presentation_time = now + 13ms
  available_render_time = next_presentation_time - max(now, update_time)
                        = (now + 13ms) - (now + 15ms)
                        = -2ms  so the next frame will be skipped.

          -3ms now        +13ms           +29ms           +45ms
        ----|--+------------|-+-------------|---------------|----
            :               : :
            :               : update_time (too late)
            :               :
  last_presentation_time  next_presentation_time (a missed frame)

```

New algorithm:
```
  min_render_time_allowed = refresh_interval / 2
                          = 8ms
  max_render_time_allowed = refresh_interval - sync_delay
                          = 14ms
  target_presentation_time = last_presentation_time + refresh_interval
                           = now - 3ms + 16ms
                           = now + 13ms
  while (target_presentation_time - min_render_time_allowed < now)
        (now + 13ms - 8ms < now)
        (5ms < 0ms)
    # loop is never entered
  update_time = target_presentation_time - max_render_time_allowed
              = now + 13ms - 14ms
              = now - 1ms
  next_presentation_time = now + 13ms
  available_render_time = next_presentation_time - max(now, update_time)
                        = (now + 13ms) - now
                        = 13ms  which is plenty of render time.

          -3ms now        +13ms           +29ms           +45ms
        ----|-++------------|---------------|---------------|----
            : :             :
            : update_time   :
            :               :
  last_presentation_time  next_presentation_time
```

The reason nobody noticed these missed frames very often was because
mutter has some accidental workarounds built-in:

 * Prior to 3.32, the offending code was only reachable in Xorg sessions.
   It was never reached in Wayland sessions because it hadn't been
   implemented yet (till e9e4b2b72).

 * Even though Wayland support is now implemented the native backend
   provides a `last_presentation_time` much faster than Xorg sessions
   (being in the same process) and so is less likely to spuriously enter
   the while loop to miss a frame.

 * For Xorg sessions we are accidentally triple buffering (#334). This
   is a good way to avoid the missed frames, but is also an accident.

 * `sync_delay` is presently just high enough (2ms by coincidence is very
   close to common values of `now - last_presentation_time`) to push the
   `update_time` into the future in some cases, which avoids entering the
   while loop. This is why the same missed frames problem was also noticed
   when experimenting with `sync_delay = 0`.

v2: adjust variable names and code style.

Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=789186
       and most of https://gitlab.gnome.org/GNOME/mutter/issues/571

https://gitlab.gnome.org/GNOME/mutter/merge_requests/520

1 files changed, 14 insertions, 3 deletions

diff --git a/clutter/clutter/cogl/clutter-stage-cogl.c b/clutter/clutter/cogl/clutter-stage-cogl.c
index 2a2a17e05..1ed7e01c4 100644
--- a/clutter/clutter/cogl/clutter-stage-cogl.c
+++ b/clutter/clutter/cogl/clutter-stage-cogl.c
@@ -152,6 +152,9 @@ clutter_stage_cogl_schedule_update (ClutterStageWindow *stage_window,
   gint64 now;
   float refresh_rate;
   gint64 refresh_interval;
+  int64_t min_render_time_allowed;
+  int64_t max_render_time_allowed;
+  int64_t next_presentation_time;
 
   if (stage_cogl->update_time != -1)
     return;
@@ -184,10 +187,18 @@ clutter_stage_cogl_schedule_update (ClutterStageWindow *stage_window,
   if (refresh_interval == 0)
     refresh_interval = 16667; /* 1/60th second */
 
-  stage_cogl->update_time = stage_cogl->last_presentation_time + 1000 * sync_delay;
+  min_render_time_allowed = refresh_interval / 2;
+  max_render_time_allowed = refresh_interval - 1000 * sync_delay;
 
-  while (stage_cogl->update_time < now)
-    stage_cogl->update_time += refresh_interval;
+  if (min_render_time_allowed > max_render_time_allowed)
+    min_render_time_allowed = max_render_time_allowed;
+
+  next_presentation_time = stage_cogl->last_presentation_time + refresh_interval;
+
+  while (next_presentation_time < now + min_render_time_allowed)
+    next_presentation_time += refresh_interval;
+
+  stage_cogl->update_time = next_presentation_time - max_render_time_allowed;
 }
 
 static gint64