[gtk/gtk-3-24: 4/11] frame-clock: Ensure we're always monotonic

[Alexander Larsson <alexl src gnome org>]

commit 1a598c32d9d19ad3ea23cdb25468bfbfe3b3e08d
Author: Alexander Larsson <alexl redhat com>
Date:   Thu May 28 17:44:51 2020 +0200

    frame-clock: Ensure we're always monotonic
    
    A call to frame gdk_frame_clock_get_frame_time() outside of the paint
    cycle could report an un-error-corrected frame time, and later a
    corrected value could be earlier than the previously reported value.
    
    We now always store the latest reported time so we can ensure
    monotonicity.
    
    (cherry picked from commit a27fed47e0d20579cd6506e4d2f90f316f7f85a2)

 gdk/gdkframeclockidle.c | 31 ++++++++++++++++++++-----------
 1 file changed, 20 insertions(+), 11 deletions(-)
---
diff --git a/gdk/gdkframeclockidle.c b/gdk/gdkframeclockidle.c
index 5b145ad6c1..c5dd5ac791 100644
--- a/gdk/gdkframeclockidle.c
+++ b/gdk/gdkframeclockidle.c
@@ -38,10 +38,11 @@
 
 struct _GdkFrameClockIdlePrivate
 {
-  gint64 frame_time;                 /* The exact time we last ran the clock cycle, or 0 if never */
-  gint64 smoothed_frame_time_base;   /* A grid-aligned version of frame_time (grid size == refresh period), 
never more than half a grid from frame_time */
-  gint64 smoothed_frame_time_period; /* The grid size that smoothed_frame_time_base is aligned to */
-  gint64 min_next_frame_time;        /* We're not synced to vblank, so wait at least until this before next 
cycle to avoid busy looping */
+  gint64 frame_time;                   /* The exact time we last ran the clock cycle, or 0 if never */
+  gint64 smoothed_frame_time_base;     /* A grid-aligned version of frame_time (grid size == refresh 
period), never more than half a grid from frame_time */
+  gint64 smoothed_frame_time_period;   /* The grid size that smoothed_frame_time_base is aligned to */
+  gint64 smoothed_frame_time_reported; /* Ensures we are always monotonic */
+  gint64 min_next_frame_time;          /* We're not synced to vblank, so wait at least until this before 
next cycle to avoid busy looping */
   gint64 sleep_serial;
 #ifdef G_ENABLE_DEBUG
   gint64 freeze_time;
@@ -220,9 +221,9 @@ compute_smooth_frame_time (GdkFrameClock *clock,
         new_smoothed_time += correction_magnitude;
     }
 
-  /* Ensure we're always strictly increasing (avoid division by zero when using time deltas) */
-  if (new_smoothed_time <= priv->smoothed_frame_time_base)
-    new_smoothed_time = priv->smoothed_frame_time_base + 1;
+  /* Ensure we're always monotonic  */
+  if (new_smoothed_time <= priv->smoothed_frame_time_reported)
+    new_smoothed_time = priv->smoothed_frame_time_reported;
 
   return new_smoothed_time;
 }
@@ -232,6 +233,7 @@ gdk_frame_clock_idle_get_frame_time (GdkFrameClock *clock)
 {
   GdkFrameClockIdlePrivate *priv = GDK_FRAME_CLOCK_IDLE (clock)->priv;
   gint64 now;
+  gint64 new_smoothed_time;
 
   /* can't change frame time during a paint */
   if (priv->phase != GDK_FRAME_CLOCK_PHASE_NONE &&
@@ -243,13 +245,19 @@ gdk_frame_clock_idle_get_frame_time (GdkFrameClock *clock)
 
   /* First time frame, just return something */
   if (priv->smoothed_frame_time_base == 0)
-    return now;
+    {
+      priv->smoothed_frame_time_reported = now;
+      return now;
+    }
 
   /* Since time is monotonic this is <= what we will pick for the next cycle, but
      more likely than not it will be equal if we're doing a constant animation. */
-  return compute_smooth_frame_time (clock, now, FALSE,
-                                    priv->smoothed_frame_time_base,
-                                    priv->smoothed_frame_time_period);
+  new_smoothed_time = compute_smooth_frame_time (clock, now, FALSE,
+                                                 priv->smoothed_frame_time_base,
+                                                 priv->smoothed_frame_time_period);
+
+  priv->smoothed_frame_time_reported = new_smoothed_time;
+  return new_smoothed_time;
 }
 
 #define RUN_FLUSH_IDLE(priv)                                            \
@@ -424,6 +432,7 @@ gdk_frame_clock_paint_idle (void *data)
                                                  priv->smoothed_frame_time_period);
                   priv->smoothed_frame_time_period = frame_interval;
                 }
+              priv->smoothed_frame_time_reported = priv->smoothed_frame_time_base;
 
               _gdk_frame_clock_begin_frame (clock);
               /* Note "current" is different now so timings != prev_timings */