[gtk+/gtk-3-14] GdkDisplayX11: Properly translate server timestamps from _NET_WM_FRAME_* messages

[Matthias Clasen <matthiasc src gnome org>]

commit 2ef204e6071b985d5b8e5d1c25ca0b0abf5f9d31
Author: Owen W. Taylor <otaylor fishsoup net>
Date:   Wed Jul 15 12:38:38 2015 -0400

    GdkDisplayX11: Properly translate server timestamps from _NET_WM_FRAME_* messages
    
    When using frame times from _NET_WM_FRAME_DRAWN and _NET_WM_FRAME_TIMINGS, we
    were treating them as local monotonic times, but they are actually extended-precision
    versions of the server time, and need to be translated to monotonic times in the
    case where the X server and client aren't running on the same system.
    
    This fixes rendering stalls when using X over a remote ssh connection.
    https://bugzilla.gnome.org/show_bug.cgi?id=741800

 gdk/x11/gdkdisplay-x11.c |   43 ++++++++++++++++++++++++++++++++++++++++++-
 gdk/x11/gdkdisplay-x11.h |    6 ++++++
 2 files changed, 48 insertions(+), 1 deletions(-)
---
diff --git a/gdk/x11/gdkdisplay-x11.c b/gdk/x11/gdkdisplay-x11.c
index 4fe7524..2fe2cd4 100644
--- a/gdk/x11/gdkdisplay-x11.c
+++ b/gdk/x11/gdkdisplay-x11.c
@@ -1096,6 +1096,47 @@ find_frame_timings (GdkFrameClock *clock,
   return NULL;
 }
 
+/* _NET_WM_FRAME_DRAWN and _NET_WM_FRAME_TIMINGS messages represent time
+ * as a "high resolution server time" - this is the server time interpolated
+ * to microsecond resolution. The advantage of this time representation
+ * is that if  X server is running on the same computer as a client, and
+ * the Xserver uses 'clock_gettime(CLOCK_MONOTONIC, ...)' for the server
+ * time, the client can detect this, and all such clients will share a
+ * a time representation with high accuracy. If there is not a common
+ * time source, then the time synchronization will be less accurate.
+ */
+gint64
+server_time_to_monotonic_time (GdkX11Display *display_x11,
+                               gint64         server_time)
+{
+  if (display_x11->server_time_query_time == 0 ||
+      (!display_x11->server_time_is_monotonic_time &&
+       server_time > display_x11->server_time_query_time + 10*1000*1000)) /* 10 seconds */
+    {
+      gint64 current_server_time = gdk_x11_get_server_time (display_x11->leader_gdk_window);
+      gint64 current_server_time_usec = (gint64)current_server_time * 1000;
+      gint64 current_monotonic_time = g_get_monotonic_time ();
+      display_x11->server_time_query_time = current_monotonic_time;
+
+      /* If the server time is within a second of the monotonic time,
+       * we assume that they are identical. This seems like a big margin,
+       * but we want to be as robust as possible even if the system
+       * is under load and our processing of the server response is
+       * delayed.
+       */
+      if (current_server_time_usec > current_monotonic_time - 1000*1000 &&
+          current_server_time_usec < current_monotonic_time + 1000*1000)
+        display_x11->server_time_is_monotonic_time = TRUE;
+
+      display_x11->server_time_offset = current_server_time_usec - current_monotonic_time;
+    }
+
+  if (display_x11->server_time_is_monotonic_time)
+    return server_time;
+  else
+    return server_time - display_x11->server_time_offset;
+}
+
 GdkFilterReturn
 _gdk_wm_protocols_filter (GdkXEvent *xev,
                          GdkEvent  *event,
@@ -1128,7 +1169,7 @@ _gdk_wm_protocols_filter (GdkXEvent *xev,
           guint32 d3 = xevent->xclient.data.l[3];
 
           guint64 serial = ((guint64)d1 << 32) | d0;
-          gint64 frame_drawn_time = ((guint64)d3 << 32) | d2;
+          gint64 frame_drawn_time = server_time_to_monotonic_time (GDK_X11_DISPLAY (display), ((guint64)d3 
<< 32) | d2);
           gint64 refresh_interval, presentation_time;
 
           GdkFrameClock *clock = gdk_window_get_frame_clock (win);
diff --git a/gdk/x11/gdkdisplay-x11.h b/gdk/x11/gdkdisplay-x11.h
index 69ba890..d9305dd 100644
--- a/gdk/x11/gdkdisplay-x11.h
+++ b/gdk/x11/gdkdisplay-x11.h
@@ -124,6 +124,12 @@ struct _GdkX11Display
   GSList *error_traps;
 
   gint wm_moveresize_button;
+
+  /* Translation between X server time and system-local monotonic time */
+  gint64 server_time_query_time;
+  gint64 server_time_offset;
+
+  guint server_time_is_monotonic_time : 1;
 };
 
 struct _GdkX11DisplayClass