X11: Better timestamp handling for _NET_ACTIVE_WINDOW

ui/events/platform/x11/x11_event_source.cc

Index: ui/events/platform/x11/x11_event_source.cc
diff --git a/ui/events/platform/x11/x11_event_source.cc b/ui/events/platform/x11/x11_event_source.cc
index a50cf739e1b06a82487b494c52029048f9440936..b395b9dd84c6d8c5d3ab18d605a4e3cb4575e4d4 100644
--- a/ui/events/platform/x11/x11_event_source.cc
+++ b/ui/events/platform/x11/x11_event_source.cc
@@ -4,10 +4,12 @@
 
 #include "ui/events/platform/x11/x11_event_source.h"
 
+#include <X11/Xatom.h>
 #include <X11/XKBlib.h>
 #include <X11/Xlib.h>
 
 #include "base/logging.h"
+#include "base/metrics/histogram_macros.h"
 #include "ui/events/devices/x11/device_data_manager_x11.h"
 #include "ui/events/devices/x11/touch_factory_x11.h"
 #include "ui/events/event_utils.h"
@@ -78,6 +80,14 @@ void UpdateDeviceList() {
   DeviceDataManagerX11::GetInstance()->UpdateDeviceList(display);
 }
 
+Bool IsPropertyNotifyForTimestamp(Display* display,
+                                  XEvent* event,
+                                  XPointer arg) {
+  return event->type == PropertyNotify &&
+         event->xproperty.window == DefaultRootWindow(display) &&
+         event->xproperty.atom == *(Atom*)arg;
+}
+
 }  // namespace
 
 X11EventSource* X11EventSource::instance_ = nullptr;
@@ -134,6 +144,43 @@ void X11EventSource::BlockUntilWindowMapped(XID window) {
   } while (event.type != MapNotify);
 }
 
+Time X11EventSource::UpdateLastSeenServerTime() {
+  base::TimeTicks start = base::TimeTicks::Now();
+
+  const char kAtomStr[] = "X11_EVENT_SOURCE_UPDATE_SERVER_TIME_DUMMY_ATOM";
+
+  DCHECK(display_);
+  Window window = DefaultRootWindow(display_);
+  const unsigned char dummy_data = 0;
+  Atom dummy_atom = XInternAtom(display_, kAtomStr, False);
+
+  XWindowAttributes attributes;
+  XGetWindowAttributes(display_, window, &attributes);

[danakj, 2016/05/12 21:32:50]

Round trips :(

Can you just use some arbitrary never-mapped window to do this timestamp
generation instead of the root?

+  long old_event_mask = attributes.your_event_mask;
+
+  // Make a no-op property change on the root window.
+  XSelectInput(display_, window, old_event_mask | PropertyChangeMask);
+  XChangeProperty(display_, window, dummy_atom, XA_INTEGER, 8, PropModeReplace,
+                  &dummy_data, 1);
+
+
+  // Observe the resulting PropertyChange to obtain the timestamp.
+  XEvent event;
+  XIfEvent(display_, &event, IsPropertyNotifyForTimestamp,
+           (XPointer)&dummy_atom);
+
+  // Clean up: don't leave a dummy property in the root window.
+  XSelectInput(display_, window, old_event_mask);
+  XDeleteProperty(display_, window, dummy_atom);
+  XFlush(display_);
+
+  last_seen_server_time_ = event.xproperty.time;
+
+  UMA_HISTOGRAM_TIMES("Event.Latency.X11EventSource.UpdateServerTime",
+                      base::TimeTicks::Now() - start);
+  return last_seen_server_time_;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // X11EventSource, protected