Avoid null poses in WebVR magic window mode

There's a race condition where the first magic window VSync could arrive before
ever getting a pose. Add a check and deferred execution to avoid this.

BUG=787196

Change-Id: If804e4d6acc3da7b903ba36108f23c00b1c8fe5b
Reviewed-on: https://chromium-review.googlesource.com/783887
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523657}

third_party/WebKit/LayoutTests/vr/getFrameData_oneframeupdate.html

@@ -17,7 +17,6 @@ vr_test( (t, mock_service) => {
     var counter = 0;
 
     function onFrame() {
-      display.requestAnimationFrame(onFrame);
       if (counter == 0) {
         t.step( () => {
           assert_false(display.getFrameData(fd));
@@ -27,14 +26,6 @@ vr_test( (t, mock_service) => {
           assert_false(display.getFrameData(fd));
         }, "Does not update within the same frame");
       } else {
-        // TODO(mthiesse): This is a workaround for crbug.com/787196. Either
-        // remove the workaround once fixed or remove this todo.
-        // Let several animation frames get triggered so we're sure to have a
-        // pose
-        if (counter <= 2) {
-          counter++;
-          return;
-        }
         t.step( () => {
           assert_true(display.getFrameData(fd));
           assert_not_equals(fd.pose, null);
@@ -49,6 +40,10 @@ vr_test( (t, mock_service) => {
         t.done();
       }
       counter++;
+      // Use rAF late so that the mock's setPose above can supply its data
+      // before RequestVSync calls GetPose. This is an artifact of the mocking
+      // framework, the real implementation doesn't have this restriction.
+      display.requestAnimationFrame(onFrame);
     }
 
     display.requestAnimationFrame(onFrame);

third_party/WebKit/LayoutTests/vr/getFrameData_samewithinframe.html

@@ -16,16 +16,9 @@ vr_test( (t, mock_service) => {
     var fd1 = new VRFrameData();
     var fd2 = new VRFrameData();
     mock_service.mockVRDisplays_[0].setPose(expected_pose);
-    var numFrames = 0;
 
     function onFrame() {
       display.requestAnimationFrame(onFrame);
-      numFrames++;
-      // Let one rAF run before checking in order to ensure we actually get
-      // frame data
-      if (numFrames == 1) {
-        return;
-      }
 
       t.step( () => {
         assert_true(display.getFrameData(fd1));

third_party/WebKit/Source/modules/vr/VRDisplay.cpp

@@ -47,6 +47,12 @@ namespace blink {
 
 namespace {
 
+// Threshold for rejecting stored magic window poses as being too old.
+// If it's exceeded, defer magic window rAF callback execution until
+// a fresh pose is received.
+constexpr WTF::TimeDelta kMagicWindowPoseAgeThreshold =
+    WTF::TimeDelta::FromMilliseconds(250);
+
 VREye StringToVREye(const String& which_eye) {
   if (which_eye == "left")
     return kVREyeLeft;
@@ -220,6 +226,7 @@ void VRDisplay::RequestVSync() {
     return;
 
   if (!is_presenting_) {
+    magic_window_vsync_waiting_for_pose_.Reset();
     magic_window_provider_->GetPose(
         WTF::Bind(&VRDisplay::OnMagicWindowPose, WrapWeakPersistent(this)));
     pending_magic_window_vsync_ = true;
@@ -1044,15 +1051,31 @@ void VRDisplay::OnMagicWindowVSync(double timestamp) {
   pending_magic_window_vsync_ = false;
   pending_magic_window_vsync_id_ = -1;
   vr_frame_id_ = -1;
-  ProcessScheduledAnimations(timestamp);
+  WTF::TimeDelta pose_age = WTF::TimeTicks::Now() - magic_window_pose_time_;
+  if (pose_age < kMagicWindowPoseAgeThreshold) {
+    ProcessScheduledAnimations(timestamp);
+  } else {
+    // The VSync got triggered before ever getting a pose, or the pose is
+    // stale. Defer the animation until a pose arrives to avoid passing null
+    // poses to the application.
+    magic_window_vsync_waiting_for_pose_ =
+        WTF::Bind(&VRDisplay::ProcessScheduledAnimations,
+                  WrapWeakPersistent(this), timestamp);
+  }
 }
 
 void VRDisplay::OnMagicWindowPose(device::mojom::blink::VRPosePtr pose) {
+  magic_window_pose_time_ = WTF::TimeTicks::Now();
   if (!in_animation_frame_) {
     frame_pose_ = std::move(pose);
   } else {
     pending_pose_ = std::move(pose);
   }
+  if (magic_window_vsync_waiting_for_pose_) {
+    // We have a vsync waiting for a pose, run it now.
+    std::move(magic_window_vsync_waiting_for_pose_).Run();
+    magic_window_vsync_waiting_for_pose_.Reset();
+  }
 }
 
 void VRDisplay::OnPresentationProviderConnectionError() {

third_party/WebKit/Source/modules/vr/VRDisplay.h

@@ -16,6 +16,7 @@
 #include "platform/Timer.h"
 #include "platform/heap/Handle.h"
 #include "platform/wtf/Forward.h"
+#include "platform/wtf/Functional.h"
 #include "platform/wtf/text/WTFString.h"
 #include "public/platform/WebGraphicsContext3DProvider.h"
 
@@ -246,6 +247,8 @@ class VRDisplay final : public EventTargetWithInlineData,
   bool pending_presenting_vsync_ = false;
   bool pending_magic_window_vsync_ = false;
   int pending_magic_window_vsync_id_ = -1;
+  base::OnceClosure magic_window_vsync_waiting_for_pose_;
+  WTF::TimeTicks magic_window_pose_time_;
   bool in_animation_frame_ = false;
   bool did_submit_this_frame_ = false;
   bool display_blurred_ = false;