Notify Blink about start of gesture scroll through a queued event.

content/browser/renderer_host/input/touch_event_queue.cc

Index: content/browser/renderer_host/input/touch_event_queue.cc
diff --git a/content/browser/renderer_host/input/touch_event_queue.cc b/content/browser/renderer_host/input/touch_event_queue.cc
index a36520fdcb1df30f6bc05fbd2274c6c61b983a30..bebf6efa57a5a4de5fa2c55ccb3a6d65873b60d4 100644
--- a/content/browser/renderer_host/input/touch_event_queue.cc
+++ b/content/browser/renderer_host/input/touch_event_queue.cc
@@ -28,8 +28,7 @@ namespace {
 const double kAsyncTouchMoveIntervalSec = .2;
 
 // A sanity check on touches received to ensure that touch movement outside
-// the platform slop region will cause scrolling, as indicated by the event's
-// |causesScrollingIfUncanceled| bit.
+// the platform slop region will cause scrolling.
 const double kMaxConceivablePlatformSlopRegionLengthDipsSquared = 60. * 60.;
 
 TouchEventWithLatencyInfo ObtainCancelEventForTouchEvent(
@@ -491,6 +490,25 @@ void TouchEventQueue::QueueEvent(const TouchEventWithLatencyInfo& event) {
   touch_queue_.push_back(new CoalescedWebTouchEvent(event, false));
 }
 
+void TouchEventQueue::PrependTouchScrollNotification() {
+  TRACE_EVENT0("input", "TouchEventQueue::PrependTouchScrollNotification");
+
+  TouchEventWithLatencyInfo touch;
+  touch.event.type = WebInputEvent::TouchScrollStarted;
+  touch.event.uniqueTouchEventId = 0;
+  touch.event.touchesLength = 0;
+
+  // Leave the head of the queue untouched since it is an in-flight event. The
+  // queue should have an in-flight event at this moment because this method is
+  // triggered by InputRouterImpl::SendGestureEvent, which is triggered by
+  // TouchEventQueue::AckTouchEventToClient, which has just received an ack for
+  // the in-flight event.

[tdresser, 2016/04/11 18:19:38]

Should we DCHECK this here?

[mustaq, 2016/04/11 18:40:41]

Didn't add a DCHECK here because I am not 100% sure that my comment covers all
the cases. To be precise, can the InputRouterImpl::SendGestureEvent() call be
triggered by some "event" from a WebGestureDeviceTouchscreen other than a touch
ack? RenderWidgetHostImpl::ForwardGestureEventWithLatencyInfo has too many
callers to be certain about it.

+  auto it = touch_queue_.begin();
+  if (it != touch_queue_.end())
+    ++it;
+  touch_queue_.insert(it, new CoalescedWebTouchEvent(touch, false));
+}
+
 void TouchEventQueue::ProcessTouchAck(InputEventAckState ack_result,
                                       const LatencyInfo& latency_info,
                                       const uint32_t unique_touch_event_id) {
@@ -525,8 +543,14 @@ void TouchEventQueue::ProcessTouchAck(InputEventAckState ack_result,
   if (touch_queue_.empty())
     return;
 
-  DCHECK_EQ(touch_queue_.front()->coalesced_event().event.uniqueTouchEventId,
-            unique_touch_event_id);
+  // We don't care about the ordering of the acks vs the ordering of the
+  // dispatched events because we can receive the ack for event B before the ack
+  // for event A even though A was sent before B. This seems to be happening
+  // when, for example, A is acked from renderer but B isn't, so the ack for B
+  // is synthesized "locally" in InputRouter.
+  //
+  // TODO(crbug.com/600773): Bring the id checks back when dispatch triggering
+  // is sane.
 
   PopTouchEventToClient(ack_result, latency_info);
   TryForwardNextEventToRenderer();
@@ -719,38 +743,45 @@ void TouchEventQueue::FlushQueue() {
 }
 
 void TouchEventQueue::PopTouchEventToClient(InputEventAckState ack_result) {
-  AckTouchEventToClient(ack_result, PopTouchEvent(), nullptr);
+  AckTouchEventToClient(ack_result, nullptr);
 }
 
 void TouchEventQueue::PopTouchEventToClient(
     InputEventAckState ack_result,
     const LatencyInfo& renderer_latency_info) {
-  AckTouchEventToClient(ack_result, PopTouchEvent(), &renderer_latency_info);
+  AckTouchEventToClient(ack_result, &renderer_latency_info);
 }
 
 void TouchEventQueue::AckTouchEventToClient(
     InputEventAckState ack_result,
-    std::unique_ptr<CoalescedWebTouchEvent> acked_event,
     const ui::LatencyInfo* optional_latency_info) {
-  DCHECK(acked_event);
   DCHECK(!dispatching_touch_ack_);
+  DCHECK(!touch_queue_.empty());
+  std::unique_ptr<CoalescedWebTouchEvent> acked_event(touch_queue_.front());
+  DCHECK(acked_event);
+
   UpdateTouchConsumerStates(acked_event->coalesced_event().event, ack_result);
 
   // Note that acking the touch-event may result in multiple gestures being sent
   // to the renderer, or touch-events being queued.
   base::AutoReset<bool> dispatching_touch_ack(&dispatching_touch_ack_, true);
-  acked_event->DispatchAckToClient(ack_result, optional_latency_info, client_);
-}
 
-std::unique_ptr<CoalescedWebTouchEvent> TouchEventQueue::PopTouchEvent() {
-  DCHECK(!touch_queue_.empty());
-  std::unique_ptr<CoalescedWebTouchEvent> event(touch_queue_.front());
+  // Skip ack for TouchScrollStarted since it was synthesized within the queue.
+  if (acked_event->coalesced_event().event.type !=
+      WebInputEvent::TouchScrollStarted) {
+    acked_event->DispatchAckToClient(ack_result, optional_latency_info,
+                                     client_);
+  }
+
   touch_queue_.pop_front();
-  return event;
 }
 
 void TouchEventQueue::SendTouchEventImmediately(
     TouchEventWithLatencyInfo* touch) {
+  // TODO(crbug.com/600773): Hack to avoid cyclic reentry to this method.
+  if (dispatching_touch_)
+    return;
+
   // For touchmove events, compare touch points position from current event
   // to last sent event and update touch points state.
   if (touch->event.type == WebInputEvent::TouchMove) {
@@ -772,10 +803,12 @@ void TouchEventQueue::SendTouchEventImmediately(
     }
   }
 
-  if (last_sent_touchevent_)
-    *last_sent_touchevent_ = touch->event;
-  else
-    last_sent_touchevent_.reset(new WebTouchEvent(touch->event));
+  if (touch->event.type != WebInputEvent::TouchScrollStarted) {
+    if (last_sent_touchevent_)
+      *last_sent_touchevent_ = touch->event;
+    else
+      last_sent_touchevent_.reset(new WebTouchEvent(touch->event));
+  }
 
   base::AutoReset<bool> dispatching_touch(&dispatching_touch_, true);
 
@@ -805,6 +838,9 @@ void TouchEventQueue::SendTouchEventImmediately(
 
 TouchEventQueue::PreFilterResult
 TouchEventQueue::FilterBeforeForwarding(const WebTouchEvent& event) {
+  if (event.type == WebInputEvent::TouchScrollStarted)
+    return FORWARD_TO_RENDERER;
+
   if (WebTouchEventTraits::IsTouchSequenceStart(event)) {
     has_handler_for_current_sequence_ = false;
     send_touch_events_async_ = false;