Missing source pointer down events are handled.

ui/events/gesture_detection/gesture_detector.cc

Index: ui/events/gesture_detection/gesture_detector.cc
diff --git a/ui/events/gesture_detection/gesture_detector.cc b/ui/events/gesture_detection/gesture_detector.cc
index ef0e71f6d5dcd14368a65ebbfceae64f6e354728..bfbd73b3885bd82c4105ccf851435e7b1589be13 100644
--- a/ui/events/gesture_detection/gesture_detector.cc
+++ b/ui/events/gesture_detection/gesture_detector.cc
@@ -246,6 +246,7 @@ bool GestureDetector::OnTouchEvent(const MotionEvent& ev) {
       handled = HandleSwipeIfNeeded(ev, vx_total / count, vy_total / count);
 
       if (two_finger_tap_allowed_for_gesture_ && ev.GetPointerCount() == 2 &&
+          secondary_pointer_down_event_ &&
           (ev.GetEventTime() - secondary_pointer_down_event_->GetEventTime() <=
            two_finger_tap_timeout_)) {
         handled = listener_->OnTwoFingerTap(*current_down_event_, ev);
@@ -554,12 +555,9 @@ bool GestureDetector::IsWithinTouchSlop(const MotionEvent& ev) {
   for (size_t i = 0; i < ev.GetPointerCount(); i++) {
     const int pointer_id = ev.GetPointerId(i);
     const MotionEvent* source_pointer_down_event = GetSourcePointerDownEvent(
-        *current_down_event_,
-        (maximum_pointer_count_ > 1 && secondary_pointer_down_event_)
-            ? *secondary_pointer_down_event_
-            : *current_down_event_,
+        current_down_event_.get(), secondary_pointer_down_event_.get(),
         pointer_id);
-    DCHECK(source_pointer_down_event);
+
     if (!source_pointer_down_event)
       return false;
 
@@ -579,18 +577,22 @@ bool GestureDetector::IsWithinTouchSlop(const MotionEvent& ev) {
 }
 
 const MotionEvent* GestureDetector::GetSourcePointerDownEvent(
-    const MotionEvent& current_down_event,
-    const MotionEvent& secondary_pointer_down_event,
+    const MotionEvent* current_down_event,
+    const MotionEvent* secondary_pointer_down_event,
     const int pointer_id) {
-  if (current_down_event.GetPointerId(0) == pointer_id)
-    return &current_down_event;
+  if (current_down_event->GetPointerId(0) == pointer_id)
+    return current_down_event;
+
+  // Secondary pointer down event is missing, the source pointer down event is
+  // not found.

[tdresser, 2017/05/11 16:57:27]

Add a link to the bug, and mention that this is sometimes missing, but we don't
have a reliable repro.

[sahel, 2017/05/11 17:26:19]

Done.

+  if (!secondary_pointer_down_event)
+    return nullptr;
 
-  for (size_t i = 0; i < secondary_pointer_down_event.GetPointerCount(); i++) {
-    if (secondary_pointer_down_event.GetPointerId(i) == pointer_id)
-      return &secondary_pointer_down_event;
+  for (size_t i = 0; i < secondary_pointer_down_event->GetPointerCount(); i++) {
+    if (secondary_pointer_down_event->GetPointerId(i) == pointer_id)
+      return secondary_pointer_down_event;
   }
 
-  NOTREACHED();
   return nullptr;
 }