Add diagnostic DumpWithoutCrashing to investigate gesture event hang.

We introduce a DumpWithoutCrashing when a new gesture event is queued
while acks are being processed if the processing has already lasted
over 2 seconds. This is to test the hypothesis that the queue is
getting stuck in a loop in GestureEventQueue::AckCompletedEvents.

Bug: 818214
Change-Id: I36daa488d3f5aa5e8eee5e4f442fdb3d8445c0a7
Reviewed-on: https://chromium-review.googlesource.com/959179
Commit-Queue: Kevin McNee <mcnee@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543158}

content/browser/renderer_host/input/gesture_event_queue.cc

@@ -5,6 +5,8 @@
 #include "content/browser/renderer_host/input/gesture_event_queue.h"
 
 #include "base/auto_reset.h"
+#include "base/debug/crash_logging.h"
+#include "base/debug/dump_without_crashing.h"
 #include "base/trace_event/trace_event.h"
 #include "content/browser/renderer_host/input/touchpad_tap_suppression_controller.h"
 #include "content/browser/renderer_host/input/touchscreen_tap_suppression_controller.h"
@@ -159,8 +161,36 @@ bool GestureEventQueue::ShouldForwardForBounceReduction(
   }
 }
 
+void GestureEventQueue::ReportPossibleHang(
+    const blink::WebGestureEvent& event) {
+  if (processing_acks_ && !did_report_hang_ &&
+      (base::TimeTicks::Now() - processing_acks_start_) >
+          base::TimeDelta::FromSeconds(2)) {
+    // Diagnostic for the hang in https://crbug.com/818214
+    static auto* type_key = base::debug::AllocateCrashKeyString(
+        "gesture-event-queue-hang-event-type",
+        base::debug::CrashKeySize::Size32);
+    base::debug::ScopedCrashKeyString type_key_value(
+        type_key, std::to_string(event.GetType()));
+    static auto* device_key = base::debug::AllocateCrashKeyString(
+        "gesture-event-queue-hang-source-device",
+        base::debug::CrashKeySize::Size32);
+    base::debug::ScopedCrashKeyString device_key_value(
+        device_key, std::to_string(event.source_device));
+    static auto* size_key = base::debug::AllocateCrashKeyString(
+        "gesture-event-queue-hang-queue-size",
+        base::debug::CrashKeySize::Size32);
+    base::debug::ScopedCrashKeyString size_key_value(
+        size_key, std::to_string(coalesced_gesture_events_.size()));
+    base::debug::DumpWithoutCrashing();
+    did_report_hang_ = true;
+  }
+}
+
 void GestureEventQueue::QueueAndForwardIfNecessary(
     const GestureEventWithLatencyInfo& gesture_event) {
+  ReportPossibleHang(gesture_event.event);
+
   if (allow_multiple_inflight_events_) {
     // Event coalescing should be handled in compositor thread event queue.
     if (gesture_event.event.GetType() == WebInputEvent::kGestureFlingCancel)
@@ -252,6 +282,7 @@ void GestureEventQueue::AckCompletedEvents() {
   if (processing_acks_)
     return;
   base::AutoReset<bool> process_acks(&processing_acks_, true);
+  processing_acks_start_ = base::TimeTicks::Now();
   while (!coalesced_gesture_events_.empty()) {
     auto iter = coalesced_gesture_events_.begin();
     if (iter->ack_state() == INPUT_EVENT_ACK_STATE_UNKNOWN)

content/browser/renderer_host/input/gesture_event_queue.h

@@ -179,6 +179,9 @@ class CONTENT_EXPORT GestureEventQueue {
   // remain at the head of the queue until ack'ed.
   size_t EventsInFlightCount() const;
 
+  // TODO(818214): Remove once the cause of the hang is identified.
+  void ReportPossibleHang(const blink::WebGestureEvent& event);
+
   // The receiver of all forwarded gesture events.
   GestureEventQueueClient* client_;
 
@@ -230,6 +233,12 @@ class CONTENT_EXPORT GestureEventQueue {
   // taps.
   FlingController fling_controller_;
 
+  // Diagnostic for the hang in https://crbug.com/818214
+  // Meaningful only when processing_acks_ is true.
+  // TODO(818214): Remove once the cause of the hang is identified.
+  base::TimeTicks processing_acks_start_;
+  bool did_report_hang_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(GestureEventQueue);
 };