Track thread activities in order to diagnose hangs.

base/synchronization/waitable_event_win.cc

Index: base/synchronization/waitable_event_win.cc
diff --git a/base/synchronization/waitable_event_win.cc b/base/synchronization/waitable_event_win.cc
index 597716337be9c92fa264fecd524da9c643d9f301..cbdebbb082cd2a2ab8f223904c0aee26dc3b74a7 100644
--- a/base/synchronization/waitable_event_win.cc
+++ b/base/synchronization/waitable_event_win.cc
@@ -9,6 +9,7 @@
 
 #include <utility>
 
+#include "base/debug/activity_tracker.h"
 #include "base/logging.h"
 #include "base/numerics/safe_conversions.h"
 #include "base/threading/thread_restrictions.h"
@@ -47,6 +48,9 @@ bool WaitableEvent::IsSignaled() {
 }
 
 void WaitableEvent::Wait() {
+  // Record the event that this thread is blocking upon (for hang diagnosis).
+  base::debug::ScopedEventWaitActivity event_activity(this);
+
   base::ThreadRestrictions::AssertWaitAllowed();
   DWORD result = WaitForSingleObject(handle_.Get(), INFINITE);
   // It is most unexpected that this should ever fail.  Help consumers learn
@@ -55,6 +59,9 @@ void WaitableEvent::Wait() {
 }
 
 bool WaitableEvent::TimedWait(const TimeDelta& max_time) {
+  // Record the event that this thread is blocking upon (for hang diagnosis).
+  base::debug::ScopedEventWaitActivity event_activity(this);
+
   base::ThreadRestrictions::AssertWaitAllowed();
   DCHECK_GE(max_time, TimeDelta());
   // Truncate the timeout to milliseconds. The API specifies that this method
@@ -77,6 +84,11 @@ bool WaitableEvent::TimedWait(const TimeDelta& max_time) {
 
 // static
 size_t WaitableEvent::WaitMany(WaitableEvent** events, size_t count) {
+  DCHECK(count) << "Cannot wait on no events";
+
+  // Record an event (the first) that this thread is blocking upon.
+  base::debug::ScopedEventWaitActivity event_activity(events[0]);
+
   base::ThreadRestrictions::AssertWaitAllowed();
   HANDLE handles[MAXIMUM_WAIT_OBJECTS];
   CHECK_LE(count, static_cast<size_t>(MAXIMUM_WAIT_OBJECTS))