[Synchronization] Fix a crash in WaitableEventWatcher

base/synchronization/waitable_event_watcher_win.cc

Index: base/synchronization/waitable_event_watcher_win.cc
diff --git a/base/synchronization/waitable_event_watcher_win.cc b/base/synchronization/waitable_event_watcher_win.cc
index f3e114e3966149c610d5579ec69fddaf32555c5f..d28b6a765743cc2cc41d49790be1ca1d0afd58ab 100644
--- a/base/synchronization/waitable_event_watcher_win.cc
+++ b/base/synchronization/waitable_event_watcher_win.cc
@@ -12,26 +12,43 @@ namespace base {
 
 WaitableEventWatcher::WaitableEventWatcher() = default;
 
-WaitableEventWatcher::~WaitableEventWatcher() {
-}
+WaitableEventWatcher::~WaitableEventWatcher() {}
 
 bool WaitableEventWatcher::StartWatching(WaitableEvent* event,
                                          EventCallback callback) {
+  DCHECK(event);
   callback_ = std::move(callback);
   event_ = event;
-  return watcher_.StartWatchingOnce(event->handle(), this);
+
+  // Duplicate and hold the event handle until a callback is returned or
+  // waiting is stopped.
+  HANDLE handle = nullptr;
+  if (!::DuplicateHandle(::GetCurrentProcess(),  // hSourceProcessHandle
+                         event->handle(),
+                         ::GetCurrentProcess(),  // hTargetProcessHandle
+                         &handle,
+                         0,      // dwDesiredAccess ignored due to SAME_ACCESS
+                         FALSE,  // !bInheritHandle
+                         DUPLICATE_SAME_ACCESS)) {
+    return false;
+  }
+  duplicated_event_handle_.Set(handle);
+  return watcher_.StartWatchingOnce(handle, this);
 }
 
 void WaitableEventWatcher::StopWatching() {
   callback_.Reset();
   event_ = NULL;
   watcher_.StopWatching();
+  duplicated_event_handle_.Close();
 }
 
 void WaitableEventWatcher::OnObjectSignaled(HANDLE h) {
+  DCHECK_EQ(duplicated_event_handle_.Get(), h);
   WaitableEvent* event = event_;
   EventCallback callback = std::move(callback_);
   event_ = NULL;
+  duplicated_event_handle_.Close();
   DCHECK(event);
 
   std::move(callback).Run(event);