[Synchronization] Fix a crash in WaitableEventWatcher

base/synchronization/waitable_event_watcher_unittest.cc

Index: base/synchronization/waitable_event_watcher_unittest.cc
diff --git a/base/synchronization/waitable_event_watcher_unittest.cc b/base/synchronization/waitable_event_watcher_unittest.cc
index 9df72892c3c3813adf8d1286c7bdf6dfefe48930..362c8f2b62945713496aee9fbf9dc5c2b88ca4e5 100644
--- a/base/synchronization/waitable_event_watcher_unittest.cc
+++ b/base/synchronization/waitable_event_watcher_unittest.cc
@@ -7,10 +7,12 @@
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/macros.h"
+#include "base/memory/ptr_util.h"
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/threading/platform_thread.h"
+#include "base/threading/sequenced_task_runner_handle.h"
 #include "build/build_config.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ -38,6 +40,7 @@ class DecrementCountContainer {
   explicit DecrementCountContainer(int* counter) : counter_(counter) {
   }
   void OnWaitableEventSignaled(WaitableEvent* object) {
+    // NOTE: |object| may be already deleted.
     --(*counter_);
   }
  private:
@@ -126,15 +129,35 @@ void RunTest_DeleteUnder(MessageLoop::Type message_loop_type) {
   {
     WaitableEventWatcher watcher;
 
-    WaitableEvent* event =
-        new WaitableEvent(WaitableEvent::ResetPolicy::AUTOMATIC,
-                          WaitableEvent::InitialState::NOT_SIGNALED);
+    auto event = new WaitableEvent(WaitableEvent::ResetPolicy::AUTOMATIC,

[danakj, 2017/05/04 16:09:37]

auto*

+                                   WaitableEvent::InitialState::NOT_SIGNALED);
 
     watcher.StartWatching(event, BindOnce(&QuitWhenSignaled));
     delete event;
   }
 }
 
+void RunTest_SignalAndDelete(MessageLoop::Type message_loop_type) {
+  // Signal and immediately delete the WaitableEvent out from under the Watcher.
+
+  MessageLoop message_loop(message_loop_type);
+
+  {
+    WaitableEventWatcher watcher;
+
+    auto event = base::MakeUnique<WaitableEvent>(
+        WaitableEvent::ResetPolicy::AUTOMATIC,
+        WaitableEvent::InitialState::NOT_SIGNALED);
+
+    watcher.StartWatching(event.get(), BindOnce(&QuitWhenSignaled));
+    event->Signal();
+    event.reset();
+
+    // Wait for the watcher callback:

[danakj, 2017/05/04 16:09:38]

nit: s/:/./

+    RunLoop().Run();
+  }
+}
+
 }  // namespace
 
 //-----------------------------------------------------------------------------
@@ -169,4 +192,10 @@ TEST(WaitableEventWatcherTest, DeleteUnder) {
   }
 }
 
+TEST(WaitableEventWatcherTest, SignalAndDelete) {
+  for (int i = 0; i < kNumTestingMessageLoops; i++) {
+    RunTest_SignalAndDelete(testing_message_loops[i]);
+  }
+}
+
 }  // namespace base