Add a warning to the top of base/memory/singleton.h

base/memory/singleton.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// PLEASE READ: Do you really need a singleton?
+//
+// Singletons make it hard to determine the lifetime of an object, which can
+// lead to buggy code and spurious crashes.
+//
+// Instead of adding another singleton into the mix, try to identify either:
+//   a) An existing singleton that can manage your object's lifetime
+//   b) Locations where you can deterministically create the object and pass
+//      into other objects
+//
+// If you absolutely need a singleton, please keep them as trivial as possible
+// and ideally a leaf dependency. Singletons get problematic when they attempt
+// to do too much in their destructor or have circular dependencies.
+
 #ifndef BASE_MEMORY_SINGLETON_H_
 #define BASE_MEMORY_SINGLETON_H_
 #pragma once
 
 #include "base/at_exit.h"
 #include "base/atomicops.h"
 #include "base/third_party/dynamic_annotations/dynamic_annotations.h"
 #include "base/threading/platform_thread.h"
 #include "base/threading/thread_restrictions.h"
 
@@ skipping 247 matching lines @@
     instance_ = 0;
   }
   static base::subtle::AtomicWord instance_;
 };
 
 template <typename Type, typename Traits, typename DifferentiatingType>
 base::subtle::AtomicWord Singleton<Type, Traits, DifferentiatingType>::
     instance_ = 0;
 
 #endif  // BASE_MEMORY_SINGLETON_H_