text: // Copyright (c) 2012 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.
  
  #ifndef BASE_THREADING_THREAD_CHECKER_H_
  #define BASE_THREADING_THREAD_CHECKER_H_
  
  #include "base/compiler_specific.h"
  #include "base/logging.h"
  #include "base/threading/thread_checker_impl.h"
  
  // ThreadChecker is a helper class used to help verify that some methods of a
  // class are called from the same thread (for thread-affinity).
  //
  // Use the macros below instead of the ThreadChecker directly so that the unused
  // member doesn't result in an extra byte (four when padded) per instance in
  // production.
  //
  // Usage of this class should be *rare* as most classes require thread-safety
  // but not thread-affinity. Prefer base::SequenceChecker to verify thread-safe
  // access.
  //
  // Thread-affinity checks should only be required in classes that use thread-
  // local-storage or a third-party API that does.
  //
  // Prefer to encode the minimum requirements of each class instead of the
  // environment it happens to run in today. e.g. if a class requires thread-
  // safety but not thread-affinity, use a SequenceChecker even if it happens to
  // run on a SingleThreadTaskRunner today. That makes it easier to understand
  // what would need to change to turn that SingleThreadTaskRunner into a
  // SequencedTaskRunner for ease of scheduling as well as minimizes side-effects
  // if that change is made.
  //
  // Usage:
  //   class MyClass {
  //    public:
  //     MyClass() {
  //       // It's sometimes useful to detach on construction for objects that are
  //       // constructed in one place and forever after used from another
  //       // thread.
  //       DETACH_FROM_THREAD(my_thread_checker_);
  //     }
  //
  //     ~MyClass() {
  //       // ThreadChecker doesn't automatically check it's destroyed on origin
  //       // thread for the same reason it's sometimes detached in the
  //       // constructor. It's okay to destroy off thread if the owner otherwise
  //       // knows usage on the associated thread is done. If you're not
  //       // detaching in the constructor, you probably want to explicitly check
  //       // in the destructor.
  //       DCHECK_CALLED_ON_VALID_THREAD(my_thread_checker_);
  //     }
  //
  //     void MyMethod() {
  //       DCHECK_CALLED_ON_VALID_THREAD(my_thread_checker_);
  //       ... (do stuff) ...
  //     }
  //
  //    private:
  //     THREAD_CHECKER(my_thread_checker_);
  //   }
  
  #if DCHECK_IS_ON()
  #define THREAD_CHECKER(name) base::ThreadChecker name
  #define DCHECK_CALLED_ON_VALID_THREAD(name) DCHECK((name).CalledOnValidThread())
  #define DETACH_FROM_THREAD(name) (name).DetachFromThread()
  #else  // DCHECK_IS_ON()
  #define THREAD_CHECKER(name) static_assert(true, "")
  #define DCHECK_CALLED_ON_VALID_THREAD(name) EAT_STREAM_PARAMETERS
  #define DETACH_FROM_THREAD(name)
  #endif  // DCHECK_IS_ON()
  
  namespace base {
  
  // Do nothing implementation, for use in release mode.
  //
  // Note: You should almost always use the ThreadChecker class (through the above
  // macros) to get the right version for your build configuration.
  class ThreadCheckerDoNothing {
   public:
    ThreadCheckerDoNothing() = default;
    bool CalledOnValidThread() const WARN_UNUSED_RESULT { return true; }
    void DetachFromThread() {}
  
   private:
    DISALLOW_COPY_AND_ASSIGN(ThreadCheckerDoNothing);
  };
  
  // Note that ThreadCheckerImpl::CalledOnValidThread() returns false when called
  // from tasks posted to SingleThreadTaskRunners bound to different sequences,
  // even if the tasks happen to run on the same thread (e.g. two independent
  // SingleThreadTaskRunners on the ThreadPool that happen to share a thread).
  #if DCHECK_IS_ON()
  class ThreadChecker : public ThreadCheckerImpl {
  };
  #else
  class ThreadChecker : public ThreadCheckerDoNothing {
  };
  #endif  // DCHECK_IS_ON()
  
  }  // namespace base
  
  #endif  // BASE_THREADING_THREAD_CHECKER_H_
  