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lazy_instance.cc

// Copyright (c) 2008 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.

#include "base/lazy_instance.h"

#include "base/at_exit.h"
#include "base/atomicops.h"
#include "base/dynamic_annotations.h"
#include "base/basictypes.h"
#include "base/platform_thread.h"

namespace base {

bool LazyInstanceHelper::NeedsInstance() {
  // Try to create the instance, if we're the first, will go from EMPTY
  // to CREATING, otherwise we've already been beaten here.
  if (base::subtle::Acquire_CompareAndSwap(
          &state_, STATE_EMPTY, STATE_CREATING) == STATE_EMPTY) {
    // Caller must create instance
    return true;
  } else {
    // It's either in the process of being created, or already created.  Spin.
    while (base::subtle::NoBarrier_Load(&state_) != STATE_CREATED)
      PlatformThread::YieldCurrentThread();
  }

  // Someone else created the instance.
  return false;
}

void LazyInstanceHelper::CompleteInstance(void* instance, void (*dtor)(void*)) {
  // See the comment to the corresponding HAPPENS_AFTER in Pointer().
  ANNOTATE_HAPPENS_BEFORE(&state_);

  // Instance is created, go from CREATING to CREATED.
  base::subtle::Release_Store(&state_, STATE_CREATED);

  // Allow reusing the LazyInstance (reset it to the initial state). This
  // makes possible calling all AtExit callbacks between tests. Assumes that
  // no other threads execute when AtExit callbacks are processed.
  base::AtExitManager::RegisterCallback(&LazyInstanceHelper::ResetState,
                                        this);

  // Make sure that the lazily instantiated object will get destroyed at exit.
  base::AtExitManager::RegisterCallback(dtor, instance);
}

// static
void LazyInstanceHelper::ResetState(void* helper) {
  reinterpret_cast<LazyInstanceHelper*>(helper)->state_ = STATE_EMPTY;
}

}  // namespace base

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