Fix EventBase destruction race in FiberManagerMap

Summary:
Previously we could be reading from thread-local FiberManagerMap while it was modified.
This is now fixed by keeping a per-thread list of EventBases which need to be removed from local maps. On the fast-path no action is taken, since list will be empty.

Reviewed By: yfeldblum

Differential Revision: D2853921

fb-gh-sync-id: f05e1924dd2b97bfb359537de1909bbe193e0cb9

folly/AtomicIntrusiveLinkedList.h

+/*
+ * Copyright 2016 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#pragma once
+
+#include <atomic>
+
+#include <glog/logging.h>
+
+namespace folly {
+
+/**
+ * A very simple atomic single-linked list primitive.
+ *
+ * Usage:
+ *
+ * class MyClass {
+ *   AtomicIntrusiveLinkedListHook<MyClass> hook_;
+ * }
+ *
+ * AtomicIntrusiveLinkedList<MyClass, &MyClass::hook_> list;
+ * list.insert(&a);
+ * list.sweep([] (MyClass* c) { doSomething(c); }
+ */
+template <class T>
+struct AtomicIntrusiveLinkedListHook {
+  T* next{nullptr};
+};
+
+template <class T, AtomicIntrusiveLinkedListHook<T> T::*HookMember>
+class AtomicIntrusiveLinkedList {
+ public:
+  AtomicIntrusiveLinkedList() {}
+  AtomicIntrusiveLinkedList(const AtomicIntrusiveLinkedList&) = delete;
+  AtomicIntrusiveLinkedList& operator=(const AtomicIntrusiveLinkedList&) =
+      delete;
+  AtomicIntrusiveLinkedList(AtomicIntrusiveLinkedList&& other) noexcept {
+    *this = std::move(other);
+  }
+  AtomicIntrusiveLinkedList& operator=(
+      AtomicIntrusiveLinkedList&& other) noexcept {
+    auto tmp = other.head_.load();
+    other.head_ = head_.load();
+    head_ = tmp;
+
+    return *this;
+  }
+
+  /**
+   * Note: list must be empty on destruction.
+   */
+  ~AtomicIntrusiveLinkedList() { DCHECK(empty()); }
+
+  bool empty() const { return head_ == nullptr; }
+
+  /**
+   * Atomically insert t at the head of the list.
+   * @return True if the inserted element is the only one in the list
+   *         after the call.
+   */
+  bool insertHead(T* t) {
+    DCHECK(next(t) == nullptr);
+
+    auto oldHead = head_.load(std::memory_order_relaxed);
+    do {
+      next(t) = oldHead;
+      /* oldHead is updated by the call below.
+
+         NOTE: we don't use next(t) instead of oldHead directly due to
+         compiler bugs (GCC prior to 4.8.3 (bug 60272), clang (bug 18899),
+         MSVC (bug 819819); source:
+         http://en.cppreference.com/w/cpp/atomic/atomic/compare_exchange */
+    } while (!head_.compare_exchange_weak(
+        oldHead, t, std::memory_order_release, std::memory_order_relaxed));
+
+    return oldHead == nullptr;
+  }
+
+  /**
+   * Repeatedly replaces the head with nullptr,
+   * and calls func() on the removed elements in the order from tail to head.
+   * Stops when the list is empty.
+   */
+  template <typename F>
+  void sweep(F&& func) {
+    while (auto head = head_.exchange(nullptr)) {
+      auto rhead = reverse(head);
+      while (rhead != nullptr) {
+        auto t = rhead;
+        rhead = next(t);
+        next(t) = nullptr;
+        func(t);
+      }
+    }
+  }
+
+ private:
+  std::atomic<T*> head_{nullptr};
+
+  static T*& next(T* t) { return (t->*HookMember).next; }
+
+  /* Reverses a linked list, returning the pointer to the new head
+     (old tail) */
+  static T* reverse(T* head) {
+    T* rhead = nullptr;
+    while (head != nullptr) {
+      auto t = head;
+      head = next(t);
+      next(t) = rhead;
+      rhead = t;
+    }
+    return rhead;
+  }
+};
+
+} // namespace folly

folly/AtomicLinkedList.h

@@ -13,13 +13,14 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-
-#ifndef FOLLY_ATOMIC_LINKED_LIST_H_
-#define FOLLY_ATOMIC_LINKED_LIST_H_
+#pragma once
 
 #include <atomic>
 #include <cassert>
 
+#include <folly/AtomicIntrusiveLinkedList.h>
+#include <folly/Memory.h>
+
 namespace folly {
 
 /**
@@ -27,112 +28,59 @@ namespace folly {
  *
  * Usage:
  *
- * class MyClass {
- *   AtomicLinkedListHook<MyClass> hook_;
- * }
- *
- * AtomicLinkedList<MyClass, &MyClass::hook_> list;
- * list.insert(&a);
- * list.sweep([] (MyClass* c) { doSomething(c); }
+ * AtomicLinkedList<MyClass> list;
+ * list.insert(a);
+ * list.sweep([] (MyClass& c) { doSomething(c); }
  */
-template <class T>
-struct AtomicLinkedListHook {
-  T* next{nullptr};
-};
 
-template <class T, AtomicLinkedListHook<T> T::* HookMember>
+template <class T>
 class AtomicLinkedList {
  public:
   AtomicLinkedList() {}
   AtomicLinkedList(const AtomicLinkedList&) = delete;
   AtomicLinkedList& operator=(const AtomicLinkedList&) = delete;
-  AtomicLinkedList(AtomicLinkedList&& other) noexcept {
-    auto tmp = other.head_.load();
-    other.head_ = head_.load();
-    head_ = tmp;
-  }
-  AtomicLinkedList& operator=(AtomicLinkedList&& other) noexcept {
-    auto tmp = other.head_.load();
-    other.head_ = head_.load();
-    head_ = tmp;
+  AtomicLinkedList(AtomicLinkedList&& other) noexcept = default;
+  AtomicLinkedList& operator=(AtomicLinkedList&& other) = default;
 
-    return *this;
-  }
-
-  /**
-   * Note: list must be empty on destruction.
-   */
   ~AtomicLinkedList() {
-    assert(empty());
+    sweep([](T&&) {});
   }
 
-  bool empty() const {
-    return head_ == nullptr;
-  }
+  bool empty() const { return list_.empty(); }
 
   /**
    * Atomically insert t at the head of the list.
    * @return True if the inserted element is the only one in the list
    *         after the call.
    */
-  bool insertHead(T* t) {
-    assert(next(t) == nullptr);
-
-    auto oldHead = head_.load(std::memory_order_relaxed);
-    do {
-      next(t) = oldHead;
-      /* oldHead is updated by the call below.
+  bool insertHead(T t) {
+    auto wrapper = folly::make_unique<Wrapper>(std::move(t));
 
-         NOTE: we don't use next(t) instead of oldHead directly due to
-         compiler bugs (GCC prior to 4.8.3 (bug 60272), clang (bug 18899),
-         MSVC (bug 819819); source:
-         http://en.cppreference.com/w/cpp/atomic/atomic/compare_exchange */
-    } while (!head_.compare_exchange_weak(oldHead, t,
-                                          std::memory_order_release,
-                                          std::memory_order_relaxed));
-
-    return oldHead == nullptr;
+    return list_.insertHead(wrapper.release());
   }
 
   /**
-   * Repeatedly replaces the head with nullptr,
+   * Repeatedly pops element from head,
    * and calls func() on the removed elements in the order from tail to head.
    * Stops when the list is empty.
    */
   template <typename F>
   void sweep(F&& func) {
-    while (auto head = head_.exchange(nullptr)) {
-      auto rhead = reverse(head);
-      while (rhead != nullptr) {
-        auto t = rhead;
-        rhead = next(t);
-        next(t) = nullptr;
-        func(t);
-      }
-    }
+    list_.sweep([&](Wrapper* wrapperPtr) mutable {
+      std::unique_ptr<Wrapper> wrapper(wrapperPtr);
+
+      func(std::move(wrapper->data));
+    });
   }
 
  private:
-  std::atomic<T*> head_{nullptr};
+  struct Wrapper {
+    explicit Wrapper(T&& t) : data(std::move(t)) {}
 
-  static T*& next(T* t) {
-    return (t->*HookMember).next;
-  }
-
-  /* Reverses a linked list, returning the pointer to the new head
-     (old tail) */
-  static T* reverse(T* head) {
-    T* rhead = nullptr;
-    while (head != nullptr) {
-      auto t = head;
-      head = next(t);
-      next(t) = rhead;
-      rhead = t;
-    }
-    return rhead;
-  }
+    AtomicIntrusiveLinkedListHook<Wrapper> hook;
+    T data;
+  };
+  AtomicIntrusiveLinkedList<Wrapper, &Wrapper::hook> list_;
 };
 
 } // namespace folly
-
-#endif

folly/Makefile.am

@@ -27,6 +27,7 @@ nobase_follyinclude_HEADERS = \
 	AtomicHashArray-inl.h \
 	AtomicHashMap.h \
 	AtomicHashMap-inl.h \
+	AtomicIntrusiveLinkedList.h \
 	AtomicLinkedList.h \
 	AtomicStruct.h \
 	AtomicUnorderedMap.h \

folly/experimental/fibers/Fiber.h

@@ -20,7 +20,7 @@
 
 #include <boost/context/all.hpp>
 #include <boost/version.hpp>
-#include <folly/AtomicLinkedList.h>
+#include <folly/AtomicIntrusiveLinkedList.h>
 #include <folly/CPortability.h>
 #include <folly/IntrusiveList.h>
 #include <folly/experimental/fibers/BoostContextCompatibility.h>
@@ -126,7 +126,7 @@ class Fiber {
   /**
    * Points to next fiber in remote ready list
    */
-  folly::AtomicLinkedListHook<Fiber> nextRemoteReady_;
+  folly::AtomicIntrusiveLinkedListHook<Fiber> nextRemoteReady_;
 
   static constexpr size_t kUserBufferSize = 256;
   std::aligned_storage<kUserBufferSize>::type userBuffer_;

folly/experimental/fibers/FiberManager.h

@@ -23,7 +23,7 @@
 #include <unordered_set>
 #include <vector>
 
-#include <folly/AtomicLinkedList.h>
+#include <folly/AtomicIntrusiveLinkedList.h>
 #include <folly/Executor.h>
 #include <folly/Likely.h>
 #include <folly/IntrusiveList.h>
@@ -307,7 +307,7 @@ class FiberManager : public ::folly::Executor {
     std::function<void()> func;
     std::unique_ptr<Fiber::LocalData> localData;
     std::shared_ptr<RequestContext> rcontext;
-    AtomicLinkedListHook<RemoteTask> nextRemoteTask;
+    AtomicIntrusiveLinkedListHook<RemoteTask> nextRemoteTask;
   };
 
   typedef folly::IntrusiveList<Fiber, &Fiber::listHook_> FiberTailQueue;
@@ -414,9 +414,10 @@ class FiberManager : public ::folly::Executor {
 
   ExceptionCallback exceptionCallback_; /**< task exception callback */
 
-  folly::AtomicLinkedList<Fiber, &Fiber::nextRemoteReady_> remoteReadyQueue_;
+  folly::AtomicIntrusiveLinkedList<Fiber, &Fiber::nextRemoteReady_>
+      remoteReadyQueue_;
 
-  folly::AtomicLinkedList<RemoteTask, &RemoteTask::nextRemoteTask>
+  folly::AtomicIntrusiveLinkedList<RemoteTask, &RemoteTask::nextRemoteTask>
       remoteTaskQueue_;
 
   std::shared_ptr<TimeoutController> timeoutManager_;

folly/experimental/fibers/FiberManagerMap.cpp

@@ -15,96 +15,140 @@
  */
 #include "FiberManagerMap.h"
 
-#include <cassert>
 #include <memory>
 #include <unordered_map>
 
+#include <folly/AtomicLinkedList.h>
 #include <folly/ThreadLocal.h>
 
 namespace folly { namespace fibers {
 
 namespace {
 
-// Leak these intentionally.  During shutdown, we may call getFiberManager, and
-// want access to the fiber managers during that time.
-class LocalFiberManagerMapTag;
-typedef folly::ThreadLocal<
-    std::unordered_map<folly::EventBase*, FiberManager*>,
-    LocalFiberManagerMapTag>
-  LocalMapType;
-LocalMapType* localFiberManagerMap() {
-  static auto ret = new LocalMapType();
-  return ret;
-}
+class OnEventBaseDestructionCallback : public EventBase::LoopCallback {
+ public:
+  explicit OnEventBaseDestructionCallback(EventBase& evb) : evb_(evb) {}
+  void runLoopCallback() noexcept override;
 
-typedef
-  std::unordered_map<folly::EventBase*, std::unique_ptr<FiberManager>>
-  MapType;
-MapType* fiberManagerMap() {
-  static auto ret = new MapType();
-  return ret;
-}
+ private:
+  EventBase& evb_;
+};
 
-std::mutex* fiberManagerMapMutex() {
-  static auto ret = new std::mutex();
-  return ret;
-}
+class GlobalCache {
+ public:
+  static FiberManager& get(EventBase& evb, const FiberManager::Options& opts) {
+    return instance().getImpl(evb, opts);
+  }
+
+  static std::unique_ptr<FiberManager> erase(EventBase& evb) {
+    return instance().eraseImpl(evb);
+  }
 
+ private:
+  GlobalCache() {}
 
-class OnEventBaseDestructionCallback : public folly::EventBase::LoopCallback {
- public:
-  explicit OnEventBaseDestructionCallback(folly::EventBase& evb)
-           : evb_(&evb) {}
-  void runLoopCallback() noexcept override {
-    for (auto& localMap : localFiberManagerMap()->accessAllThreads()) {
-      localMap.erase(evb_);
+  // Leak this intentionally. During shutdown, we may call getFiberManager,
+  // and want access to the fiber managers during that time.
+  static GlobalCache& instance() {
+    static auto ret = new GlobalCache();
+    return *ret;
+  }
+
+  FiberManager& getImpl(EventBase& evb, const FiberManager::Options& opts) {
+    std::lock_guard<std::mutex> lg(mutex_);
+
+    auto& fmPtrRef = map_[&evb];
+
+    if (!fmPtrRef) {
+      auto loopController = make_unique<EventBaseLoopController>();
+      loopController->attachEventBase(evb);
+      evb.runOnDestruction(new OnEventBaseDestructionCallback(evb));
+
+      fmPtrRef = make_unique<FiberManager>(std::move(loopController), opts);
     }
-    std::unique_ptr<FiberManager> fm;
-    {
-      std::lock_guard<std::mutex> lg(*fiberManagerMapMutex());
-      auto it = fiberManagerMap()->find(evb_);
-      assert(it != fiberManagerMap()->end());
-      fm = std::move(it->second);
-      fiberManagerMap()->erase(it);
+
+    return *fmPtrRef;
+  }
+
+  std::unique_ptr<FiberManager> eraseImpl(EventBase& evb) {
+    std::lock_guard<std::mutex> lg(mutex_);
+
+    DCHECK(map_.find(&evb) != map_.end());
+
+    auto ret = std::move(map_[&evb]);
+    map_.erase(&evb);
+    return ret;
+  }
+
+  std::mutex mutex_;
+  std::unordered_map<EventBase*, std::unique_ptr<FiberManager>> map_;
+};
+
+class LocalCache {
+ public:
+  static FiberManager& get(EventBase& evb, const FiberManager::Options& opts) {
+    return instance()->getImpl(evb, opts);
+  }
+
+  static void erase(EventBase& evb) {
+    for (auto& localInstance : instance().accessAllThreads()) {
+      localInstance.removedEvbs_.insertHead(&evb);
     }
-    assert(fm.get() != nullptr);
-    fm->loopUntilNoReady();
-    delete this;
   }
+
  private:
-  folly::EventBase* evb_;
-};
+  LocalCache() {}
 
-FiberManager* getFiberManagerThreadSafe(folly::EventBase& evb,
-                                        const FiberManager::Options& opts) {
-  std::lock_guard<std::mutex> lg(*fiberManagerMapMutex());
+  struct LocalCacheTag {};
+  using ThreadLocalCache = ThreadLocal<LocalCache, LocalCacheTag>;
 
-  auto it = fiberManagerMap()->find(&evb);
-  if (LIKELY(it != fiberManagerMap()->end())) {
-    return it->second.get();
+  // Leak this intentionally. During shutdown, we may call getFiberManager,
+  // and want access to the fiber managers during that time.
+  static ThreadLocalCache& instance() {
+    static auto ret = new ThreadLocalCache([]() { return new LocalCache(); });
+    return *ret;
   }
 
-  auto loopController = folly::make_unique<EventBaseLoopController>();
-  loopController->attachEventBase(evb);
-  auto fiberManager =
-      folly::make_unique<FiberManager>(std::move(loopController), opts);
-  auto result = fiberManagerMap()->emplace(&evb, std::move(fiberManager));
-  evb.runOnDestruction(new OnEventBaseDestructionCallback(evb));
-  return result.first->second.get();
+  FiberManager& getImpl(EventBase& evb, const FiberManager::Options& opts) {
+    eraseImpl();
+
+    auto& fmPtrRef = map_[&evb];
+    if (!fmPtrRef) {
+      fmPtrRef = &GlobalCache::get(evb, opts);
+    }
+
+    DCHECK(fmPtrRef != nullptr);
+
+    return *fmPtrRef;
+  }
+
+  void eraseImpl() {
+    if (removedEvbs_.empty()) {
+      return;
+    }
+
+    removedEvbs_.sweep([&](EventBase* evb) { map_.erase(evb); });
+  }
+
+  std::unordered_map<EventBase*, FiberManager*> map_;
+  AtomicLinkedList<EventBase*> removedEvbs_;
+};
+
+void OnEventBaseDestructionCallback::runLoopCallback() noexcept {
+  auto fm = GlobalCache::erase(evb_);
+  DCHECK(fm.get() != nullptr);
+  LocalCache::erase(evb_);
+
+  fm->loopUntilNoReady();
+
+  delete this;
 }
 
 } // namespace
 
-FiberManager& getFiberManager(folly::EventBase& evb,
+FiberManager& getFiberManager(EventBase& evb,
                               const FiberManager::Options& opts) {
-  auto it = (*localFiberManagerMap())->find(&evb);
-  if (LIKELY(it != (*localFiberManagerMap())->end())) {
-    return *(it->second);
-  }
-
-  auto fm = getFiberManagerThreadSafe(evb, opts);
-  (*localFiberManagerMap())->emplace(&evb, fm);
-  return *fm;
+  return LocalCache::get(evb, opts);
 }
 
 }}

folly/test/AtomicLinkedListTest.cpp

+/*
+ * Copyright 2016 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <thread>
+
+#include <gtest/gtest.h>
+
+#include <folly/AtomicLinkedList.h>
+
+class TestIntrusiveObject {
+ public:
+  explicit TestIntrusiveObject(size_t id__) : id_(id__) {}
+  size_t id() { return id_; }
+
+ private:
+  folly::AtomicIntrusiveLinkedListHook<TestIntrusiveObject> hook_;
+  size_t id_;
+
+ public:
+  using List = folly::AtomicIntrusiveLinkedList<TestIntrusiveObject,
+                                                &TestIntrusiveObject::hook_>;
+};
+
+TEST(AtomicIntrusiveLinkedList, Basic) {
+  TestIntrusiveObject a(1), b(2), c(3);
+
+  TestIntrusiveObject::List list;
+
+  EXPECT_TRUE(list.empty());
+
+  {
+    EXPECT_TRUE(list.insertHead(&a));
+    EXPECT_FALSE(list.insertHead(&b));
+
+    EXPECT_FALSE(list.empty());
+
+    size_t id = 0;
+    list.sweep(
+        [&](TestIntrusiveObject* obj) mutable { EXPECT_EQ(++id, obj->id()); });
+
+    EXPECT_TRUE(list.empty());
+  }
+
+  // Try re-inserting the same item (b) and a new item (c)
+  {
+    EXPECT_TRUE(list.insertHead(&b));
+    EXPECT_FALSE(list.insertHead(&c));
+
+    EXPECT_FALSE(list.empty());
+
+    size_t id = 1;
+    list.sweep(
+        [&](TestIntrusiveObject* obj) mutable { EXPECT_EQ(++id, obj->id()); });
+
+    EXPECT_TRUE(list.empty());
+  }
+
+  TestIntrusiveObject::List movedList = std::move(list);
+}
+
+TEST(AtomicIntrusiveLinkedList, Move) {
+  TestIntrusiveObject a(1), b(2);
+
+  TestIntrusiveObject::List list1;
+
+  EXPECT_TRUE(list1.insertHead(&a));
+  EXPECT_FALSE(list1.insertHead(&b));
+
+  EXPECT_FALSE(list1.empty());
+
+  TestIntrusiveObject::List list2(std::move(list1));
+
+  EXPECT_TRUE(list1.empty());
+  EXPECT_FALSE(list2.empty());
+
+  TestIntrusiveObject::List list3;
+
+  EXPECT_TRUE(list3.empty());
+
+  list3 = std::move(list2);
+
+  EXPECT_TRUE(list2.empty());
+  EXPECT_FALSE(list3.empty());
+
+  size_t id = 0;
+  list3.sweep(
+      [&](TestIntrusiveObject* obj) mutable { EXPECT_EQ(++id, obj->id()); });
+}
+
+TEST(AtomicIntrusiveLinkedList, Stress) {
+  constexpr size_t kNumThreads = 32;
+  constexpr size_t kNumElements = 100000;
+
+  std::vector<TestIntrusiveObject> elements;
+  for (size_t i = 0; i < kNumThreads * kNumElements; ++i) {
+    elements.emplace_back(i);
+  }
+
+  TestIntrusiveObject::List list;
+
+  std::vector<std::thread> threads;
+  for (size_t threadId = 0; threadId < kNumThreads; ++threadId) {
+    threads.emplace_back(
+        [threadId, kNumThreads, kNumElements, &list, &elements]() {
+          for (size_t id = 0; id < kNumElements; ++id) {
+            list.insertHead(&elements[threadId + kNumThreads * id]);
+          }
+        });
+  }
+
+  std::vector<size_t> ids;
+  TestIntrusiveObject* prev{nullptr};
+
+  while (ids.size() < kNumThreads * kNumThreads) {
+    list.sweep([&](TestIntrusiveObject* current) {
+      ids.push_back(current->id());
+
+      if (prev && prev->id() % kNumThreads == current->id() % kNumThreads) {
+        EXPECT_EQ(prev->id() + kNumThreads, current->id());
+      }
+
+      prev = current;
+    });
+  }
+
+  std::sort(ids.begin(), ids.end());
+
+  for (size_t i = 0; i < kNumThreads * kNumElements; ++i) {
+    EXPECT_EQ(i, ids[i]);
+  }
+
+  for (auto& thread : threads) {
+    thread.join();
+  }
+}
+
+class TestObject {
+ public:
+  TestObject(size_t id__, std::shared_ptr<void> ptr) : id_(id__), ptr_(ptr) {}
+
+  size_t id() { return id_; }
+
+ private:
+  size_t id_;
+  std::shared_ptr<void> ptr_;
+};
+
+TEST(AtomicLinkedList, Basic) {
+  constexpr size_t kNumElements = 10;
+
+  using List = folly::AtomicLinkedList<TestObject>;
+  List list;
+
+  std::shared_ptr<void> ptr = std::make_shared<int>(42);
+
+  for (size_t id = 0; id < kNumElements; ++id) {
+    list.insertHead({id, ptr});
+  }
+
+  size_t counter = 0;
+
+  list.sweep([&](TestObject object) {
+    EXPECT_EQ(counter, object.id());
+
+    EXPECT_EQ(1 + kNumElements - counter, ptr.use_count());
+
+    ++counter;
+  });
+
+  EXPECT_TRUE(ptr.unique());
+}