folly::SingletonVault 분석 — 등록·소멸·의존성

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class SingletonVault {
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  // 등록된 모든 singleton (type → entry)
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  folly::F14FastMap<TypeDescriptor, std::shared_ptr<SingletonHolderBase>> singletons_;
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  // 등록 순서 (역순 소멸용)
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  std::vector<TypeDescriptor> creationOrder_;
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  // 라이프사이클 상태
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  enum class State { Running, Quiescing, Dead };
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  std::atomic<State> state_;
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  // pthread_atfork 훅 등록 여부
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  bool atForkInstalled_;
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};

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static SingletonVault* SingletonVault::singleton(...);

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template <typename T>
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class Singleton {
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  Singleton() {
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    SingletonVault::singleton()->registerSingleton(
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      TypeDescriptor::create<T>(), holder_);
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  }
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};
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namespace {
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  folly::Singleton<Database> kDatabase; // ← 이 시점에 vault에 등록됨
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}

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int main() {
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  folly::SingletonVault::singleton()->registrationComplete();
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  ...
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}

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// lazy (default) — 첫 try_get 시 생성
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folly::Singleton<Foo> kFoo;
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// eager — registrationComplete에서 생성
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folly::Singleton<Foo> kFoo = folly::Singleton<Foo>().shouldEagerInit();

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void destroyInstances() {
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  state_ = State::Quiescing;
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  // creationOrder_를 역순으로 소멸
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  for (auto it = creationOrder_.rbegin(); it != creationOrder_.rend(); ++it) {
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    auto& holder = singletons_[*it];
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    holder->destroyInstance();
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  }
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  state_ = State::Dead;
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}

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// vault 초기화 시 한 번
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pthread_atfork(
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  /*prepare*/ []{ vault->lockAllSingletons(); },
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  /*parent*/  []{ vault->unlockAllSingletons(); },
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  /*child*/   []{ vault->reenableInstances(); }
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);

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// 핫패스에서 자주 쓰면
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folly::Singleton<Cache> kCache = folly::Singleton<Cache>().shouldEagerInit();
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// 가끔만 쓰면 lazy
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folly::Singleton<DebugTool> kDebug;

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class MyTest : public ::testing::Test {
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  void SetUp() override {
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    folly::SingletonVault::singleton()->destroyInstances();
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    folly::SingletonVault::singleton()->reenableInstances();
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  }
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};

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folly::Singleton<A> kA;
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folly::Singleton<B> kB([]{
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  // A를 try_get하면 vault가 dependency를 추적
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  auto a = folly::Singleton<A>::try_get();
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  return new B(a.get());
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});

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// 회피
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registrationComplete(); // first
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// ...
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registrationComplete(); // throw

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// 회피
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struct GlobalCleanup {
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  ~GlobalCleanup() {
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    folly::Singleton<Logger>::try_get()->log("bye"); // Logger 이미 소멸됨
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  }
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};
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GlobalCleanup g;

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// 회피
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static auto kLogger = folly::Singleton<Logger>::try_get();
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// vault가 Logger를 영원히 못 destroy

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// 회피
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int main() {
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  folly::Singleton<Foo>::try_get(); // 항상 nullptr
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  folly::SingletonVault::singleton()->registrationComplete();
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}