folly::Singleton try_get·try_get_fast — TLS-cached 접근

1
auto p = folly::Singleton<Service>::try_get();

1
auto* p = folly::Singleton<Service>::try_get_fast(); // Service*, nullable
2
if (p) {
3
  p->doWork();
4
}

1
template <typename T>
2
class Singleton {
3
  static T* try_get_fast() noexcept {
4
    thread_local T* cached = nullptr;
5
    if (FOLLY_LIKELY(cached != nullptr)) {
6
      return cached;
7
    }
8
    // slow path
9
    auto sp = try_get();
10
    cached = sp.get();
11
    return cached;
12
  }
13
};

1
// 회피 — 핫패스마다 shared_ptr copy
2
folly::Future<Response> handle(Request req) {
3
  auto cache = folly::Singleton<Cache>::try_get();
4
  if (!cache) return errorResponse();
5
  return cache->lookup(req.key);
6
}
7

8
// Good — TLS 캐시
9
folly::Future<Response> handle(Request req) {
10
  auto* cache = folly::Singleton<Cache>::try_get_fast();
11
  if (!cache) return errorResponse();
12
  return cache->lookup(req.key);
13
}

1
// 회피
2
class Worker {
3
  Cache* cache_ = folly::Singleton<Cache>::try_get_fast();
4
  // ↑ vault destroy 후엔 dangling
5
};
6

7
// Good — 매번 try_get_fast
8
class Worker {
9
  void doWork() {
10
    auto* cache = folly::Singleton<Cache>::try_get_fast();
11
    if (!cache) return;
12
    cache->lookup(...);
13
  }
14
};

1
// 회피
2
~Worker() {
3
  folly::Singleton<Logger>::try_get_fast()->log("bye");
4
  // shutdown 중이면 nullptr (체크 안 함)
5
}
6

7
// Good — try_get 또는 null check
8
~Worker() {
9
  if (auto* logger = folly::Singleton<Logger>::try_get_fast()) {
10
    logger->log("bye");
11
  }
12
}

1
// 명시적 주석
2
// HOT PATH — try_get_fast로 ns 단위 access
3
auto* svc = folly::Singleton<Service>::try_get_fast();
4

5
// COLD PATH — init 단계, shared_ptr 안전
6
auto svc = folly::Singleton<Service>::try_get();

1
// 회피
2
folly::Singleton<X>::try_get_fast()->method(); // shutdown 직후 crash

1
// async에 stash
2
auto svc = folly::Singleton<Service>::try_get(); // shared_ptr
3
folly::makeFuture().thenValue([svc](auto) {
4
  svc->doWork(); // future 끝날 때까지 svc 유효
5
});

1
BENCHMARK(try_get) {
2
  auto sp = folly::Singleton<Foo>::try_get();
3
  doNotOptimizeAway(sp);
4
}
5

6
BENCHMARK(try_get_fast) {
7
  auto* p = folly::Singleton<Foo>::try_get_fast();
8
  doNotOptimizeAway(p);
9
}

1
// 회피 — async 콜백 안에서 raw ptr capture
2
folly::makeFuture().thenValue([cache = folly::Singleton<Cache>::try_get_fast()](auto) {
3
  cache->lookup(...); // future 실행 시점에 cache가 살아있는지?
4
});

1
// 회피
2
class Handler {
3
  Cache* cache_ = folly::Singleton<Cache>::try_get_fast(); // 한 번만 호출
4
};

1
// 회피
2
void readOnly() const {
3
  auto* c = folly::Singleton<Cache>::try_get_fast();
4
  c->mutate(); // 의도와 다른 mutation
5
}