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17 // SingletonVault - a library to manage the creation and destruction
18 // of interdependent singletons.
20 // Basic usage of this class is very simple; suppose you have a class
21 // called MyExpensiveService, and you only want to construct one (ie,
22 // it's a singleton), but you only want to construct it if it is used.
25 // class MyExpensiveService { ... };
28 // namespace { folly::Singleton<MyExpensiveService> the_singleton; }
30 // Code can access it via:
32 // MyExpensiveService* instance = Singleton<MyExpensiveService>::get();
34 // std::weak_ptr<MyExpensiveService> instance =
35 // Singleton<MyExpensiveService>::get_weak();
37 // You also can directly access it by the variable defining the
38 // singleton rather than via get(), and even treat that variable like
39 // a smart pointer (dereferencing it or using the -> operator).
41 // Please note, however, that all non-weak_ptr interfaces are
42 // inherently subject to races with destruction. Use responsibly.
44 // The singleton will be created on demand. If the constructor for
45 // MyExpensiveService actually makes use of *another* Singleton, then
46 // the right thing will happen -- that other singleton will complete
47 // construction before get() returns. However, in the event of a
48 // circular dependency, a runtime error will occur.
50 // You can have multiple singletons of the same underlying type, but
51 // each must be given a unique tag. If no tag is specified - default tag is used
56 // folly::Singleton<MyExpensiveService> s_default;
57 // folly::Singleton<MyExpensiveService, Tag1> s1;
58 // folly::Singleton<MyExpensiveService, Tag2> s2;
61 // MyExpensiveService* svc_default = s_default.get();
62 // MyExpensiveService* svc1 = s1.get();
63 // MyExpensiveService* svc2 = s2.get();
65 // By default, the singleton instance is constructed via new and
66 // deleted via delete, but this is configurable:
68 // namespace { folly::Singleton<MyExpensiveService> the_singleton(create,
71 // Where create and destroy are functions, Singleton<T>::CreateFunc
72 // Singleton<T>::TeardownFunc.
74 // What if you need to destroy all of your singletons? Say, some of
75 // your singletons manage threads, but you need to fork? Or your unit
76 // test wants to clean up all global state? Then you can call
77 // SingletonVault::singleton()->destroyInstances(), which invokes the
78 // TeardownFunc for each singleton, in the reverse order they were
79 // created. It is your responsibility to ensure your singletons can
80 // handle cases where the singletons they depend on go away, however.
81 // Singletons won't be recreated after destroyInstances call. If you
82 // want to re-enable singleton creation (say after fork was called) you
83 // should call reenableInstances.
86 #include <folly/Baton.h>
87 #include <folly/Exception.h>
88 #include <folly/Hash.h>
89 #include <folly/Memory.h>
90 #include <folly/RWSpinLock.h>
91 #include <folly/Demangle.h>
92 #include <folly/io/async/Request.h>
98 #include <condition_variable>
100 #include <unordered_map>
101 #include <functional>
105 #include <glog/logging.h>
109 // For actual usage, please see the Singleton<T> class at the bottom
110 // of this file; that is what you will actually interact with.
112 // SingletonVault is the class that manages singleton instances. It
113 // is unaware of the underlying types of singletons, and simply
114 // manages lifecycles and invokes CreateFunc and TeardownFunc when
115 // appropriate. In general, you won't need to interact with the
116 // SingletonVault itself.
118 // A vault goes through a few stages of life:
120 // 1. Registration phase; singletons can be registered, but no
121 // singleton can be created.
122 // 2. registrationComplete() has been called; singletons can no
123 // longer be registered, but they can be created.
124 // 3. A vault can return to stage 1 when destroyInstances is called.
126 // In general, you don't need to worry about any of the above; just
127 // ensure registrationComplete() is called near the top of your main()
128 // function, otherwise no singletons can be instantiated.
130 class SingletonVault;
134 struct DefaultTag {};
136 // A TypeDescriptor is the unique handle for a given singleton. It is
137 // a combinaiton of the type and of the optional name, and is used as
138 // a key in unordered_maps.
139 class TypeDescriptor {
141 TypeDescriptor(const std::type_info& ti,
142 const std::type_info& tag_ti)
143 : ti_(ti), tag_ti_(tag_ti) {
146 TypeDescriptor(const TypeDescriptor& other)
147 : ti_(other.ti_), tag_ti_(other.tag_ti_) {
150 TypeDescriptor& operator=(const TypeDescriptor& other) {
151 if (this != &other) {
153 tag_ti_ = other.tag_ti_;
159 std::string name() const {
160 auto ret = demangle(ti_.name());
161 if (tag_ti_ != std::type_index(typeid(DefaultTag))) {
163 ret += demangle(tag_ti_.name());
165 return ret.toStdString();
168 friend class TypeDescriptorHasher;
170 bool operator==(const TypeDescriptor& other) const {
171 return ti_ == other.ti_ && tag_ti_ == other.tag_ti_;
176 std::type_index tag_ti_;
179 class TypeDescriptorHasher {
181 size_t operator()(const TypeDescriptor& ti) const {
182 return folly::hash::hash_combine(ti.ti_, ti.tag_ti_);
186 // This interface is used by SingletonVault to interact with SingletonHolders.
187 // Having a non-template interface allows SingletonVault to keep a list of all
189 class SingletonHolderBase {
191 virtual ~SingletonHolderBase() = default;
193 virtual TypeDescriptor type() = 0;
194 virtual bool hasLiveInstance() = 0;
195 virtual void destroyInstance() = 0;
198 static constexpr std::chrono::seconds kDestroyWaitTime{5};
201 // An actual instance of a singleton, tracking the instance itself,
202 // its state as described above, and the create and teardown
204 template <typename T>
205 struct SingletonHolder : public SingletonHolderBase {
207 typedef std::function<void(T*)> TeardownFunc;
208 typedef std::function<T*(void)> CreateFunc;
210 template <typename Tag, typename VaultTag>
211 inline static SingletonHolder<T>& singleton();
214 inline std::weak_ptr<T> get_weak();
216 void registerSingleton(CreateFunc c, TeardownFunc t);
217 void registerSingletonMock(CreateFunc c, TeardownFunc t);
218 virtual TypeDescriptor type();
219 virtual bool hasLiveInstance();
220 virtual void destroyInstance();
223 SingletonHolder(TypeDescriptor type, SingletonVault& vault);
225 void createInstance();
227 enum class SingletonHolderState {
233 TypeDescriptor type_;
234 SingletonVault& vault_;
236 // mutex protects the entire entry during construction/destruction
239 // State of the singleton entry. If state is Living, instance_ptr and
240 // instance_weak can be safely accessed w/o synchronization.
241 std::atomic<SingletonHolderState> state_{SingletonHolderState::NotRegistered};
243 // the thread creating the singleton (only valid while creating an object)
244 std::thread::id creating_thread_;
246 // The singleton itself and related functions.
248 // holds a shared_ptr to singleton instance, set when state is changed from
249 // Dead to Living. Reset when state is changed from Living to Dead.
250 std::shared_ptr<T> instance_;
251 // weak_ptr to the singleton instance, set when state is changed from Dead
252 // to Living. We never write to this object after initialization, so it is
253 // safe to read it from different threads w/o synchronization if we know
254 // that state is set to Living
255 std::weak_ptr<T> instance_weak_;
256 // Time we wait on destroy_baton after releasing Singleton shared_ptr.
257 std::shared_ptr<folly::Baton<>> destroy_baton_;
258 T* instance_ptr_ = nullptr;
259 CreateFunc create_ = nullptr;
260 TeardownFunc teardown_ = nullptr;
262 std::shared_ptr<std::atomic<bool>> print_destructor_stack_trace_;
264 SingletonHolder(const SingletonHolder&) = delete;
265 SingletonHolder& operator=(const SingletonHolder&) = delete;
266 SingletonHolder& operator=(SingletonHolder&&) = delete;
267 SingletonHolder(SingletonHolder&&) = delete;
272 class SingletonVault {
274 enum class Type { Strict, Relaxed };
276 explicit SingletonVault(Type type = Type::Relaxed) : type_(type) {}
278 // Destructor is only called by unit tests to check destroyInstances.
281 typedef std::function<void(void*)> TeardownFunc;
282 typedef std::function<void*(void)> CreateFunc;
284 // Ensure that Singleton has not been registered previously and that
285 // registration is not complete. If validations succeeds,
286 // register a singleton of a given type with the create and teardown
288 void registerSingleton(detail::SingletonHolderBase* entry) {
289 RWSpinLock::ReadHolder rh(&stateMutex_);
291 stateCheck(SingletonVaultState::Running);
293 if (UNLIKELY(registrationComplete_)) {
294 throw std::logic_error(
295 "Registering singleton after registrationComplete().");
298 RWSpinLock::ReadHolder rhMutex(&mutex_);
299 CHECK_THROW(singletons_.find(entry->type()) == singletons_.end(),
302 RWSpinLock::UpgradedHolder wh(&mutex_);
303 singletons_[entry->type()] = entry;
306 // Mark registration is complete; no more singletons can be
307 // registered at this point.
308 void registrationComplete() {
309 RequestContext::saveContext();
310 std::atexit([](){ SingletonVault::singleton()->destroyInstances(); });
312 RWSpinLock::WriteHolder wh(&stateMutex_);
314 stateCheck(SingletonVaultState::Running);
316 if (type_ == Type::Strict) {
317 for (const auto& p: singletons_) {
318 if (p.second->hasLiveInstance()) {
319 throw std::runtime_error(
320 "Singleton created before registration was complete.");
325 registrationComplete_ = true;
328 // Destroy all singletons; when complete, the vault can't create
329 // singletons once again until reenableInstances() is called.
330 void destroyInstances();
332 // Enable re-creating singletons after destroyInstances() was called.
333 void reenableInstances();
335 // For testing; how many registered and living singletons we have.
336 size_t registeredSingletonCount() const {
337 RWSpinLock::ReadHolder rh(&mutex_);
339 return singletons_.size();
342 size_t livingSingletonCount() const {
343 RWSpinLock::ReadHolder rh(&mutex_);
346 for (const auto& p : singletons_) {
347 if (p.second->hasLiveInstance()) {
355 // A well-known vault; you can actually have others, but this is the
357 static SingletonVault* singleton() {
358 return singleton<>();
361 // Gets singleton vault for any Tag. Non-default tag should be used in unit
363 template <typename VaultTag = detail::DefaultTag>
364 static SingletonVault* singleton() {
365 static SingletonVault* vault = new SingletonVault();
369 typedef std::string(*StackTraceGetterPtr)();
371 static std::atomic<StackTraceGetterPtr>& stackTraceGetter() {
372 static std::atomic<StackTraceGetterPtr> stackTraceGetterPtr;
373 return stackTraceGetterPtr;
377 template <typename T>
378 friend struct detail::SingletonHolder;
380 // The two stages of life for a vault, as mentioned in the class comment.
381 enum class SingletonVaultState {
386 // Each singleton in the vault can be in two states: dead
387 // (registered but never created), living (CreateFunc returned an instance).
389 void stateCheck(SingletonVaultState expected,
390 const char* msg="Unexpected singleton state change") {
391 if (expected != state_) {
392 throw std::logic_error(msg);
396 // This method only matters if registrationComplete() is never called.
397 // Otherwise destroyInstances is scheduled to be executed atexit.
399 // Initializes static object, which calls destroyInstances on destruction.
400 // Used to have better deletion ordering with singleton not managed by
401 // folly::Singleton. The desruction will happen in the following order:
402 // 1. Singletons, not managed by folly::Singleton, which were created after
403 // any of the singletons managed by folly::Singleton was requested.
404 // 2. All singletons managed by folly::Singleton
405 // 3. Singletons, not managed by folly::Singleton, which were created before
406 // any of the singletons managed by folly::Singleton was requested.
407 static void scheduleDestroyInstances();
409 typedef std::unordered_map<detail::TypeDescriptor,
410 detail::SingletonHolderBase*,
411 detail::TypeDescriptorHasher> SingletonMap;
413 mutable folly::RWSpinLock mutex_;
414 SingletonMap singletons_;
415 std::vector<detail::TypeDescriptor> creation_order_;
416 SingletonVaultState state_{SingletonVaultState::Running};
417 bool registrationComplete_{false};
418 folly::RWSpinLock stateMutex_;
419 Type type_{Type::Relaxed};
422 // This is the wrapper class that most users actually interact with.
423 // It allows for simple access to registering and instantiating
424 // singletons. Create instances of this class in the global scope of
425 // type Singleton<T> to register your singleton for later access via
426 // Singleton<T>::get().
427 template <typename T,
428 typename Tag = detail::DefaultTag,
429 typename VaultTag = detail::DefaultTag /* for testing */>
432 typedef std::function<T*(void)> CreateFunc;
433 typedef std::function<void(T*)> TeardownFunc;
435 // Generally your program life cycle should be fine with calling
436 // get() repeatedly rather than saving the reference, and then not
437 // call get() during process shutdown.
439 return getEntry().get();
442 // If, however, you do need to hold a reference to the specific
443 // singleton, you can try to do so with a weak_ptr. Avoid this when
444 // possible but the inability to lock the weak pointer can be a
445 // signal that the vault has been destroyed.
446 static std::weak_ptr<T> get_weak() {
447 return getEntry().get_weak();
450 // Allow the Singleton<t> instance to also retrieve the underlying
451 // singleton, if desired.
452 T& operator*() { return *get(); }
453 T* operator->() { return get(); }
455 explicit Singleton(std::nullptr_t _ = nullptr,
456 typename Singleton::TeardownFunc t = nullptr) :
457 Singleton ([]() { return new T; }, std::move(t)) {
460 explicit Singleton(typename Singleton::CreateFunc c,
461 typename Singleton::TeardownFunc t = nullptr) {
463 throw std::logic_error(
464 "nullptr_t should be passed if you want T to be default constructed");
467 auto vault = SingletonVault::singleton<VaultTag>();
468 getEntry().registerSingleton(std::move(c), getTeardownFunc(std::move(t)));
469 vault->registerSingleton(&getEntry());
473 * Construct and inject a mock singleton which should be used only from tests.
474 * Unlike regular singletons which are initialized once per process lifetime,
475 * mock singletons live for the duration of a test. This means that one process
476 * running multiple tests can initialize and register the same singleton
477 * multiple times. This functionality should be used only from tests
478 * since it relaxes validation and performance in order to be able to perform
479 * the injection. The returned mock singleton is functionality identical to
480 * regular singletons.
482 static void make_mock(std::nullptr_t c = nullptr,
483 typename Singleton<T>::TeardownFunc t = nullptr) {
484 make_mock([]() { return new T; }, t);
487 static void make_mock(CreateFunc c,
488 typename Singleton<T>::TeardownFunc t = nullptr) {
490 throw std::logic_error(
491 "nullptr_t should be passed if you want T to be default constructed");
494 auto& entry = getEntry();
496 entry.registerSingletonMock(c, getTeardownFunc(t));
500 inline static detail::SingletonHolder<T>& getEntry() {
501 return detail::SingletonHolder<T>::template singleton<Tag, VaultTag>();
504 // Construct TeardownFunc.
505 static typename detail::SingletonHolder<T>::TeardownFunc getTeardownFunc(
508 return [](T* v) { delete v; };
517 #include <folly/Singleton-inl.h>