2 * Copyright 2015 Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
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13 * See the License for the specific language governing permissions and
14 * limitations under the License.
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>
107 // use this guard to handleSingleton breaking change in 3rd party code
108 #ifndef FOLLY_SINGLETON_TRY_GET
109 #define FOLLY_SINGLETON_TRY_GET
114 // For actual usage, please see the Singleton<T> class at the bottom
115 // of this file; that is what you will actually interact with.
117 // SingletonVault is the class that manages singleton instances. It
118 // is unaware of the underlying types of singletons, and simply
119 // manages lifecycles and invokes CreateFunc and TeardownFunc when
120 // appropriate. In general, you won't need to interact with the
121 // SingletonVault itself.
123 // A vault goes through a few stages of life:
125 // 1. Registration phase; singletons can be registered, but no
126 // singleton can be created.
127 // 2. registrationComplete() has been called; singletons can no
128 // longer be registered, but they can be created.
129 // 3. A vault can return to stage 1 when destroyInstances is called.
131 // In general, you don't need to worry about any of the above; just
132 // ensure registrationComplete() is called near the top of your main()
133 // function, otherwise no singletons can be instantiated.
135 class SingletonVault;
139 struct DefaultTag {};
141 // A TypeDescriptor is the unique handle for a given singleton. It is
142 // a combinaiton of the type and of the optional name, and is used as
143 // a key in unordered_maps.
144 class TypeDescriptor {
146 TypeDescriptor(const std::type_info& ti,
147 const std::type_info& tag_ti)
148 : ti_(ti), tag_ti_(tag_ti) {
151 TypeDescriptor(const TypeDescriptor& other)
152 : ti_(other.ti_), tag_ti_(other.tag_ti_) {
155 TypeDescriptor& operator=(const TypeDescriptor& other) {
156 if (this != &other) {
158 tag_ti_ = other.tag_ti_;
164 std::string name() const {
165 auto ret = demangle(ti_.name());
166 if (tag_ti_ != std::type_index(typeid(DefaultTag))) {
168 ret += demangle(tag_ti_.name());
170 return ret.toStdString();
173 friend class TypeDescriptorHasher;
175 bool operator==(const TypeDescriptor& other) const {
176 return ti_ == other.ti_ && tag_ti_ == other.tag_ti_;
181 std::type_index tag_ti_;
184 class TypeDescriptorHasher {
186 size_t operator()(const TypeDescriptor& ti) const {
187 return folly::hash::hash_combine(ti.ti_, ti.tag_ti_);
191 // This interface is used by SingletonVault to interact with SingletonHolders.
192 // Having a non-template interface allows SingletonVault to keep a list of all
194 class SingletonHolderBase {
196 virtual ~SingletonHolderBase() = default;
198 virtual TypeDescriptor type() = 0;
199 virtual bool hasLiveInstance() = 0;
200 virtual void destroyInstance() = 0;
203 static constexpr std::chrono::seconds kDestroyWaitTime{5};
206 // An actual instance of a singleton, tracking the instance itself,
207 // its state as described above, and the create and teardown
209 template <typename T>
210 struct SingletonHolder : public SingletonHolderBase {
212 typedef std::function<void(T*)> TeardownFunc;
213 typedef std::function<T*(void)> CreateFunc;
215 template <typename Tag, typename VaultTag>
216 inline static SingletonHolder<T>& singleton();
219 inline std::weak_ptr<T> get_weak();
221 void registerSingleton(CreateFunc c, TeardownFunc t);
222 void registerSingletonMock(CreateFunc c, TeardownFunc t);
223 virtual TypeDescriptor type();
224 virtual bool hasLiveInstance();
225 virtual void destroyInstance();
228 SingletonHolder(TypeDescriptor type, SingletonVault& vault);
230 void createInstance();
232 enum class SingletonHolderState {
238 TypeDescriptor type_;
239 SingletonVault& vault_;
241 // mutex protects the entire entry during construction/destruction
244 // State of the singleton entry. If state is Living, instance_ptr and
245 // instance_weak can be safely accessed w/o synchronization.
246 std::atomic<SingletonHolderState> state_{SingletonHolderState::NotRegistered};
248 // the thread creating the singleton (only valid while creating an object)
249 std::thread::id creating_thread_;
251 // The singleton itself and related functions.
253 // holds a shared_ptr to singleton instance, set when state is changed from
254 // Dead to Living. Reset when state is changed from Living to Dead.
255 std::shared_ptr<T> instance_;
256 // weak_ptr to the singleton instance, set when state is changed from Dead
257 // to Living. We never write to this object after initialization, so it is
258 // safe to read it from different threads w/o synchronization if we know
259 // that state is set to Living
260 std::weak_ptr<T> instance_weak_;
261 // Time we wait on destroy_baton after releasing Singleton shared_ptr.
262 std::shared_ptr<folly::Baton<>> destroy_baton_;
263 T* instance_ptr_ = nullptr;
264 CreateFunc create_ = nullptr;
265 TeardownFunc teardown_ = nullptr;
267 std::shared_ptr<std::atomic<bool>> print_destructor_stack_trace_;
269 SingletonHolder(const SingletonHolder&) = delete;
270 SingletonHolder& operator=(const SingletonHolder&) = delete;
271 SingletonHolder& operator=(SingletonHolder&&) = delete;
272 SingletonHolder(SingletonHolder&&) = delete;
277 class SingletonVault {
279 enum class Type { Strict, Relaxed };
281 explicit SingletonVault(Type type = Type::Relaxed) : type_(type) {}
283 // Destructor is only called by unit tests to check destroyInstances.
286 typedef std::function<void(void*)> TeardownFunc;
287 typedef std::function<void*(void)> CreateFunc;
289 // Ensure that Singleton has not been registered previously and that
290 // registration is not complete. If validations succeeds,
291 // register a singleton of a given type with the create and teardown
293 void registerSingleton(detail::SingletonHolderBase* entry) {
294 RWSpinLock::ReadHolder rh(&stateMutex_);
296 stateCheck(SingletonVaultState::Running);
298 if (UNLIKELY(registrationComplete_)) {
299 throw std::logic_error(
300 "Registering singleton after registrationComplete().");
303 RWSpinLock::ReadHolder rhMutex(&mutex_);
304 CHECK_THROW(singletons_.find(entry->type()) == singletons_.end(),
307 RWSpinLock::UpgradedHolder wh(&mutex_);
308 singletons_[entry->type()] = entry;
311 // Mark registration is complete; no more singletons can be
312 // registered at this point.
313 void registrationComplete() {
314 RequestContext::saveContext();
315 std::atexit([](){ SingletonVault::singleton()->destroyInstances(); });
317 RWSpinLock::WriteHolder wh(&stateMutex_);
319 stateCheck(SingletonVaultState::Running);
321 if (type_ == Type::Strict) {
322 for (const auto& p: singletons_) {
323 if (p.second->hasLiveInstance()) {
324 throw std::runtime_error(
325 "Singleton created before registration was complete.");
330 registrationComplete_ = true;
333 // Destroy all singletons; when complete, the vault can't create
334 // singletons once again until reenableInstances() is called.
335 void destroyInstances();
337 // Enable re-creating singletons after destroyInstances() was called.
338 void reenableInstances();
340 // For testing; how many registered and living singletons we have.
341 size_t registeredSingletonCount() const {
342 RWSpinLock::ReadHolder rh(&mutex_);
344 return singletons_.size();
347 size_t livingSingletonCount() const {
348 RWSpinLock::ReadHolder rh(&mutex_);
351 for (const auto& p : singletons_) {
352 if (p.second->hasLiveInstance()) {
360 // A well-known vault; you can actually have others, but this is the
362 static SingletonVault* singleton() {
363 return singleton<>();
366 // Gets singleton vault for any Tag. Non-default tag should be used in unit
368 template <typename VaultTag = detail::DefaultTag>
369 static SingletonVault* singleton() {
370 static SingletonVault* vault = new SingletonVault();
374 typedef std::string(*StackTraceGetterPtr)();
376 static std::atomic<StackTraceGetterPtr>& stackTraceGetter() {
377 static std::atomic<StackTraceGetterPtr> stackTraceGetterPtr;
378 return stackTraceGetterPtr;
382 template <typename T>
383 friend struct detail::SingletonHolder;
385 // The two stages of life for a vault, as mentioned in the class comment.
386 enum class SingletonVaultState {
391 // Each singleton in the vault can be in two states: dead
392 // (registered but never created), living (CreateFunc returned an instance).
394 void stateCheck(SingletonVaultState expected,
395 const char* msg="Unexpected singleton state change") {
396 if (expected != state_) {
397 throw std::logic_error(msg);
401 // This method only matters if registrationComplete() is never called.
402 // Otherwise destroyInstances is scheduled to be executed atexit.
404 // Initializes static object, which calls destroyInstances on destruction.
405 // Used to have better deletion ordering with singleton not managed by
406 // folly::Singleton. The desruction will happen in the following order:
407 // 1. Singletons, not managed by folly::Singleton, which were created after
408 // any of the singletons managed by folly::Singleton was requested.
409 // 2. All singletons managed by folly::Singleton
410 // 3. Singletons, not managed by folly::Singleton, which were created before
411 // any of the singletons managed by folly::Singleton was requested.
412 static void scheduleDestroyInstances();
414 typedef std::unordered_map<detail::TypeDescriptor,
415 detail::SingletonHolderBase*,
416 detail::TypeDescriptorHasher> SingletonMap;
418 mutable folly::RWSpinLock mutex_;
419 SingletonMap singletons_;
420 std::vector<detail::TypeDescriptor> creation_order_;
421 SingletonVaultState state_{SingletonVaultState::Running};
422 bool registrationComplete_{false};
423 folly::RWSpinLock stateMutex_;
424 Type type_{Type::Relaxed};
427 // This is the wrapper class that most users actually interact with.
428 // It allows for simple access to registering and instantiating
429 // singletons. Create instances of this class in the global scope of
430 // type Singleton<T> to register your singleton for later access via
431 // Singleton<T>::try_get().
432 template <typename T,
433 typename Tag = detail::DefaultTag,
434 typename VaultTag = detail::DefaultTag /* for testing */>
437 typedef std::function<T*(void)> CreateFunc;
438 typedef std::function<void(T*)> TeardownFunc;
440 // Generally your program life cycle should be fine with calling
441 // get() repeatedly rather than saving the reference, and then not
442 // call get() during process shutdown.
443 static T* get() __attribute__ ((__deprecated__("Replaced by try_get"))) {
444 return getEntry().get();
447 // If, however, you do need to hold a reference to the specific
448 // singleton, you can try to do so with a weak_ptr. Avoid this when
449 // possible but the inability to lock the weak pointer can be a
450 // signal that the vault has been destroyed.
451 static std::weak_ptr<T> get_weak() {
452 return getEntry().get_weak();
455 // Preferred alternative to get_weak, it returns shared_ptr that can be
456 // stored; a singleton won't be destroyed unless shared_ptr is destroyed.
457 // Avoid holding these shared_ptrs beyond the scope of a function;
458 // don't put them in member variables, always use try_get() instead
459 static std::shared_ptr<T> try_get() {
460 auto ret = get_weak().lock();
463 "folly::Singleton<" << getEntry().type().name() <<
464 ">::get_weak() called on destructed singleton; "
465 "returning nullptr, possible segfault coming";
470 explicit Singleton(std::nullptr_t _ = nullptr,
471 typename Singleton::TeardownFunc t = nullptr) :
472 Singleton ([]() { return new T; }, std::move(t)) {
475 explicit Singleton(typename Singleton::CreateFunc c,
476 typename Singleton::TeardownFunc t = nullptr) {
478 throw std::logic_error(
479 "nullptr_t should be passed if you want T to be default constructed");
482 auto vault = SingletonVault::singleton<VaultTag>();
483 getEntry().registerSingleton(std::move(c), getTeardownFunc(std::move(t)));
484 vault->registerSingleton(&getEntry());
488 * Construct and inject a mock singleton which should be used only from tests.
489 * Unlike regular singletons which are initialized once per process lifetime,
490 * mock singletons live for the duration of a test. This means that one process
491 * running multiple tests can initialize and register the same singleton
492 * multiple times. This functionality should be used only from tests
493 * since it relaxes validation and performance in order to be able to perform
494 * the injection. The returned mock singleton is functionality identical to
495 * regular singletons.
497 static void make_mock(std::nullptr_t c = nullptr,
498 typename Singleton<T>::TeardownFunc t = nullptr) {
499 make_mock([]() { return new T; }, t);
502 static void make_mock(CreateFunc c,
503 typename Singleton<T>::TeardownFunc t = nullptr) {
505 throw std::logic_error(
506 "nullptr_t should be passed if you want T to be default constructed");
509 auto& entry = getEntry();
511 entry.registerSingletonMock(c, getTeardownFunc(t));
515 inline static detail::SingletonHolder<T>& getEntry() {
516 return detail::SingletonHolder<T>::template singleton<Tag, VaultTag>();
519 // Construct TeardownFunc.
520 static typename detail::SingletonHolder<T>::TeardownFunc getTeardownFunc(
523 return [](T* v) { delete v; };
532 #include <folly/Singleton-inl.h>