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4 * Licensed under the Apache License, Version 2.0 (the "License");
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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 // Within same compilation unit you should directly access it by the variable
38 // defining the singleton via get_fast()/get_weak_fast(), and even treat that
39 // variable like a smart pointer (dereferencing it or using the -> operator):
41 // MyExpensiveService* instance = the_singleton.get_fast();
43 // std::weak_ptr<MyExpensiveService> instance = the_singleton.get_weak_fast();
45 // the_singleton->doSomething();
47 // *_fast() accessors are faster than static accessors, and have performance
48 // similar to Meyers singletons/static objects.
50 // Please note, however, that all non-weak_ptr interfaces are
51 // inherently subject to races with destruction. Use responsibly.
53 // The singleton will be created on demand. If the constructor for
54 // MyExpensiveService actually makes use of *another* Singleton, then
55 // the right thing will happen -- that other singleton will complete
56 // construction before get() returns. However, in the event of a
57 // circular dependency, a runtime error will occur.
59 // You can have multiple singletons of the same underlying type, but
60 // each must be given a unique tag. If no tag is specified - default tag is used
65 // folly::Singleton<MyExpensiveService> s_default();
66 // folly::Singleton<MyExpensiveService, Tag1> s1();
67 // folly::Singleton<MyExpensiveService, Tag2> s2();
70 // MyExpensiveService* svc_default = s_default.get_fast();
71 // MyExpensiveService* svc1 = s1.get_fast();
72 // MyExpensiveService* svc2 = s2.get_fast();
74 // By default, the singleton instance is constructed via new and
75 // deleted via delete, but this is configurable:
77 // namespace { folly::Singleton<MyExpensiveService> the_singleton(create,
80 // Where create and destroy are functions, Singleton<T>::CreateFunc
81 // Singleton<T>::TeardownFunc.
83 // What if you need to destroy all of your singletons? Say, some of
84 // your singletons manage threads, but you need to fork? Or your unit
85 // test wants to clean up all global state? Then you can call
86 // SingletonVault::singleton()->destroyInstances(), which invokes the
87 // TeardownFunc for each singleton, in the reverse order they were
88 // created. It is your responsibility to ensure your singletons can
89 // handle cases where the singletons they depend on go away, however.
90 // Singletons won't be recreated after destroyInstances call. If you
91 // want to re-enable singleton creation (say after fork was called) you
92 // should call reenableInstances.
95 #include <folly/Baton.h>
96 #include <folly/Exception.h>
97 #include <folly/Hash.h>
98 #include <folly/Memory.h>
99 #include <folly/RWSpinLock.h>
100 #include <folly/Demangle.h>
101 #include <folly/io/async/Request.h>
107 #include <condition_variable>
109 #include <unordered_map>
110 #include <functional>
114 #include <glog/logging.h>
118 // For actual usage, please see the Singleton<T> class at the bottom
119 // of this file; that is what you will actually interact with.
121 // SingletonVault is the class that manages singleton instances. It
122 // is unaware of the underlying types of singletons, and simply
123 // manages lifecycles and invokes CreateFunc and TeardownFunc when
124 // appropriate. In general, you won't need to interact with the
125 // SingletonVault itself.
127 // A vault goes through a few stages of life:
129 // 1. Registration phase; singletons can be registered, but no
130 // singleton can be created.
131 // 2. registrationComplete() has been called; singletons can no
132 // longer be registered, but they can be created.
133 // 3. A vault can return to stage 1 when destroyInstances is called.
135 // In general, you don't need to worry about any of the above; just
136 // ensure registrationComplete() is called near the top of your main()
137 // function, otherwise no singletons can be instantiated.
139 class SingletonVault;
143 struct DefaultTag {};
145 // A TypeDescriptor is the unique handle for a given singleton. It is
146 // a combinaiton of the type and of the optional name, and is used as
147 // a key in unordered_maps.
148 class TypeDescriptor {
150 TypeDescriptor(const std::type_info& ti,
151 const std::type_info& tag_ti)
152 : ti_(ti), tag_ti_(tag_ti) {
155 TypeDescriptor(const TypeDescriptor& other)
156 : ti_(other.ti_), tag_ti_(other.tag_ti_) {
159 TypeDescriptor& operator=(const TypeDescriptor& other) {
160 if (this != &other) {
162 tag_ti_ = other.tag_ti_;
168 std::string name() const {
169 auto ret = demangle(ti_.name());
170 if (tag_ti_ != std::type_index(typeid(DefaultTag))) {
172 ret += demangle(tag_ti_.name());
174 return ret.toStdString();
177 friend class TypeDescriptorHasher;
179 bool operator==(const TypeDescriptor& other) const {
180 return ti_ == other.ti_ && tag_ti_ == other.tag_ti_;
185 std::type_index tag_ti_;
188 class TypeDescriptorHasher {
190 size_t operator()(const TypeDescriptor& ti) const {
191 return folly::hash::hash_combine(ti.ti_, ti.tag_ti_);
195 // This interface is used by SingletonVault to interact with SingletonHolders.
196 // Having a non-template interface allows SingletonVault to keep a list of all
198 class SingletonHolderBase {
200 virtual ~SingletonHolderBase() {}
202 virtual TypeDescriptor type() = 0;
203 virtual bool hasLiveInstance() = 0;
204 virtual void destroyInstance() = 0;
207 static constexpr std::chrono::seconds kDestroyWaitTime{5};
210 // An actual instance of a singleton, tracking the instance itself,
211 // its state as described above, and the create and teardown
213 template <typename T>
214 struct SingletonHolder : public SingletonHolderBase {
216 typedef std::function<void(T*)> TeardownFunc;
217 typedef std::function<T*(void)> CreateFunc;
219 template <typename Tag, typename VaultTag>
220 inline static SingletonHolder<T>& singleton();
223 inline std::weak_ptr<T> get_weak();
225 void registerSingleton(CreateFunc c, TeardownFunc t);
226 void registerSingletonMock(CreateFunc c, TeardownFunc t);
227 virtual TypeDescriptor type();
228 virtual bool hasLiveInstance();
229 virtual void destroyInstance();
232 SingletonHolder(TypeDescriptor type, SingletonVault& vault);
234 void createInstance();
236 enum class SingletonHolderState {
242 TypeDescriptor type_;
243 SingletonVault& vault_;
245 // mutex protects the entire entry during construction/destruction
248 // State of the singleton entry. If state is Living, instance_ptr and
249 // instance_weak can be safely accessed w/o synchronization.
250 std::atomic<SingletonHolderState> state_{SingletonHolderState::NotRegistered};
252 // the thread creating the singleton (only valid while creating an object)
253 std::thread::id creating_thread_;
255 // The singleton itself and related functions.
257 // holds a shared_ptr to singleton instance, set when state is changed from
258 // Dead to Living. Reset when state is changed from Living to Dead.
259 std::shared_ptr<T> instance_;
260 // weak_ptr to the singleton instance, set when state is changed from Dead
261 // to Living. We never write to this object after initialization, so it is
262 // safe to read it from different threads w/o synchronization if we know
263 // that state is set to Living
264 std::weak_ptr<T> instance_weak_;
265 // Time we wait on destroy_baton after releasing Singleton shared_ptr.
266 std::shared_ptr<folly::Baton<>> destroy_baton_;
267 T* instance_ptr_ = nullptr;
268 CreateFunc create_ = nullptr;
269 TeardownFunc teardown_ = nullptr;
271 SingletonHolder(const SingletonHolder&) = delete;
272 SingletonHolder& operator=(const SingletonHolder&) = delete;
273 SingletonHolder& operator=(SingletonHolder&&) = delete;
274 SingletonHolder(SingletonHolder&&) = delete;
279 class SingletonVault {
281 enum class Type { Strict, Relaxed };
283 explicit SingletonVault(Type type = Type::Relaxed) : type_(type) {}
285 // Destructor is only called by unit tests to check destroyInstances.
288 typedef std::function<void(void*)> TeardownFunc;
289 typedef std::function<void*(void)> CreateFunc;
291 // Ensure that Singleton has not been registered previously and that
292 // registration is not complete. If validations succeeds,
293 // register a singleton of a given type with the create and teardown
295 void registerSingleton(detail::SingletonHolderBase* entry) {
296 RWSpinLock::ReadHolder rh(&stateMutex_);
298 stateCheck(SingletonVaultState::Running);
300 if (UNLIKELY(registrationComplete_)) {
301 throw std::logic_error(
302 "Registering singleton after registrationComplete().");
305 RWSpinLock::ReadHolder rhMutex(&mutex_);
306 CHECK_THROW(singletons_.find(entry->type()) == singletons_.end(),
309 RWSpinLock::UpgradedHolder wh(&mutex_);
310 singletons_[entry->type()] = entry;
313 // Mark registration is complete; no more singletons can be
314 // registered at this point.
315 void registrationComplete() {
316 RequestContext::getStaticContext();
317 std::atexit([](){ SingletonVault::singleton()->destroyInstances(); });
319 RWSpinLock::WriteHolder wh(&stateMutex_);
321 stateCheck(SingletonVaultState::Running);
323 if (type_ == Type::Strict) {
324 for (const auto& p: singletons_) {
325 if (p.second->hasLiveInstance()) {
326 throw std::runtime_error(
327 "Singleton created before registration was complete.");
332 registrationComplete_ = true;
335 // Destroy all singletons; when complete, the vault can't create
336 // singletons once again until reenableInstances() is called.
337 void destroyInstances();
339 // Enable re-creating singletons after destroyInstances() was called.
340 void reenableInstances();
342 // For testing; how many registered and living singletons we have.
343 size_t registeredSingletonCount() const {
344 RWSpinLock::ReadHolder rh(&mutex_);
346 return singletons_.size();
349 size_t livingSingletonCount() const {
350 RWSpinLock::ReadHolder rh(&mutex_);
353 for (const auto& p : singletons_) {
354 if (p.second->hasLiveInstance()) {
362 // A well-known vault; you can actually have others, but this is the
364 static SingletonVault* singleton() {
365 return singleton<>();
368 // Gets singleton vault for any Tag. Non-default tag should be used in unit
370 template <typename VaultTag = detail::DefaultTag>
371 static SingletonVault* singleton() {
372 static SingletonVault* vault = new SingletonVault();
377 template <typename T>
378 friend class 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 // Same as get, but should be preffered to it in the same compilation
443 // unit, where Singleton is registered.
448 // If, however, you do need to hold a reference to the specific
449 // singleton, you can try to do so with a weak_ptr. Avoid this when
450 // possible but the inability to lock the weak pointer can be a
451 // signal that the vault has been destroyed.
452 static std::weak_ptr<T> get_weak() {
453 return getEntry().get_weak();
456 // Same as get_weak, but should be preffered to it in the same compilation
457 // unit, where Singleton is registered.
458 std::weak_ptr<T> get_weak_fast() {
459 return entry_.get_weak();
462 // Allow the Singleton<t> instance to also retrieve the underlying
463 // singleton, if desired.
464 T* ptr() { return get_fast(); }
465 T& operator*() { return *ptr(); }
466 T* operator->() { return ptr(); }
468 explicit Singleton(std::nullptr_t _ = nullptr,
469 Singleton::TeardownFunc t = nullptr) :
470 Singleton ([]() { return new T; }, std::move(t)) {
473 explicit Singleton(Singleton::CreateFunc c,
474 Singleton::TeardownFunc t = nullptr) : entry_(getEntry()) {
476 throw std::logic_error(
477 "nullptr_t should be passed if you want T to be default constructed");
480 auto vault = SingletonVault::singleton<VaultTag>();
481 entry_.registerSingleton(std::move(c), getTeardownFunc(std::move(t)));
482 vault->registerSingleton(&entry_);
486 * Construct and inject a mock singleton which should be used only from tests.
487 * Unlike regular singletons which are initialized once per process lifetime,
488 * mock singletons live for the duration of a test. This means that one process
489 * running multiple tests can initialize and register the same singleton
490 * multiple times. This functionality should be used only from tests
491 * since it relaxes validation and performance in order to be able to perform
492 * the injection. The returned mock singleton is functionality identical to
493 * regular singletons.
495 static void make_mock(std::nullptr_t c = nullptr,
496 typename Singleton<T>::TeardownFunc t = nullptr) {
497 make_mock([]() { return new T; }, t);
500 static void make_mock(CreateFunc c,
501 typename Singleton<T>::TeardownFunc t = nullptr) {
503 throw std::logic_error(
504 "nullptr_t should be passed if you want T to be default constructed");
507 auto& entry = getEntry();
509 entry.registerSingletonMock(c, getTeardownFunc(t));
513 inline static detail::SingletonHolder<T>& getEntry() {
514 return detail::SingletonHolder<T>::template singleton<Tag, VaultTag>();
517 // Construct TeardownFunc.
518 static typename detail::SingletonHolder<T>::TeardownFunc getTeardownFunc(
521 return [](T* v) { delete v; };
527 // This is pointing to SingletonHolder paired with this singleton object. This
528 // is never reset, so each SingletonHolder should never be destroyed.
529 // We rely on the fact that Singleton destructor won't reset this pointer, so
530 // it can be "safely" used even after static Singleton object is destroyed.
531 detail::SingletonHolder<T>& entry_;
536 #include <folly/experimental/Singleton-inl.h>