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 // The above examples detail a situation where an expensive singleton is loaded
75 // on-demand (thus only if needed). However if there is an expensive singleton
76 // that will likely be needed, and initialization takes a potentially long time,
77 // e.g. while initializing, parsing some files, talking to remote services,
78 // making uses of other singletons, and so on, the initialization of those can
79 // be scheduled up front, or "eagerly".
81 // In that case the singleton can be declared this way:
84 // auto the_singleton =
85 // folly::Singleton<MyExpensiveService>(/* optional create, destroy args */)
86 // .shouldEagerInit();
89 // This way the singleton's instance is built at program initialization,
90 // if the program opted-in to that feature by calling "doEagerInit" or
91 // "doEagerInitVia" during its startup.
93 // What if you need to destroy all of your singletons? Say, some of
94 // your singletons manage threads, but you need to fork? Or your unit
95 // test wants to clean up all global state? Then you can call
96 // SingletonVault::singleton()->destroyInstances(), which invokes the
97 // TeardownFunc for each singleton, in the reverse order they were
98 // created. It is your responsibility to ensure your singletons can
99 // handle cases where the singletons they depend on go away, however.
100 // Singletons won't be recreated after destroyInstances call. If you
101 // want to re-enable singleton creation (say after fork was called) you
102 // should call reenableInstances.
105 #include <folly/Baton.h>
106 #include <folly/Exception.h>
107 #include <folly/Hash.h>
108 #include <folly/Memory.h>
109 #include <folly/RWSpinLock.h>
110 #include <folly/Demangle.h>
111 #include <folly/Executor.h>
112 #include <folly/experimental/ReadMostlySharedPtr.h>
116 #include <condition_variable>
117 #include <functional>
124 #include <unordered_map>
125 #include <unordered_set>
128 #include <glog/logging.h>
130 // use this guard to handleSingleton breaking change in 3rd party code
131 #ifndef FOLLY_SINGLETON_TRY_GET
132 #define FOLLY_SINGLETON_TRY_GET
137 // For actual usage, please see the Singleton<T> class at the bottom
138 // of this file; that is what you will actually interact with.
140 // SingletonVault is the class that manages singleton instances. It
141 // is unaware of the underlying types of singletons, and simply
142 // manages lifecycles and invokes CreateFunc and TeardownFunc when
143 // appropriate. In general, you won't need to interact with the
144 // SingletonVault itself.
146 // A vault goes through a few stages of life:
148 // 1. Registration phase; singletons can be registered:
149 // a) Strict: no singleton can be created in this stage.
150 // b) Relaxed: singleton can be created (the default vault is Relaxed).
151 // 2. registrationComplete() has been called; singletons can no
152 // longer be registered, but they can be created.
153 // 3. A vault can return to stage 1 when destroyInstances is called.
155 // In general, you don't need to worry about any of the above; just
156 // ensure registrationComplete() is called near the top of your main()
157 // function, otherwise no singletons can be instantiated.
159 class SingletonVault;
163 struct DefaultTag {};
165 // A TypeDescriptor is the unique handle for a given singleton. It is
166 // a combinaiton of the type and of the optional name, and is used as
167 // a key in unordered_maps.
168 class TypeDescriptor {
170 TypeDescriptor(const std::type_info& ti,
171 const std::type_info& tag_ti)
172 : ti_(ti), tag_ti_(tag_ti) {
175 TypeDescriptor(const TypeDescriptor& other)
176 : ti_(other.ti_), tag_ti_(other.tag_ti_) {
179 TypeDescriptor& operator=(const TypeDescriptor& other) {
180 if (this != &other) {
182 tag_ti_ = other.tag_ti_;
188 std::string name() const {
189 auto ret = demangle(ti_.name());
190 if (tag_ti_ != std::type_index(typeid(DefaultTag))) {
192 ret += demangle(tag_ti_.name());
194 return ret.toStdString();
197 friend class TypeDescriptorHasher;
199 bool operator==(const TypeDescriptor& other) const {
200 return ti_ == other.ti_ && tag_ti_ == other.tag_ti_;
205 std::type_index tag_ti_;
208 class TypeDescriptorHasher {
210 size_t operator()(const TypeDescriptor& ti) const {
211 return folly::hash::hash_combine(ti.ti_, ti.tag_ti_);
215 // This interface is used by SingletonVault to interact with SingletonHolders.
216 // Having a non-template interface allows SingletonVault to keep a list of all
218 class SingletonHolderBase {
220 virtual ~SingletonHolderBase() = default;
222 virtual TypeDescriptor type() = 0;
223 virtual bool hasLiveInstance() = 0;
224 virtual void createInstance() = 0;
225 virtual bool creationStarted() = 0;
226 virtual void destroyInstance() = 0;
229 static constexpr std::chrono::seconds kDestroyWaitTime{5};
232 // An actual instance of a singleton, tracking the instance itself,
233 // its state as described above, and the create and teardown
235 template <typename T>
236 struct SingletonHolder : public SingletonHolderBase {
238 typedef std::function<void(T*)> TeardownFunc;
239 typedef std::function<T*(void)> CreateFunc;
241 template <typename Tag, typename VaultTag>
242 inline static SingletonHolder<T>& singleton();
245 inline std::weak_ptr<T> get_weak();
246 inline std::shared_ptr<T> try_get();
247 inline folly::ReadMostlySharedPtr<T> try_get_fast();
249 void registerSingleton(CreateFunc c, TeardownFunc t);
250 void registerSingletonMock(CreateFunc c, TeardownFunc t);
251 virtual TypeDescriptor type() override;
252 virtual bool hasLiveInstance() override;
253 virtual void createInstance() override;
254 virtual bool creationStarted() override;
255 virtual void destroyInstance() override;
258 SingletonHolder(TypeDescriptor type, SingletonVault& vault);
260 enum class SingletonHolderState {
266 TypeDescriptor type_;
267 SingletonVault& vault_;
269 // mutex protects the entire entry during construction/destruction
272 // State of the singleton entry. If state is Living, instance_ptr and
273 // instance_weak can be safely accessed w/o synchronization.
274 std::atomic<SingletonHolderState> state_{SingletonHolderState::NotRegistered};
276 // the thread creating the singleton (only valid while creating an object)
277 std::atomic<std::thread::id> creating_thread_;
279 // The singleton itself and related functions.
281 // holds a ReadMostlyMainPtr to singleton instance, set when state is changed
282 // from Dead to Living. Reset when state is changed from Living to Dead.
283 folly::ReadMostlyMainPtr<T> instance_;
284 // weak_ptr to the singleton instance, set when state is changed from Dead
285 // to Living. We never write to this object after initialization, so it is
286 // safe to read it from different threads w/o synchronization if we know
287 // that state is set to Living
288 std::weak_ptr<T> instance_weak_;
289 // Fast equivalent of instance_weak_
290 folly::ReadMostlyWeakPtr<T> instance_weak_fast_;
291 // Time we wait on destroy_baton after releasing Singleton shared_ptr.
292 std::shared_ptr<folly::Baton<>> destroy_baton_;
293 T* instance_ptr_ = nullptr;
294 CreateFunc create_ = nullptr;
295 TeardownFunc teardown_ = nullptr;
297 std::shared_ptr<std::atomic<bool>> print_destructor_stack_trace_;
299 SingletonHolder(const SingletonHolder&) = delete;
300 SingletonHolder& operator=(const SingletonHolder&) = delete;
301 SingletonHolder& operator=(SingletonHolder&&) = delete;
302 SingletonHolder(SingletonHolder&&) = delete;
307 class SingletonVault {
310 Strict, // Singletons can't be created before registrationComplete()
311 Relaxed, // Singletons can be created before registrationComplete()
315 * Clears all singletons in the given vault at ctor and dtor times.
316 * Useful for unit-tests that need to clear the world.
318 * This need can arise when a unit-test needs to swap out an object used by a
319 * singleton for a test-double, but the singleton needing its dependency to be
320 * swapped has a type or a tag local to some other translation unit and
321 * unavailable in the current translation unit.
323 * Other, better approaches to this need are "plz 2 refactor" ....
325 struct ScopedExpunger {
326 SingletonVault* vault;
327 explicit ScopedExpunger(SingletonVault* v) : vault(v) { expunge(); }
328 ~ScopedExpunger() { expunge(); }
330 vault->destroyInstances();
331 vault->reenableInstances();
335 explicit SingletonVault(Type type = Type::Relaxed) : type_(type) {}
337 // Destructor is only called by unit tests to check destroyInstances.
340 typedef std::function<void(void*)> TeardownFunc;
341 typedef std::function<void*(void)> CreateFunc;
343 // Ensure that Singleton has not been registered previously and that
344 // registration is not complete. If validations succeeds,
345 // register a singleton of a given type with the create and teardown
347 void registerSingleton(detail::SingletonHolderBase* entry);
350 * Called by `Singleton<T>.shouldEagerInit()` to ensure the instance
351 * is built when `doEagerInit[Via]` is called; see those methods
354 void addEagerInitSingleton(detail::SingletonHolderBase* entry);
356 // Mark registration is complete; no more singletons can be
357 // registered at this point.
358 void registrationComplete();
361 * Initialize all singletons which were marked as eager-initialized
362 * (using `shouldEagerInit()`). No return value. Propagates exceptions
363 * from constructors / create functions, as is the usual case when calling
364 * for example `Singleton<Foo>::get_weak()`.
369 * Schedule eager singletons' initializations through the given executor.
370 * If baton ptr is not null, its `post` method is called after all
371 * early initialization has completed.
373 * If exceptions are thrown during initialization, this method will still
374 * `post` the baton to indicate completion. The exception will not propagate
375 * and future attempts to `try_get` or `get_weak` the failed singleton will
376 * retry initialization.
380 * wangle::IOThreadPoolExecutor executor(max_concurrency_level);
381 * folly::Baton<> done;
382 * doEagerInitVia(executor, &done);
383 * done.wait(); // or 'timed_wait', or spin with 'try_wait'
386 void doEagerInitVia(Executor& exe, folly::Baton<>* done = nullptr);
388 // Destroy all singletons; when complete, the vault can't create
389 // singletons once again until reenableInstances() is called.
390 void destroyInstances();
392 // Enable re-creating singletons after destroyInstances() was called.
393 void reenableInstances();
395 // For testing; how many registered and living singletons we have.
396 size_t registeredSingletonCount() const {
397 RWSpinLock::ReadHolder rh(&mutex_);
399 return singletons_.size();
403 * Flips to true if eager initialization was used, and has completed.
404 * Never set to true if "doEagerInit()" or "doEagerInitVia" never called.
406 bool eagerInitComplete() const;
408 size_t livingSingletonCount() const {
409 RWSpinLock::ReadHolder rh(&mutex_);
412 for (const auto& p : singletons_) {
413 if (p.second->hasLiveInstance()) {
421 // A well-known vault; you can actually have others, but this is the
423 static SingletonVault* singleton() {
424 return singleton<>();
427 // Gets singleton vault for any Tag. Non-default tag should be used in unit
429 template <typename VaultTag = detail::DefaultTag>
430 static SingletonVault* singleton() {
431 static SingletonVault* vault = new SingletonVault();
435 typedef std::string(*StackTraceGetterPtr)();
437 static std::atomic<StackTraceGetterPtr>& stackTraceGetter() {
438 static std::atomic<StackTraceGetterPtr> stackTraceGetterPtr;
439 return stackTraceGetterPtr;
443 template <typename T>
444 friend struct detail::SingletonHolder;
446 // The two stages of life for a vault, as mentioned in the class comment.
447 enum class SingletonVaultState {
452 // Each singleton in the vault can be in two states: dead
453 // (registered but never created), living (CreateFunc returned an instance).
455 void stateCheck(SingletonVaultState expected,
456 const char* msg="Unexpected singleton state change") {
457 if (expected != state_) {
458 throw std::logic_error(msg);
462 // This method only matters if registrationComplete() is never called.
463 // Otherwise destroyInstances is scheduled to be executed atexit.
465 // Initializes static object, which calls destroyInstances on destruction.
466 // Used to have better deletion ordering with singleton not managed by
467 // folly::Singleton. The desruction will happen in the following order:
468 // 1. Singletons, not managed by folly::Singleton, which were created after
469 // any of the singletons managed by folly::Singleton was requested.
470 // 2. All singletons managed by folly::Singleton
471 // 3. Singletons, not managed by folly::Singleton, which were created before
472 // any of the singletons managed by folly::Singleton was requested.
473 static void scheduleDestroyInstances();
475 typedef std::unordered_map<detail::TypeDescriptor,
476 detail::SingletonHolderBase*,
477 detail::TypeDescriptorHasher> SingletonMap;
479 mutable folly::RWSpinLock mutex_;
480 SingletonMap singletons_;
481 std::unordered_set<detail::SingletonHolderBase*> eagerInitSingletons_;
482 std::vector<detail::TypeDescriptor> creation_order_;
483 SingletonVaultState state_{SingletonVaultState::Running};
484 bool registrationComplete_{false};
485 folly::RWSpinLock stateMutex_;
486 Type type_{Type::Relaxed};
489 // This is the wrapper class that most users actually interact with.
490 // It allows for simple access to registering and instantiating
491 // singletons. Create instances of this class in the global scope of
492 // type Singleton<T> to register your singleton for later access via
493 // Singleton<T>::try_get().
494 template <typename T,
495 typename Tag = detail::DefaultTag,
496 typename VaultTag = detail::DefaultTag /* for testing */>
499 typedef std::function<T*(void)> CreateFunc;
500 typedef std::function<void(T*)> TeardownFunc;
502 // Generally your program life cycle should be fine with calling
503 // get() repeatedly rather than saving the reference, and then not
504 // call get() during process shutdown.
505 FOLLY_DEPRECATED("Replaced by try_get")
506 static T* get() { return getEntry().get(); }
508 // If, however, you do need to hold a reference to the specific
509 // singleton, you can try to do so with a weak_ptr. Avoid this when
510 // possible but the inability to lock the weak pointer can be a
511 // signal that the vault has been destroyed.
512 FOLLY_DEPRECATED("Replaced by try_get")
513 static std::weak_ptr<T> get_weak() { return getEntry().get_weak(); }
515 // Preferred alternative to get_weak, it returns shared_ptr that can be
516 // stored; a singleton won't be destroyed unless shared_ptr is destroyed.
517 // Avoid holding these shared_ptrs beyond the scope of a function;
518 // don't put them in member variables, always use try_get() instead
520 // try_get() can return nullptr if the singleton was destroyed, caller is
521 // responsible for handling nullptr return
522 static std::shared_ptr<T> try_get() {
523 return getEntry().try_get();
526 static folly::ReadMostlySharedPtr<T> try_get_fast() {
527 return getEntry().try_get_fast();
530 explicit Singleton(std::nullptr_t /* _ */ = nullptr,
531 typename Singleton::TeardownFunc t = nullptr)
532 : Singleton([]() { return new T; }, std::move(t)) {}
534 explicit Singleton(typename Singleton::CreateFunc c,
535 typename Singleton::TeardownFunc t = nullptr) {
537 throw std::logic_error(
538 "nullptr_t should be passed if you want T to be default constructed");
541 auto vault = SingletonVault::singleton<VaultTag>();
542 getEntry().registerSingleton(std::move(c), getTeardownFunc(std::move(t)));
543 vault->registerSingleton(&getEntry());
547 * Should be instantiated as soon as "doEagerInit[Via]" is called.
548 * Singletons are usually lazy-loaded (built on-demand) but for those which
549 * are known to be needed, to avoid the potential lag for objects that take
550 * long to construct during runtime, there is an option to make sure these
551 * are built up-front.
554 * Singleton<Foo> gFooInstance = Singleton<Foo>(...).shouldEagerInit();
556 * Or alternately, define the singleton as usual, and say
557 * gFooInstance.shouldEagerInit();
559 * at some point prior to calling registrationComplete().
560 * Then doEagerInit() or doEagerInitVia(Executor*) can be called.
562 Singleton& shouldEagerInit() {
563 auto vault = SingletonVault::singleton<VaultTag>();
564 vault->addEagerInitSingleton(&getEntry());
569 * Construct and inject a mock singleton which should be used only from tests.
570 * Unlike regular singletons which are initialized once per process lifetime,
571 * mock singletons live for the duration of a test. This means that one process
572 * running multiple tests can initialize and register the same singleton
573 * multiple times. This functionality should be used only from tests
574 * since it relaxes validation and performance in order to be able to perform
575 * the injection. The returned mock singleton is functionality identical to
576 * regular singletons.
578 static void make_mock(std::nullptr_t /* c */ = nullptr,
579 typename Singleton<T>::TeardownFunc t = nullptr) {
580 make_mock([]() { return new T; }, t);
583 static void make_mock(CreateFunc c,
584 typename Singleton<T>::TeardownFunc t = nullptr) {
586 throw std::logic_error(
587 "nullptr_t should be passed if you want T to be default constructed");
590 auto& entry = getEntry();
592 entry.registerSingletonMock(c, getTeardownFunc(t));
596 inline static detail::SingletonHolder<T>& getEntry() {
597 return detail::SingletonHolder<T>::template singleton<Tag, VaultTag>();
600 // Construct TeardownFunc.
601 static typename detail::SingletonHolder<T>::TeardownFunc getTeardownFunc(
604 return [](T* v) { delete v; };
611 template <typename T, typename Tag = detail::DefaultTag>
612 class LeakySingleton {
614 using CreateFunc = std::function<T*()>;
616 LeakySingleton() : LeakySingleton([] { return new T(); }) {}
618 explicit LeakySingleton(CreateFunc createFunc) {
619 auto& entry = entryInstance();
620 if (entry.state != State::NotRegistered) {
621 LOG(FATAL) << "Double registration of singletons of the same "
622 << "underlying type; check for multiple definitions "
623 << "of type folly::LeakySingleton<" + entry.type_.name() + ">";
625 entry.createFunc = createFunc;
626 entry.state = State::Dead;
629 static T& get() { return instance(); }
632 enum class State { NotRegistered, Dead, Living };
636 Entry(const Entry&) = delete;
637 Entry& operator=(const Entry&) = delete;
639 std::atomic<State> state{State::NotRegistered};
641 CreateFunc createFunc;
643 detail::TypeDescriptor type_{typeid(T), typeid(Tag)};
646 static Entry& entryInstance() {
647 static auto entry = new Entry();
651 static T& instance() {
652 auto& entry = entryInstance();
653 if (UNLIKELY(entry.state != State::Living)) {
660 static void createInstance() {
661 auto& entry = entryInstance();
663 std::lock_guard<std::mutex> lg(entry.mutex);
664 if (entry.state == State::Living) {
668 if (entry.state == State::NotRegistered) {
669 auto ptr = SingletonVault::stackTraceGetter().load();
670 LOG(FATAL) << "Creating instance for unregistered singleton: "
671 << entry.type_.name() << "\n"
673 << "\n" << (ptr ? (*ptr)() : "(not available)");
676 entry.ptr = entry.createFunc();
677 entry.state = State::Living;
682 #include <folly/Singleton-inl.h>