2 * Copyright 2016 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.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
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11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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 // For example, if you need to pass arguments to your class's constructor:
77 // X(int a1, std::string a2);
80 // Make your singleton like this:
81 // folly::Singleton<X> singleton_x([]() { return new X(42, "foo"); });
83 // The above examples detail a situation where an expensive singleton is loaded
84 // on-demand (thus only if needed). However if there is an expensive singleton
85 // that will likely be needed, and initialization takes a potentially long time,
86 // e.g. while initializing, parsing some files, talking to remote services,
87 // making uses of other singletons, and so on, the initialization of those can
88 // be scheduled up front, or "eagerly".
90 // In that case the singleton can be declared this way:
93 // auto the_singleton =
94 // folly::Singleton<MyExpensiveService>(/* optional create, destroy args */)
95 // .shouldEagerInit();
98 // This way the singleton's instance is built at program initialization,
99 // if the program opted-in to that feature by calling "doEagerInit" or
100 // "doEagerInitVia" during its startup.
102 // What if you need to destroy all of your singletons? Say, some of
103 // your singletons manage threads, but you need to fork? Or your unit
104 // test wants to clean up all global state? Then you can call
105 // SingletonVault::singleton()->destroyInstances(), which invokes the
106 // TeardownFunc for each singleton, in the reverse order they were
107 // created. It is your responsibility to ensure your singletons can
108 // handle cases where the singletons they depend on go away, however.
109 // Singletons won't be recreated after destroyInstances call. If you
110 // want to re-enable singleton creation (say after fork was called) you
111 // should call reenableInstances.
114 #include <folly/Baton.h>
115 #include <folly/Demangle.h>
116 #include <folly/Exception.h>
117 #include <folly/Executor.h>
118 #include <folly/Hash.h>
119 #include <folly/Memory.h>
120 #include <folly/RWSpinLock.h>
121 #include <folly/Synchronized.h>
122 #include <folly/detail/StaticSingletonManager.h>
123 #include <folly/experimental/ReadMostlySharedPtr.h>
127 #include <condition_variable>
128 #include <functional>
135 #include <unordered_map>
136 #include <unordered_set>
139 #include <glog/logging.h>
141 // use this guard to handleSingleton breaking change in 3rd party code
142 #ifndef FOLLY_SINGLETON_TRY_GET
143 #define FOLLY_SINGLETON_TRY_GET
148 // For actual usage, please see the Singleton<T> class at the bottom
149 // of this file; that is what you will actually interact with.
151 // SingletonVault is the class that manages singleton instances. It
152 // is unaware of the underlying types of singletons, and simply
153 // manages lifecycles and invokes CreateFunc and TeardownFunc when
154 // appropriate. In general, you won't need to interact with the
155 // SingletonVault itself.
157 // A vault goes through a few stages of life:
159 // 1. Registration phase; singletons can be registered:
160 // a) Strict: no singleton can be created in this stage.
161 // b) Relaxed: singleton can be created (the default vault is Relaxed).
162 // 2. registrationComplete() has been called; singletons can no
163 // longer be registered, but they can be created.
164 // 3. A vault can return to stage 1 when destroyInstances is called.
166 // In general, you don't need to worry about any of the above; just
167 // ensure registrationComplete() is called near the top of your main()
168 // function, otherwise no singletons can be instantiated.
170 class SingletonVault;
174 struct DefaultTag {};
176 // A TypeDescriptor is the unique handle for a given singleton. It is
177 // a combinaiton of the type and of the optional name, and is used as
178 // a key in unordered_maps.
179 class TypeDescriptor {
181 TypeDescriptor(const std::type_info& ti,
182 const std::type_info& tag_ti)
183 : ti_(ti), tag_ti_(tag_ti) {
186 TypeDescriptor(const TypeDescriptor& other)
187 : ti_(other.ti_), tag_ti_(other.tag_ti_) {
190 TypeDescriptor& operator=(const TypeDescriptor& other) {
191 if (this != &other) {
193 tag_ti_ = other.tag_ti_;
199 std::string name() const {
200 auto ret = demangle(ti_.name());
201 if (tag_ti_ != std::type_index(typeid(DefaultTag))) {
203 ret += demangle(tag_ti_.name());
205 return ret.toStdString();
208 friend class TypeDescriptorHasher;
210 bool operator==(const TypeDescriptor& other) const {
211 return ti_ == other.ti_ && tag_ti_ == other.tag_ti_;
216 std::type_index tag_ti_;
219 class TypeDescriptorHasher {
221 size_t operator()(const TypeDescriptor& ti) const {
222 return folly::hash::hash_combine(ti.ti_, ti.tag_ti_);
226 // This interface is used by SingletonVault to interact with SingletonHolders.
227 // Having a non-template interface allows SingletonVault to keep a list of all
229 class SingletonHolderBase {
231 explicit SingletonHolderBase(TypeDescriptor typeDesc) : type_(typeDesc) {}
232 virtual ~SingletonHolderBase() = default;
234 TypeDescriptor type() const {
237 virtual bool hasLiveInstance() = 0;
238 virtual void createInstance() = 0;
239 virtual bool creationStarted() = 0;
240 virtual void preDestroyInstance(ReadMostlyMainPtrDeleter<>&) = 0;
241 virtual void destroyInstance() = 0;
244 TypeDescriptor type_;
247 // An actual instance of a singleton, tracking the instance itself,
248 // its state as described above, and the create and teardown
250 template <typename T>
251 struct SingletonHolder : public SingletonHolderBase {
253 typedef std::function<void(T*)> TeardownFunc;
254 typedef std::function<T*(void)> CreateFunc;
256 template <typename Tag, typename VaultTag>
257 inline static SingletonHolder<T>& singleton();
260 inline std::weak_ptr<T> get_weak();
261 inline std::shared_ptr<T> try_get();
262 inline folly::ReadMostlySharedPtr<T> try_get_fast();
264 void registerSingleton(CreateFunc c, TeardownFunc t);
265 void registerSingletonMock(CreateFunc c, TeardownFunc t);
266 virtual bool hasLiveInstance() override;
267 virtual void createInstance() override;
268 virtual bool creationStarted() override;
269 virtual void preDestroyInstance(ReadMostlyMainPtrDeleter<>&) override;
270 virtual void destroyInstance() override;
273 SingletonHolder(TypeDescriptor type, SingletonVault& vault);
275 enum class SingletonHolderState {
281 SingletonVault& vault_;
283 // mutex protects the entire entry during construction/destruction
286 // State of the singleton entry. If state is Living, instance_ptr and
287 // instance_weak can be safely accessed w/o synchronization.
288 std::atomic<SingletonHolderState> state_{SingletonHolderState::NotRegistered};
290 // the thread creating the singleton (only valid while creating an object)
291 std::atomic<std::thread::id> creating_thread_;
293 // The singleton itself and related functions.
295 // holds a ReadMostlyMainPtr to singleton instance, set when state is changed
296 // from Dead to Living. Reset when state is changed from Living to Dead.
297 folly::ReadMostlyMainPtr<T> instance_;
298 // used to release all ReadMostlyMainPtrs at once
299 folly::ReadMostlySharedPtr<T> instance_copy_;
300 // weak_ptr to the singleton instance, set when state is changed from Dead
301 // to Living. We never write to this object after initialization, so it is
302 // safe to read it from different threads w/o synchronization if we know
303 // that state is set to Living
304 std::weak_ptr<T> instance_weak_;
305 // Fast equivalent of instance_weak_
306 folly::ReadMostlyWeakPtr<T> instance_weak_fast_;
307 // Time we wait on destroy_baton after releasing Singleton shared_ptr.
308 std::shared_ptr<folly::Baton<>> destroy_baton_;
309 T* instance_ptr_ = nullptr;
310 CreateFunc create_ = nullptr;
311 TeardownFunc teardown_ = nullptr;
313 std::shared_ptr<std::atomic<bool>> print_destructor_stack_trace_;
315 SingletonHolder(const SingletonHolder&) = delete;
316 SingletonHolder& operator=(const SingletonHolder&) = delete;
317 SingletonHolder& operator=(SingletonHolder&&) = delete;
318 SingletonHolder(SingletonHolder&&) = delete;
323 class SingletonVault {
326 Strict, // Singletons can't be created before registrationComplete()
327 Relaxed, // Singletons can be created before registrationComplete()
331 * Clears all singletons in the given vault at ctor and dtor times.
332 * Useful for unit-tests that need to clear the world.
334 * This need can arise when a unit-test needs to swap out an object used by a
335 * singleton for a test-double, but the singleton needing its dependency to be
336 * swapped has a type or a tag local to some other translation unit and
337 * unavailable in the current translation unit.
339 * Other, better approaches to this need are "plz 2 refactor" ....
341 struct ScopedExpunger {
342 SingletonVault* vault;
343 explicit ScopedExpunger(SingletonVault* v) : vault(v) { expunge(); }
344 ~ScopedExpunger() { expunge(); }
346 vault->destroyInstances();
347 vault->reenableInstances();
351 explicit SingletonVault(Type type = Type::Strict) : type_(type) {}
353 // Destructor is only called by unit tests to check destroyInstances.
356 typedef std::function<void(void*)> TeardownFunc;
357 typedef std::function<void*(void)> CreateFunc;
359 // Ensure that Singleton has not been registered previously and that
360 // registration is not complete. If validations succeeds,
361 // register a singleton of a given type with the create and teardown
363 void registerSingleton(detail::SingletonHolderBase* entry);
366 * Called by `Singleton<T>.shouldEagerInit()` to ensure the instance
367 * is built when `doEagerInit[Via]` is called; see those methods
370 void addEagerInitSingleton(detail::SingletonHolderBase* entry);
372 // Mark registration is complete; no more singletons can be
373 // registered at this point.
374 void registrationComplete();
377 * Initialize all singletons which were marked as eager-initialized
378 * (using `shouldEagerInit()`). No return value. Propagates exceptions
379 * from constructors / create functions, as is the usual case when calling
380 * for example `Singleton<Foo>::get_weak()`.
385 * Schedule eager singletons' initializations through the given executor.
386 * If baton ptr is not null, its `post` method is called after all
387 * early initialization has completed.
389 * If exceptions are thrown during initialization, this method will still
390 * `post` the baton to indicate completion. The exception will not propagate
391 * and future attempts to `try_get` or `get_weak` the failed singleton will
392 * retry initialization.
396 * wangle::IOThreadPoolExecutor executor(max_concurrency_level);
397 * folly::Baton<> done;
398 * doEagerInitVia(executor, &done);
399 * done.wait(); // or 'timed_wait', or spin with 'try_wait'
402 void doEagerInitVia(Executor& exe, folly::Baton<>* done = nullptr);
404 // Destroy all singletons; when complete, the vault can't create
405 // singletons once again until reenableInstances() is called.
406 void destroyInstances();
408 // Enable re-creating singletons after destroyInstances() was called.
409 void reenableInstances();
411 // For testing; how many registered and living singletons we have.
412 size_t registeredSingletonCount() const {
413 return singletons_.rlock()->size();
417 * Flips to true if eager initialization was used, and has completed.
418 * Never set to true if "doEagerInit()" or "doEagerInitVia" never called.
420 bool eagerInitComplete() const;
422 size_t livingSingletonCount() const {
423 auto singletons = singletons_.rlock();
426 for (const auto& p : *singletons) {
427 if (p.second->hasLiveInstance()) {
435 // A well-known vault; you can actually have others, but this is the
437 static SingletonVault* singleton() {
438 return singleton<>();
441 // Gets singleton vault for any Tag. Non-default tag should be used in unit
443 template <typename VaultTag = detail::DefaultTag>
444 static SingletonVault* singleton() {
445 /* library-local */ static auto vault =
446 detail::createGlobal<SingletonVault, VaultTag>();
450 typedef std::string(*StackTraceGetterPtr)();
452 static std::atomic<StackTraceGetterPtr>& stackTraceGetter() {
453 /* library-local */ static auto stackTraceGetterPtr = detail::
454 createGlobal<std::atomic<StackTraceGetterPtr>, SingletonVault>();
455 return *stackTraceGetterPtr;
458 void setType(Type type) {
463 template <typename T>
464 friend struct detail::SingletonHolder;
466 // The two stages of life for a vault, as mentioned in the class comment.
467 enum class SingletonVaultState {
473 SingletonVaultState state{SingletonVaultState::Running};
474 bool registrationComplete{false};
477 // Each singleton in the vault can be in two states: dead
478 // (registered but never created), living (CreateFunc returned an instance).
480 static void stateCheck(
481 SingletonVaultState expected,
483 const char* msg = "Unexpected singleton state change") {
484 if (expected != state.state) {
485 throw std::logic_error(msg);
489 // This method only matters if registrationComplete() is never called.
490 // Otherwise destroyInstances is scheduled to be executed atexit.
492 // Initializes static object, which calls destroyInstances on destruction.
493 // Used to have better deletion ordering with singleton not managed by
494 // folly::Singleton. The desruction will happen in the following order:
495 // 1. Singletons, not managed by folly::Singleton, which were created after
496 // any of the singletons managed by folly::Singleton was requested.
497 // 2. All singletons managed by folly::Singleton
498 // 3. Singletons, not managed by folly::Singleton, which were created before
499 // any of the singletons managed by folly::Singleton was requested.
500 static void scheduleDestroyInstances();
502 typedef std::unordered_map<detail::TypeDescriptor,
503 detail::SingletonHolderBase*,
504 detail::TypeDescriptorHasher> SingletonMap;
505 folly::Synchronized<SingletonMap> singletons_;
506 folly::Synchronized<std::unordered_set<detail::SingletonHolderBase*>>
507 eagerInitSingletons_;
508 folly::Synchronized<std::vector<detail::TypeDescriptor>> creationOrder_;
510 // Using SharedMutexReadPriority is important here, because we want to make
511 // sure we don't block nested singleton creation happening concurrently with
512 // destroyInstances().
513 folly::Synchronized<State, folly::SharedMutexReadPriority> state_;
518 // This is the wrapper class that most users actually interact with.
519 // It allows for simple access to registering and instantiating
520 // singletons. Create instances of this class in the global scope of
521 // type Singleton<T> to register your singleton for later access via
522 // Singleton<T>::try_get().
523 template <typename T,
524 typename Tag = detail::DefaultTag,
525 typename VaultTag = detail::DefaultTag /* for testing */>
528 typedef std::function<T*(void)> CreateFunc;
529 typedef std::function<void(T*)> TeardownFunc;
531 // Generally your program life cycle should be fine with calling
532 // get() repeatedly rather than saving the reference, and then not
533 // call get() during process shutdown.
534 FOLLY_DEPRECATED("Replaced by try_get")
535 static T* get() { return getEntry().get(); }
537 // If, however, you do need to hold a reference to the specific
538 // singleton, you can try to do so with a weak_ptr. Avoid this when
539 // possible but the inability to lock the weak pointer can be a
540 // signal that the vault has been destroyed.
541 FOLLY_DEPRECATED("Replaced by try_get")
542 static std::weak_ptr<T> get_weak() { return getEntry().get_weak(); }
544 // Preferred alternative to get_weak, it returns shared_ptr that can be
545 // stored; a singleton won't be destroyed unless shared_ptr is destroyed.
546 // Avoid holding these shared_ptrs beyond the scope of a function;
547 // don't put them in member variables, always use try_get() instead
549 // try_get() can return nullptr if the singleton was destroyed, caller is
550 // responsible for handling nullptr return
551 static std::shared_ptr<T> try_get() {
552 return getEntry().try_get();
555 static folly::ReadMostlySharedPtr<T> try_get_fast() {
556 return getEntry().try_get_fast();
559 explicit Singleton(std::nullptr_t /* _ */ = nullptr,
560 typename Singleton::TeardownFunc t = nullptr)
561 : Singleton([]() { return new T; }, std::move(t)) {}
563 explicit Singleton(typename Singleton::CreateFunc c,
564 typename Singleton::TeardownFunc t = nullptr) {
566 throw std::logic_error(
567 "nullptr_t should be passed if you want T to be default constructed");
570 auto vault = SingletonVault::singleton<VaultTag>();
571 getEntry().registerSingleton(std::move(c), getTeardownFunc(std::move(t)));
572 vault->registerSingleton(&getEntry());
576 * Should be instantiated as soon as "doEagerInit[Via]" is called.
577 * Singletons are usually lazy-loaded (built on-demand) but for those which
578 * are known to be needed, to avoid the potential lag for objects that take
579 * long to construct during runtime, there is an option to make sure these
580 * are built up-front.
583 * Singleton<Foo> gFooInstance = Singleton<Foo>(...).shouldEagerInit();
585 * Or alternately, define the singleton as usual, and say
586 * gFooInstance.shouldEagerInit();
588 * at some point prior to calling registrationComplete().
589 * Then doEagerInit() or doEagerInitVia(Executor*) can be called.
591 Singleton& shouldEagerInit() {
592 auto vault = SingletonVault::singleton<VaultTag>();
593 vault->addEagerInitSingleton(&getEntry());
598 * Construct and inject a mock singleton which should be used only from tests.
599 * Unlike regular singletons which are initialized once per process lifetime,
600 * mock singletons live for the duration of a test. This means that one process
601 * running multiple tests can initialize and register the same singleton
602 * multiple times. This functionality should be used only from tests
603 * since it relaxes validation and performance in order to be able to perform
604 * the injection. The returned mock singleton is functionality identical to
605 * regular singletons.
607 static void make_mock(std::nullptr_t /* c */ = nullptr,
608 typename Singleton<T>::TeardownFunc t = nullptr) {
609 make_mock([]() { return new T; }, t);
612 static void make_mock(CreateFunc c,
613 typename Singleton<T>::TeardownFunc t = nullptr) {
615 throw std::logic_error(
616 "nullptr_t should be passed if you want T to be default constructed");
619 auto& entry = getEntry();
621 entry.registerSingletonMock(c, getTeardownFunc(t));
625 inline static detail::SingletonHolder<T>& getEntry() {
626 return detail::SingletonHolder<T>::template singleton<Tag, VaultTag>();
629 // Construct TeardownFunc.
630 static typename detail::SingletonHolder<T>::TeardownFunc getTeardownFunc(
633 return [](T* v) { delete v; };
640 template <typename T, typename Tag = detail::DefaultTag>
641 class LeakySingleton {
643 using CreateFunc = std::function<T*()>;
645 LeakySingleton() : LeakySingleton([] { return new T(); }) {}
647 explicit LeakySingleton(CreateFunc createFunc) {
648 auto& entry = entryInstance();
649 if (entry.state != State::NotRegistered) {
650 LOG(FATAL) << "Double registration of singletons of the same "
651 << "underlying type; check for multiple definitions "
652 << "of type folly::LeakySingleton<" + entry.type_.name() + ">";
654 entry.createFunc = createFunc;
655 entry.state = State::Dead;
658 static T& get() { return instance(); }
661 enum class State { NotRegistered, Dead, Living };
665 Entry(const Entry&) = delete;
666 Entry& operator=(const Entry&) = delete;
668 std::atomic<State> state{State::NotRegistered};
670 CreateFunc createFunc;
672 detail::TypeDescriptor type_{typeid(T), typeid(Tag)};
675 static Entry& entryInstance() {
676 /* library-local */ static auto entry = detail::createGlobal<Entry, Tag>();
680 static T& instance() {
681 auto& entry = entryInstance();
682 if (UNLIKELY(entry.state != State::Living)) {
689 static void createInstance() {
690 auto& entry = entryInstance();
692 std::lock_guard<std::mutex> lg(entry.mutex);
693 if (entry.state == State::Living) {
697 if (entry.state == State::NotRegistered) {
698 auto ptr = SingletonVault::stackTraceGetter().load();
699 LOG(FATAL) << "Creating instance for unregistered singleton: "
700 << entry.type_.name() << "\n"
702 << "\n" << (ptr ? (*ptr)() : "(not available)");
705 entry.ptr = entry.createFunc();
706 entry.state = State::Living;
711 #include <folly/Singleton-inl.h>