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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/io/async/Request.h>
118 #include <condition_variable>
120 #include <unordered_map>
121 #include <unordered_set>
122 #include <functional>
126 #include <glog/logging.h>
128 // use this guard to handleSingleton breaking change in 3rd party code
129 #ifndef FOLLY_SINGLETON_TRY_GET
130 #define FOLLY_SINGLETON_TRY_GET
135 // For actual usage, please see the Singleton<T> class at the bottom
136 // of this file; that is what you will actually interact with.
138 // SingletonVault is the class that manages singleton instances. It
139 // is unaware of the underlying types of singletons, and simply
140 // manages lifecycles and invokes CreateFunc and TeardownFunc when
141 // appropriate. In general, you won't need to interact with the
142 // SingletonVault itself.
144 // A vault goes through a few stages of life:
146 // 1. Registration phase; singletons can be registered:
147 // a) Strict: no singleton can be created in this stage.
148 // b) Relaxed: singleton can be created (the default vault is Relaxed).
149 // 2. registrationComplete() has been called; singletons can no
150 // longer be registered, but they can be created.
151 // 3. A vault can return to stage 1 when destroyInstances is called.
153 // In general, you don't need to worry about any of the above; just
154 // ensure registrationComplete() is called near the top of your main()
155 // function, otherwise no singletons can be instantiated.
157 class SingletonVault;
161 struct DefaultTag {};
163 // A TypeDescriptor is the unique handle for a given singleton. It is
164 // a combinaiton of the type and of the optional name, and is used as
165 // a key in unordered_maps.
166 class TypeDescriptor {
168 TypeDescriptor(const std::type_info& ti,
169 const std::type_info& tag_ti)
170 : ti_(ti), tag_ti_(tag_ti) {
173 TypeDescriptor(const TypeDescriptor& other)
174 : ti_(other.ti_), tag_ti_(other.tag_ti_) {
177 TypeDescriptor& operator=(const TypeDescriptor& other) {
178 if (this != &other) {
180 tag_ti_ = other.tag_ti_;
186 std::string name() const {
187 auto ret = demangle(ti_.name());
188 if (tag_ti_ != std::type_index(typeid(DefaultTag))) {
190 ret += demangle(tag_ti_.name());
192 return ret.toStdString();
195 friend class TypeDescriptorHasher;
197 bool operator==(const TypeDescriptor& other) const {
198 return ti_ == other.ti_ && tag_ti_ == other.tag_ti_;
203 std::type_index tag_ti_;
206 class TypeDescriptorHasher {
208 size_t operator()(const TypeDescriptor& ti) const {
209 return folly::hash::hash_combine(ti.ti_, ti.tag_ti_);
213 // This interface is used by SingletonVault to interact with SingletonHolders.
214 // Having a non-template interface allows SingletonVault to keep a list of all
216 class SingletonHolderBase {
218 virtual ~SingletonHolderBase() = default;
220 virtual TypeDescriptor type() = 0;
221 virtual bool hasLiveInstance() = 0;
222 virtual void createInstance() = 0;
223 virtual bool creationStarted() = 0;
224 virtual void destroyInstance() = 0;
227 static constexpr std::chrono::seconds kDestroyWaitTime{5};
230 // An actual instance of a singleton, tracking the instance itself,
231 // its state as described above, and the create and teardown
233 template <typename T>
234 struct SingletonHolder : public SingletonHolderBase {
236 typedef std::function<void(T*)> TeardownFunc;
237 typedef std::function<T*(void)> CreateFunc;
239 template <typename Tag, typename VaultTag>
240 inline static SingletonHolder<T>& singleton();
243 inline std::weak_ptr<T> get_weak();
245 void registerSingleton(CreateFunc c, TeardownFunc t);
246 void registerSingletonMock(CreateFunc c, TeardownFunc t);
247 virtual TypeDescriptor type() override;
248 virtual bool hasLiveInstance() override;
249 virtual void createInstance() override;
250 virtual bool creationStarted() override;
251 virtual void destroyInstance() override;
254 SingletonHolder(TypeDescriptor type, SingletonVault& vault);
256 enum class SingletonHolderState {
262 TypeDescriptor type_;
263 SingletonVault& vault_;
265 // mutex protects the entire entry during construction/destruction
268 // State of the singleton entry. If state is Living, instance_ptr and
269 // instance_weak can be safely accessed w/o synchronization.
270 std::atomic<SingletonHolderState> state_{SingletonHolderState::NotRegistered};
272 // the thread creating the singleton (only valid while creating an object)
273 std::atomic<std::thread::id> creating_thread_;
275 // The singleton itself and related functions.
277 // holds a shared_ptr to singleton instance, set when state is changed from
278 // Dead to Living. Reset when state is changed from Living to Dead.
279 std::shared_ptr<T> instance_;
280 // weak_ptr to the singleton instance, set when state is changed from Dead
281 // to Living. We never write to this object after initialization, so it is
282 // safe to read it from different threads w/o synchronization if we know
283 // that state is set to Living
284 std::weak_ptr<T> instance_weak_;
285 // Time we wait on destroy_baton after releasing Singleton shared_ptr.
286 std::shared_ptr<folly::Baton<>> destroy_baton_;
287 T* instance_ptr_ = nullptr;
288 CreateFunc create_ = nullptr;
289 TeardownFunc teardown_ = nullptr;
291 std::shared_ptr<std::atomic<bool>> print_destructor_stack_trace_;
293 SingletonHolder(const SingletonHolder&) = delete;
294 SingletonHolder& operator=(const SingletonHolder&) = delete;
295 SingletonHolder& operator=(SingletonHolder&&) = delete;
296 SingletonHolder(SingletonHolder&&) = delete;
301 class SingletonVault {
304 Strict, // Singletons can't be created before registrationComplete()
305 Relaxed, // Singletons can be created before registrationComplete()
308 explicit SingletonVault(Type type = Type::Relaxed) : type_(type) {}
310 // Destructor is only called by unit tests to check destroyInstances.
313 typedef std::function<void(void*)> TeardownFunc;
314 typedef std::function<void*(void)> CreateFunc;
316 // Ensure that Singleton has not been registered previously and that
317 // registration is not complete. If validations succeeds,
318 // register a singleton of a given type with the create and teardown
320 void registerSingleton(detail::SingletonHolderBase* entry);
323 * Called by `Singleton<T>.shouldEagerInit()` to ensure the instance
324 * is built when `doEagerInit[Via]` is called; see those methods
327 void addEagerInitSingleton(detail::SingletonHolderBase* entry);
329 // Mark registration is complete; no more singletons can be
330 // registered at this point.
331 void registrationComplete();
334 * Initialize all singletons which were marked as eager-initialized
335 * (using `shouldEagerInit()`). No return value. Propagates exceptions
336 * from constructors / create functions, as is the usual case when calling
337 * for example `Singleton<Foo>::get_weak()`.
342 * Schedule eager singletons' initializations through the given executor.
344 void doEagerInitVia(Executor* exe);
346 // Destroy all singletons; when complete, the vault can't create
347 // singletons once again until reenableInstances() is called.
348 void destroyInstances();
350 // Enable re-creating singletons after destroyInstances() was called.
351 void reenableInstances();
353 // For testing; how many registered and living singletons we have.
354 size_t registeredSingletonCount() const {
355 RWSpinLock::ReadHolder rh(&mutex_);
357 return singletons_.size();
360 size_t livingSingletonCount() const {
361 RWSpinLock::ReadHolder rh(&mutex_);
364 for (const auto& p : singletons_) {
365 if (p.second->hasLiveInstance()) {
373 // A well-known vault; you can actually have others, but this is the
375 static SingletonVault* singleton() {
376 return singleton<>();
379 // Gets singleton vault for any Tag. Non-default tag should be used in unit
381 template <typename VaultTag = detail::DefaultTag>
382 static SingletonVault* singleton() {
383 static SingletonVault* vault = new SingletonVault();
387 typedef std::string(*StackTraceGetterPtr)();
389 static std::atomic<StackTraceGetterPtr>& stackTraceGetter() {
390 static std::atomic<StackTraceGetterPtr> stackTraceGetterPtr;
391 return stackTraceGetterPtr;
395 template <typename T>
396 friend struct detail::SingletonHolder;
398 // The two stages of life for a vault, as mentioned in the class comment.
399 enum class SingletonVaultState {
404 // Each singleton in the vault can be in two states: dead
405 // (registered but never created), living (CreateFunc returned an instance).
407 void stateCheck(SingletonVaultState expected,
408 const char* msg="Unexpected singleton state change") {
409 if (expected != state_) {
410 throw std::logic_error(msg);
414 // This method only matters if registrationComplete() is never called.
415 // Otherwise destroyInstances is scheduled to be executed atexit.
417 // Initializes static object, which calls destroyInstances on destruction.
418 // Used to have better deletion ordering with singleton not managed by
419 // folly::Singleton. The desruction will happen in the following order:
420 // 1. Singletons, not managed by folly::Singleton, which were created after
421 // any of the singletons managed by folly::Singleton was requested.
422 // 2. All singletons managed by folly::Singleton
423 // 3. Singletons, not managed by folly::Singleton, which were created before
424 // any of the singletons managed by folly::Singleton was requested.
425 static void scheduleDestroyInstances();
427 typedef std::unordered_map<detail::TypeDescriptor,
428 detail::SingletonHolderBase*,
429 detail::TypeDescriptorHasher> SingletonMap;
431 mutable folly::RWSpinLock mutex_;
432 SingletonMap singletons_;
433 std::unordered_set<detail::SingletonHolderBase*> eagerInitSingletons_;
434 std::vector<detail::TypeDescriptor> creation_order_;
435 SingletonVaultState state_{SingletonVaultState::Running};
436 bool registrationComplete_{false};
437 folly::RWSpinLock stateMutex_;
438 Type type_{Type::Relaxed};
441 // This is the wrapper class that most users actually interact with.
442 // It allows for simple access to registering and instantiating
443 // singletons. Create instances of this class in the global scope of
444 // type Singleton<T> to register your singleton for later access via
445 // Singleton<T>::try_get().
446 template <typename T,
447 typename Tag = detail::DefaultTag,
448 typename VaultTag = detail::DefaultTag /* for testing */>
451 typedef std::function<T*(void)> CreateFunc;
452 typedef std::function<void(T*)> TeardownFunc;
454 // Generally your program life cycle should be fine with calling
455 // get() repeatedly rather than saving the reference, and then not
456 // call get() during process shutdown.
457 static T* get() __attribute__ ((__deprecated__("Replaced by try_get"))) {
458 return getEntry().get();
461 // If, however, you do need to hold a reference to the specific
462 // singleton, you can try to do so with a weak_ptr. Avoid this when
463 // possible but the inability to lock the weak pointer can be a
464 // signal that the vault has been destroyed.
465 static std::weak_ptr<T> get_weak() {
466 return getEntry().get_weak();
469 // Preferred alternative to get_weak, it returns shared_ptr that can be
470 // stored; a singleton won't be destroyed unless shared_ptr is destroyed.
471 // Avoid holding these shared_ptrs beyond the scope of a function;
472 // don't put them in member variables, always use try_get() instead
473 static std::shared_ptr<T> try_get() {
474 auto ret = get_weak().lock();
477 "folly::Singleton<" << getEntry().type().name() <<
478 ">::get_weak() called on destructed singleton; "
479 "returning nullptr, possible segfault coming";
484 explicit Singleton(std::nullptr_t _ = nullptr,
485 typename Singleton::TeardownFunc t = nullptr) :
486 Singleton ([]() { return new T; }, std::move(t)) {
489 explicit Singleton(typename Singleton::CreateFunc c,
490 typename Singleton::TeardownFunc t = nullptr) {
492 throw std::logic_error(
493 "nullptr_t should be passed if you want T to be default constructed");
496 auto vault = SingletonVault::singleton<VaultTag>();
497 getEntry().registerSingleton(std::move(c), getTeardownFunc(std::move(t)));
498 vault->registerSingleton(&getEntry());
502 * Should be instantiated as soon as "doEagerInit[Via]" is called.
503 * Singletons are usually lazy-loaded (built on-demand) but for those which
504 * are known to be needed, to avoid the potential lag for objects that take
505 * long to construct during runtime, there is an option to make sure these
506 * are built up-front.
509 * Singleton<Foo> gFooInstance = Singleton<Foo>(...).shouldEagerInit();
511 * Or alternately, define the singleton as usual, and say
512 * gFooInstance.shouldEagerInit();
514 * at some point prior to calling registrationComplete().
515 * Then doEagerInit() or doEagerInitVia(Executor*) can be called.
517 Singleton& shouldEagerInit() {
518 auto vault = SingletonVault::singleton<VaultTag>();
519 vault->addEagerInitSingleton(&getEntry());
524 * Construct and inject a mock singleton which should be used only from tests.
525 * Unlike regular singletons which are initialized once per process lifetime,
526 * mock singletons live for the duration of a test. This means that one process
527 * running multiple tests can initialize and register the same singleton
528 * multiple times. This functionality should be used only from tests
529 * since it relaxes validation and performance in order to be able to perform
530 * the injection. The returned mock singleton is functionality identical to
531 * regular singletons.
533 static void make_mock(std::nullptr_t c = nullptr,
534 typename Singleton<T>::TeardownFunc t = nullptr) {
535 make_mock([]() { return new T; }, t);
538 static void make_mock(CreateFunc c,
539 typename Singleton<T>::TeardownFunc t = nullptr) {
541 throw std::logic_error(
542 "nullptr_t should be passed if you want T to be default constructed");
545 auto& entry = getEntry();
547 entry.registerSingletonMock(c, getTeardownFunc(t));
551 inline static detail::SingletonHolder<T>& getEntry() {
552 return detail::SingletonHolder<T>::template singleton<Tag, VaultTag>();
555 // Construct TeardownFunc.
556 static typename detail::SingletonHolder<T>::TeardownFunc getTeardownFunc(
559 return [](T* v) { delete v; };
568 #include <folly/Singleton-inl.h>