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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8 * http://www.apache.org/licenses/LICENSE-2.0
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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>
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();
248 void registerSingleton(CreateFunc c, TeardownFunc t);
249 void registerSingletonMock(CreateFunc c, TeardownFunc t);
250 virtual TypeDescriptor type() override;
251 virtual bool hasLiveInstance() override;
252 virtual void createInstance() override;
253 virtual bool creationStarted() override;
254 virtual void destroyInstance() override;
257 SingletonHolder(TypeDescriptor type, SingletonVault& vault);
259 enum class SingletonHolderState {
265 TypeDescriptor type_;
266 SingletonVault& vault_;
268 // mutex protects the entire entry during construction/destruction
271 // State of the singleton entry. If state is Living, instance_ptr and
272 // instance_weak can be safely accessed w/o synchronization.
273 std::atomic<SingletonHolderState> state_{SingletonHolderState::NotRegistered};
275 // the thread creating the singleton (only valid while creating an object)
276 std::atomic<std::thread::id> creating_thread_;
278 // The singleton itself and related functions.
280 // holds a shared_ptr to singleton instance, set when state is changed from
281 // Dead to Living. Reset when state is changed from Living to Dead.
282 std::shared_ptr<T> instance_;
283 // weak_ptr to the singleton instance, set when state is changed from Dead
284 // to Living. We never write to this object after initialization, so it is
285 // safe to read it from different threads w/o synchronization if we know
286 // that state is set to Living
287 std::weak_ptr<T> instance_weak_;
288 // Time we wait on destroy_baton after releasing Singleton shared_ptr.
289 std::shared_ptr<folly::Baton<>> destroy_baton_;
290 T* instance_ptr_ = nullptr;
291 CreateFunc create_ = nullptr;
292 TeardownFunc teardown_ = nullptr;
294 std::shared_ptr<std::atomic<bool>> print_destructor_stack_trace_;
296 SingletonHolder(const SingletonHolder&) = delete;
297 SingletonHolder& operator=(const SingletonHolder&) = delete;
298 SingletonHolder& operator=(SingletonHolder&&) = delete;
299 SingletonHolder(SingletonHolder&&) = delete;
304 class SingletonVault {
307 Strict, // Singletons can't be created before registrationComplete()
308 Relaxed, // Singletons can be created before registrationComplete()
311 explicit SingletonVault(Type type = Type::Relaxed) : type_(type) {}
313 // Destructor is only called by unit tests to check destroyInstances.
316 typedef std::function<void(void*)> TeardownFunc;
317 typedef std::function<void*(void)> CreateFunc;
319 // Ensure that Singleton has not been registered previously and that
320 // registration is not complete. If validations succeeds,
321 // register a singleton of a given type with the create and teardown
323 void registerSingleton(detail::SingletonHolderBase* entry);
326 * Called by `Singleton<T>.shouldEagerInit()` to ensure the instance
327 * is built when `doEagerInit[Via]` is called; see those methods
330 void addEagerInitSingleton(detail::SingletonHolderBase* entry);
332 // Mark registration is complete; no more singletons can be
333 // registered at this point.
334 void registrationComplete();
337 * Initialize all singletons which were marked as eager-initialized
338 * (using `shouldEagerInit()`). No return value. Propagates exceptions
339 * from constructors / create functions, as is the usual case when calling
340 * for example `Singleton<Foo>::get_weak()`.
345 * Schedule eager singletons' initializations through the given executor.
346 * If baton ptr is not null, its `post` method is called after all
347 * early initialization has completed.
349 * If exceptions are thrown during initialization, this method will still
350 * `post` the baton to indicate completion. The exception will not propagate
351 * and future attempts to `try_get` or `get_weak` the failed singleton will
352 * retry initialization.
356 * wangle::IOThreadPoolExecutor executor(max_concurrency_level);
357 * folly::Baton<> done;
358 * doEagerInitVia(executor, &done);
359 * done.wait(); // or 'timed_wait', or spin with 'try_wait'
362 void doEagerInitVia(Executor& exe, folly::Baton<>* done = nullptr);
364 // Destroy all singletons; when complete, the vault can't create
365 // singletons once again until reenableInstances() is called.
366 void destroyInstances();
368 // Enable re-creating singletons after destroyInstances() was called.
369 void reenableInstances();
371 // For testing; how many registered and living singletons we have.
372 size_t registeredSingletonCount() const {
373 RWSpinLock::ReadHolder rh(&mutex_);
375 return singletons_.size();
379 * Flips to true if eager initialization was used, and has completed.
380 * Never set to true if "doEagerInit()" or "doEagerInitVia" never called.
382 bool eagerInitComplete() const;
384 size_t livingSingletonCount() const {
385 RWSpinLock::ReadHolder rh(&mutex_);
388 for (const auto& p : singletons_) {
389 if (p.second->hasLiveInstance()) {
397 // A well-known vault; you can actually have others, but this is the
399 static SingletonVault* singleton() {
400 return singleton<>();
403 // Gets singleton vault for any Tag. Non-default tag should be used in unit
405 template <typename VaultTag = detail::DefaultTag>
406 static SingletonVault* singleton() {
407 static SingletonVault* vault = new SingletonVault();
411 typedef std::string(*StackTraceGetterPtr)();
413 static std::atomic<StackTraceGetterPtr>& stackTraceGetter() {
414 static std::atomic<StackTraceGetterPtr> stackTraceGetterPtr;
415 return stackTraceGetterPtr;
419 template <typename T>
420 friend struct detail::SingletonHolder;
422 // The two stages of life for a vault, as mentioned in the class comment.
423 enum class SingletonVaultState {
428 // Each singleton in the vault can be in two states: dead
429 // (registered but never created), living (CreateFunc returned an instance).
431 void stateCheck(SingletonVaultState expected,
432 const char* msg="Unexpected singleton state change") {
433 if (expected != state_) {
434 throw std::logic_error(msg);
438 // This method only matters if registrationComplete() is never called.
439 // Otherwise destroyInstances is scheduled to be executed atexit.
441 // Initializes static object, which calls destroyInstances on destruction.
442 // Used to have better deletion ordering with singleton not managed by
443 // folly::Singleton. The desruction will happen in the following order:
444 // 1. Singletons, not managed by folly::Singleton, which were created after
445 // any of the singletons managed by folly::Singleton was requested.
446 // 2. All singletons managed by folly::Singleton
447 // 3. Singletons, not managed by folly::Singleton, which were created before
448 // any of the singletons managed by folly::Singleton was requested.
449 static void scheduleDestroyInstances();
451 typedef std::unordered_map<detail::TypeDescriptor,
452 detail::SingletonHolderBase*,
453 detail::TypeDescriptorHasher> SingletonMap;
455 mutable folly::RWSpinLock mutex_;
456 SingletonMap singletons_;
457 std::unordered_set<detail::SingletonHolderBase*> eagerInitSingletons_;
458 std::vector<detail::TypeDescriptor> creation_order_;
459 SingletonVaultState state_{SingletonVaultState::Running};
460 bool registrationComplete_{false};
461 folly::RWSpinLock stateMutex_;
462 Type type_{Type::Relaxed};
465 // This is the wrapper class that most users actually interact with.
466 // It allows for simple access to registering and instantiating
467 // singletons. Create instances of this class in the global scope of
468 // type Singleton<T> to register your singleton for later access via
469 // Singleton<T>::try_get().
470 template <typename T,
471 typename Tag = detail::DefaultTag,
472 typename VaultTag = detail::DefaultTag /* for testing */>
475 typedef std::function<T*(void)> CreateFunc;
476 typedef std::function<void(T*)> TeardownFunc;
478 // Generally your program life cycle should be fine with calling
479 // get() repeatedly rather than saving the reference, and then not
480 // call get() during process shutdown.
481 static T* get() __attribute__ ((__deprecated__("Replaced by try_get"))) {
482 return getEntry().get();
485 // If, however, you do need to hold a reference to the specific
486 // singleton, you can try to do so with a weak_ptr. Avoid this when
487 // possible but the inability to lock the weak pointer can be a
488 // signal that the vault has been destroyed.
489 static std::weak_ptr<T> get_weak() {
490 return getEntry().get_weak();
493 // Preferred alternative to get_weak, it returns shared_ptr that can be
494 // stored; a singleton won't be destroyed unless shared_ptr is destroyed.
495 // Avoid holding these shared_ptrs beyond the scope of a function;
496 // don't put them in member variables, always use try_get() instead
498 // try_get() can return nullptr if the singleton was destroyed, caller is
499 // responsible for handling nullptr return
500 static std::shared_ptr<T> try_get() {
501 return getEntry().try_get();
504 explicit Singleton(std::nullptr_t _ = nullptr,
505 typename Singleton::TeardownFunc t = nullptr) :
506 Singleton ([]() { return new T; }, std::move(t)) {
509 explicit Singleton(typename Singleton::CreateFunc c,
510 typename Singleton::TeardownFunc t = nullptr) {
512 throw std::logic_error(
513 "nullptr_t should be passed if you want T to be default constructed");
516 auto vault = SingletonVault::singleton<VaultTag>();
517 getEntry().registerSingleton(std::move(c), getTeardownFunc(std::move(t)));
518 vault->registerSingleton(&getEntry());
522 * Should be instantiated as soon as "doEagerInit[Via]" is called.
523 * Singletons are usually lazy-loaded (built on-demand) but for those which
524 * are known to be needed, to avoid the potential lag for objects that take
525 * long to construct during runtime, there is an option to make sure these
526 * are built up-front.
529 * Singleton<Foo> gFooInstance = Singleton<Foo>(...).shouldEagerInit();
531 * Or alternately, define the singleton as usual, and say
532 * gFooInstance.shouldEagerInit();
534 * at some point prior to calling registrationComplete().
535 * Then doEagerInit() or doEagerInitVia(Executor*) can be called.
537 Singleton& shouldEagerInit() {
538 auto vault = SingletonVault::singleton<VaultTag>();
539 vault->addEagerInitSingleton(&getEntry());
544 * Construct and inject a mock singleton which should be used only from tests.
545 * Unlike regular singletons which are initialized once per process lifetime,
546 * mock singletons live for the duration of a test. This means that one process
547 * running multiple tests can initialize and register the same singleton
548 * multiple times. This functionality should be used only from tests
549 * since it relaxes validation and performance in order to be able to perform
550 * the injection. The returned mock singleton is functionality identical to
551 * regular singletons.
553 static void make_mock(std::nullptr_t c = nullptr,
554 typename Singleton<T>::TeardownFunc t = nullptr) {
555 make_mock([]() { return new T; }, t);
558 static void make_mock(CreateFunc c,
559 typename Singleton<T>::TeardownFunc t = nullptr) {
561 throw std::logic_error(
562 "nullptr_t should be passed if you want T to be default constructed");
565 auto& entry = getEntry();
567 entry.registerSingletonMock(c, getTeardownFunc(t));
571 inline static detail::SingletonHolder<T>& getEntry() {
572 return detail::SingletonHolder<T>::template singleton<Tag, VaultTag>();
575 // Construct TeardownFunc.
576 static typename detail::SingletonHolder<T>::TeardownFunc getTeardownFunc(
579 return [](T* v) { delete v; };
588 #include <folly/Singleton-inl.h>