1/*
2 * Copyright 2014-present Facebook, Inc.
3 *
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
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
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.
15 */
16// SingletonVault - a library to manage the creation and destruction
17// of interdependent singletons.
18//
19// Recommended usage of this class: suppose you have a class
20// called MyExpensiveService, and you only want to construct one (ie,
21// it's a singleton), but you only want to construct it if it is used.
22//
23// In your .h file:
24// class MyExpensiveService {
25// // Caution - may return a null ptr during startup and shutdown.
26// static std::shared_ptr<MyExpensiveService> getInstance();
27// ....
28// };
29//
30// In your .cpp file:
31// namespace { struct PrivateTag {}; }
32// static folly::Singleton<MyExpensiveService, PrivateTag> the_singleton;
33// std::shared_ptr<MyExpensiveService> MyExpensiveService::getInstance() {
34// return the_singleton.try_get();
35// }
36//
37// Code in other modules can access it via:
38//
39// auto instance = MyExpensiveService::getInstance();
40//
41// Advanced usage and notes:
42//
43// You can also access a singleton instance with
44// `Singleton<ObjectType, TagType>::try_get()`. We recommend
45// that you prefer the form `the_singleton.try_get()` because it ensures that
46// `the_singleton` is used and cannot be garbage-collected during linking: this
47// is necessary because the constructor of `the_singleton` is what registers it
48// to the SingletonVault.
49//
50// The singleton will be created on demand. If the constructor for
51// MyExpensiveService actually makes use of *another* Singleton, then
52// the right thing will happen -- that other singleton will complete
53// construction before get() returns. However, in the event of a
54// circular dependency, a runtime error will occur.
55//
56// You can have multiple singletons of the same underlying type, but
57// each must be given a unique tag. If no tag is specified a default tag is
58// used. We recommend that you use a tag from an anonymous namespace private to
59// your implementation file, as this ensures that the singleton is only
60// available via your interface and not also through Singleton<T>::try_get()
61//
62// namespace {
63// struct Tag1 {};
64// struct Tag2 {};
65// folly::Singleton<MyExpensiveService> s_default;
66// folly::Singleton<MyExpensiveService, Tag1> s1;
67// folly::Singleton<MyExpensiveService, Tag2> s2;
68// }
69// ...
70// MyExpensiveService* svc_default = s_default.get();
71// MyExpensiveService* svc1 = s1.get();
72// MyExpensiveService* svc2 = s2.get();
73//
74// By default, the singleton instance is constructed via new and
75// deleted via delete, but this is configurable:
76//
77// namespace { folly::Singleton<MyExpensiveService> the_singleton(create,
78// destroy); }
79//
80// Where create and destroy are functions, Singleton<T>::CreateFunc
81// Singleton<T>::TeardownFunc.
82//
83// For example, if you need to pass arguments to your class's constructor:
84// class X {
85// public:
86// X(int a1, std::string a2);
87// // ...
88// }
89// Make your singleton like this:
90// folly::Singleton<X> singleton_x([]() { return new X(42, "foo"); });
91//
92// The above examples detail a situation where an expensive singleton is loaded
93// on-demand (thus only if needed). However if there is an expensive singleton
94// that will likely be needed, and initialization takes a potentially long time,
95// e.g. while initializing, parsing some files, talking to remote services,
96// making uses of other singletons, and so on, the initialization of those can
97// be scheduled up front, or "eagerly".
98//
99// In that case the singleton can be declared this way:
100//
101// namespace {
102// auto the_singleton =
103// folly::Singleton<MyExpensiveService>(/* optional create, destroy args */)
104// .shouldEagerInit();
105// }
106//
107// This way the singleton's instance is built at program initialization,
108// if the program opted-in to that feature by calling "doEagerInit" or
109// "doEagerInitVia" during its startup.
110//
111// What if you need to destroy all of your singletons? Say, some of
112// your singletons manage threads, but you need to fork? Or your unit
113// test wants to clean up all global state? Then you can call
114// SingletonVault::singleton()->destroyInstances(), which invokes the
115// TeardownFunc for each singleton, in the reverse order they were
116// created. It is your responsibility to ensure your singletons can
117// handle cases where the singletons they depend on go away, however.
118// Singletons won't be recreated after destroyInstances call. If you
119// want to re-enable singleton creation (say after fork was called) you
120// should call reenableInstances.
121
122#pragma once
123
124#include <folly/Exception.h>
125#include <folly/Executor.h>
126#include <folly/Memory.h>
127#include <folly/Synchronized.h>
128#include <folly/detail/Singleton.h>
129#include <folly/detail/StaticSingletonManager.h>
130#include <folly/experimental/ReadMostlySharedPtr.h>
131#include <folly/hash/Hash.h>
132#include <folly/lang/Exception.h>
133#include <folly/synchronization/Baton.h>
134#include <folly/synchronization/RWSpinLock.h>
135
136#include <algorithm>
137#include <atomic>
138#include <condition_variable>
139#include <functional>
140#include <list>
141#include <memory>
142#include <mutex>
143#include <string>
144#include <thread>
145#include <typeindex>
146#include <typeinfo>
147#include <unordered_map>
148#include <unordered_set>
149#include <vector>
150
151#include <glog/logging.h>
152
153// use this guard to handleSingleton breaking change in 3rd party code
154#ifndef FOLLY_SINGLETON_TRY_GET
155#define FOLLY_SINGLETON_TRY_GET
156#endif
157
158namespace folly {
159
160// For actual usage, please see the Singleton<T> class at the bottom
161// of this file; that is what you will actually interact with.
162
163// SingletonVault is the class that manages singleton instances. It
164// is unaware of the underlying types of singletons, and simply
165// manages lifecycles and invokes CreateFunc and TeardownFunc when
166// appropriate. In general, you won't need to interact with the
167// SingletonVault itself.
168//
169// A vault goes through a few stages of life:
170//
171// 1. Registration phase; singletons can be registered:
172// a) Strict: no singleton can be created in this stage.
173// b) Relaxed: singleton can be created (the default vault is Relaxed).
174// 2. registrationComplete() has been called; singletons can no
175// longer be registered, but they can be created.
176// 3. A vault can return to stage 1 when destroyInstances is called.
177//
178// In general, you don't need to worry about any of the above; just
179// ensure registrationComplete() is called near the top of your main()
180// function, otherwise no singletons can be instantiated.
181
182class SingletonVault;
183
184namespace detail {
185
186// A TypeDescriptor is the unique handle for a given singleton. It is
187// a combinaiton of the type and of the optional name, and is used as
188// a key in unordered_maps.
189class TypeDescriptor {
190 public:
191 TypeDescriptor(const std::type_info& ti, const std::type_info& tag_ti)
192 : ti_(ti), tag_ti_(tag_ti) {}
193
194 TypeDescriptor(const TypeDescriptor& other)
195 : ti_(other.ti_), tag_ti_(other.tag_ti_) {}
196
197 TypeDescriptor& operator=(const TypeDescriptor& other) {
198 if (this != &other) {
199 ti_ = other.ti_;
200 tag_ti_ = other.tag_ti_;
201 }
202
203 return *this;
204 }
205
206 std::string name() const;
207
208 friend class TypeDescriptorHasher;
209
210 bool operator==(const TypeDescriptor& other) const {
211 return ti_ == other.ti_ && tag_ti_ == other.tag_ti_;
212 }
213
214 private:
215 std::type_index ti_;
216 std::type_index tag_ti_;
217};
218
219class TypeDescriptorHasher {
220 public:
221 size_t operator()(const TypeDescriptor& ti) const {
222 return folly::hash::hash_combine(ti.ti_, ti.tag_ti_);
223 }
224};
225
226[[noreturn]] void singletonWarnLeakyDoubleRegistrationAndAbort(
227 const TypeDescriptor& type);
228
229[[noreturn]] void singletonWarnLeakyInstantiatingNotRegisteredAndAbort(
230 const TypeDescriptor& type);
231
232[[noreturn]] void singletonWarnRegisterMockEarlyAndAbort(
233 const TypeDescriptor& type);
234
235void singletonWarnDestroyInstanceLeak(
236 const TypeDescriptor& type,
237 const void* ptr);
238
239[[noreturn]] void singletonWarnCreateCircularDependencyAndAbort(
240 const TypeDescriptor& type);
241
242[[noreturn]] void singletonWarnCreateUnregisteredAndAbort(
243 const TypeDescriptor& type);
244
245[[noreturn]] void singletonWarnCreateBeforeRegistrationCompleteAndAbort(
246 const TypeDescriptor& type);
247
248void singletonPrintDestructionStackTrace(const TypeDescriptor& type);
249
250[[noreturn]] void singletonThrowNullCreator(const std::type_info& type);
251
252[[noreturn]] void singletonThrowGetInvokedAfterDestruction(
253 const TypeDescriptor& type);
254
255struct SingletonVaultState {
256 // The two stages of life for a vault, as mentioned in the class comment.
257 enum class Type {
258 Running,
259 Quiescing,
260 };
261
262 Type state{Type::Running};
263 bool registrationComplete{false};
264
265 // Each singleton in the vault can be in two states: dead
266 // (registered but never created), living (CreateFunc returned an instance).
267
268 void check(
269 Type expected,
270 const char* msg = "Unexpected singleton state change") const {
271 if (expected != state) {
272 throw_exception<std::logic_error>(msg);
273 }
274 }
275};
276
277// This interface is used by SingletonVault to interact with SingletonHolders.
278// Having a non-template interface allows SingletonVault to keep a list of all
279// SingletonHolders.
280class SingletonHolderBase {
281 public:
282 explicit SingletonHolderBase(TypeDescriptor typeDesc) : type_(typeDesc) {}
283 virtual ~SingletonHolderBase() = default;
284
285 TypeDescriptor type() const {
286 return type_;
287 }
288 virtual bool hasLiveInstance() = 0;
289 virtual void createInstance() = 0;
290 virtual bool creationStarted() = 0;
291 virtual void preDestroyInstance(ReadMostlyMainPtrDeleter<>&) = 0;
292 virtual void destroyInstance() = 0;
293
294 private:
295 TypeDescriptor type_;
296};
297
298// An actual instance of a singleton, tracking the instance itself,
299// its state as described above, and the create and teardown
300// functions.
301template <typename T>
302struct SingletonHolder : public SingletonHolderBase {
303 public:
304 typedef std::function<void(T*)> TeardownFunc;
305 typedef std::function<T*(void)> CreateFunc;
306
307 template <typename Tag, typename VaultTag>
308 inline static SingletonHolder<T>& singleton();
309
310 inline T* get();
311 inline std::weak_ptr<T> get_weak();
312 inline std::shared_ptr<T> try_get();
313 inline folly::ReadMostlySharedPtr<T> try_get_fast();
314 inline void vivify();
315
316 void registerSingleton(CreateFunc c, TeardownFunc t);
317 void registerSingletonMock(CreateFunc c, TeardownFunc t);
318 bool hasLiveInstance() override;
319 void createInstance() override;
320 bool creationStarted() override;
321 void preDestroyInstance(ReadMostlyMainPtrDeleter<>&) override;
322 void destroyInstance() override;
323
324 private:
325 template <typename Tag, typename VaultTag>
326 struct Impl;
327
328 SingletonHolder(TypeDescriptor type, SingletonVault& vault);
329
330 enum class SingletonHolderState {
331 NotRegistered,
332 Dead,
333 Living,
334 };
335
336 SingletonVault& vault_;
337
338 // mutex protects the entire entry during construction/destruction
339 std::mutex mutex_;
340
341 // State of the singleton entry. If state is Living, instance_ptr and
342 // instance_weak can be safely accessed w/o synchronization.
343 std::atomic<SingletonHolderState> state_{SingletonHolderState::NotRegistered};
344
345 // the thread creating the singleton (only valid while creating an object)
346 std::atomic<std::thread::id> creating_thread_{};
347
348 // The singleton itself and related functions.
349
350 // holds a ReadMostlyMainPtr to singleton instance, set when state is changed
351 // from Dead to Living. Reset when state is changed from Living to Dead.
352 folly::ReadMostlyMainPtr<T> instance_;
353 // used to release all ReadMostlyMainPtrs at once
354 folly::ReadMostlySharedPtr<T> instance_copy_;
355 // weak_ptr to the singleton instance, set when state is changed from Dead
356 // to Living. We never write to this object after initialization, so it is
357 // safe to read it from different threads w/o synchronization if we know
358 // that state is set to Living
359 std::weak_ptr<T> instance_weak_;
360 // Fast equivalent of instance_weak_
361 folly::ReadMostlyWeakPtr<T> instance_weak_fast_;
362 // Time we wait on destroy_baton after releasing Singleton shared_ptr.
363 std::shared_ptr<folly::Baton<>> destroy_baton_;
364 T* instance_ptr_ = nullptr;
365 CreateFunc create_ = nullptr;
366 TeardownFunc teardown_ = nullptr;
367
368 std::shared_ptr<std::atomic<bool>> print_destructor_stack_trace_;
369
370 SingletonHolder(const SingletonHolder&) = delete;
371 SingletonHolder& operator=(const SingletonHolder&) = delete;
372 SingletonHolder& operator=(SingletonHolder&&) = delete;
373 SingletonHolder(SingletonHolder&&) = delete;
374};
375
376} // namespace detail
377
378class SingletonVault {
379 public:
380 enum class Type {
381 Strict, // Singletons can't be created before registrationComplete()
382 Relaxed, // Singletons can be created before registrationComplete()
383 };
384
385 /**
386 * Clears all singletons in the given vault at ctor and dtor times.
387 * Useful for unit-tests that need to clear the world.
388 *
389 * This need can arise when a unit-test needs to swap out an object used by a
390 * singleton for a test-double, but the singleton needing its dependency to be
391 * swapped has a type or a tag local to some other translation unit and
392 * unavailable in the current translation unit.
393 *
394 * Other, better approaches to this need are "plz 2 refactor" ....
395 */
396 struct ScopedExpunger {
397 SingletonVault* vault;
398 explicit ScopedExpunger(SingletonVault* v) : vault(v) {
399 expunge();
400 }
401 ~ScopedExpunger() {
402 expunge();
403 }
404 void expunge() {
405 vault->destroyInstances();
406 vault->reenableInstances();
407 }
408 };
409
410 static Type defaultVaultType();
411
412 explicit SingletonVault(Type type = defaultVaultType()) : type_(type) {}
413
414 // Destructor is only called by unit tests to check destroyInstances.
415 ~SingletonVault();
416
417 typedef std::function<void(void*)> TeardownFunc;
418 typedef std::function<void*(void)> CreateFunc;
419
420 // Ensure that Singleton has not been registered previously and that
421 // registration is not complete. If validations succeeds,
422 // register a singleton of a given type with the create and teardown
423 // functions.
424 void registerSingleton(detail::SingletonHolderBase* entry);
425
426 /**
427 * Called by `Singleton<T>.shouldEagerInit()` to ensure the instance
428 * is built when `doEagerInit[Via]` is called; see those methods
429 * for more info.
430 */
431 void addEagerInitSingleton(detail::SingletonHolderBase* entry);
432
433 // Mark registration is complete; no more singletons can be
434 // registered at this point.
435 void registrationComplete();
436
437 /**
438 * Initialize all singletons which were marked as eager-initialized
439 * (using `shouldEagerInit()`). No return value. Propagates exceptions
440 * from constructors / create functions, as is the usual case when calling
441 * for example `Singleton<Foo>::get_weak()`.
442 */
443 void doEagerInit();
444
445 /**
446 * Schedule eager singletons' initializations through the given executor.
447 * If baton ptr is not null, its `post` method is called after all
448 * early initialization has completed.
449 *
450 * If exceptions are thrown during initialization, this method will still
451 * `post` the baton to indicate completion. The exception will not propagate
452 * and future attempts to `try_get` or `get_weak` the failed singleton will
453 * retry initialization.
454 *
455 * Sample usage:
456 *
457 * folly::IOThreadPoolExecutor executor(max_concurrency_level);
458 * folly::Baton<> done;
459 * doEagerInitVia(executor, &done);
460 * done.wait(); // or 'try_wait_for', etc.
461 *
462 */
463 void doEagerInitVia(Executor& exe, folly::Baton<>* done = nullptr);
464
465 // Destroy all singletons; when complete, the vault can't create
466 // singletons once again until reenableInstances() is called.
467 void destroyInstances();
468
469 // Enable re-creating singletons after destroyInstances() was called.
470 void reenableInstances();
471
472 // For testing; how many registered and living singletons we have.
473 size_t registeredSingletonCount() const {
474 return singletons_.rlock()->size();
475 }
476
477 /**
478 * Flips to true if eager initialization was used, and has completed.
479 * Never set to true if "doEagerInit()" or "doEagerInitVia" never called.
480 */
481 bool eagerInitComplete() const;
482
483 size_t livingSingletonCount() const {
484 auto singletons = singletons_.rlock();
485
486 size_t ret = 0;
487 for (const auto& p : *singletons) {
488 if (p.second->hasLiveInstance()) {
489 ++ret;
490 }
491 }
492
493 return ret;
494 }
495
496 // A well-known vault; you can actually have others, but this is the
497 // default.
498 static SingletonVault* singleton() {
499 return singleton<>();
500 }
501
502 // Gets singleton vault for any Tag. Non-default tag should be used in unit
503 // tests only.
504 template <typename VaultTag = detail::DefaultTag>
505 static SingletonVault* singleton() {
506 return &detail::createGlobal<SingletonVault, VaultTag>();
507 }
508
509 void setType(Type type) {
510 type_ = type;
511 }
512
513 private:
514 template <typename T>
515 friend struct detail::SingletonHolder;
516
517 // This method only matters if registrationComplete() is never called.
518 // Otherwise destroyInstances is scheduled to be executed atexit.
519 //
520 // Initializes static object, which calls destroyInstances on destruction.
521 // Used to have better deletion ordering with singleton not managed by
522 // folly::Singleton. The desruction will happen in the following order:
523 // 1. Singletons, not managed by folly::Singleton, which were created after
524 // any of the singletons managed by folly::Singleton was requested.
525 // 2. All singletons managed by folly::Singleton
526 // 3. Singletons, not managed by folly::Singleton, which were created before
527 // any of the singletons managed by folly::Singleton was requested.
528 static void scheduleDestroyInstances();
529
530 typedef std::unordered_map<
531 detail::TypeDescriptor,
532 detail::SingletonHolderBase*,
533 detail::TypeDescriptorHasher>
534 SingletonMap;
535
536 // Use SharedMutexSuppressTSAN to suppress noisy lock inversions when building
537 // with TSAN. If TSAN is not enabled, SharedMutexSuppressTSAN is equivalent
538 // to a normal SharedMutex.
539 Synchronized<SingletonMap, SharedMutexSuppressTSAN> singletons_;
540 Synchronized<
541 std::unordered_set<detail::SingletonHolderBase*>,
542 SharedMutexSuppressTSAN>
543 eagerInitSingletons_;
544 Synchronized<std::vector<detail::TypeDescriptor>, SharedMutexSuppressTSAN>
545 creationOrder_;
546
547 // Using SharedMutexReadPriority is important here, because we want to make
548 // sure we don't block nested singleton creation happening concurrently with
549 // destroyInstances().
550 Synchronized<detail::SingletonVaultState, SharedMutexReadPriority> state_;
551
552 Type type_;
553};
554
555// This is the wrapper class that most users actually interact with.
556// It allows for simple access to registering and instantiating
557// singletons. Create instances of this class in the global scope of
558// type Singleton<T> to register your singleton for later access via
559// Singleton<T>::try_get().
560template <
561 typename T,
562 typename Tag = detail::DefaultTag,
563 typename VaultTag = detail::DefaultTag /* for testing */>
564class Singleton {
565 public:
566 typedef std::function<T*(void)> CreateFunc;
567 typedef std::function<void(T*)> TeardownFunc;
568
569 // Generally your program life cycle should be fine with calling
570 // get() repeatedly rather than saving the reference, and then not
571 // call get() during process shutdown.
572 [[deprecated("Replaced by try_get")]] static T* get() {
573 return getEntry().get();
574 }
575
576 // If, however, you do need to hold a reference to the specific
577 // singleton, you can try to do so with a weak_ptr. Avoid this when
578 // possible but the inability to lock the weak pointer can be a
579 // signal that the vault has been destroyed.
580 [[deprecated("Replaced by try_get")]] static std::weak_ptr<T> get_weak() {
581 return getEntry().get_weak();
582 }
583
584 // Preferred alternative to get_weak, it returns shared_ptr that can be
585 // stored; a singleton won't be destroyed unless shared_ptr is destroyed.
586 // Avoid holding these shared_ptrs beyond the scope of a function;
587 // don't put them in member variables, always use try_get() instead
588 //
589 // try_get() can return nullptr if the singleton was destroyed, caller is
590 // responsible for handling nullptr return
591 static std::shared_ptr<T> try_get() {
592 return getEntry().try_get();
593 }
594
595 static folly::ReadMostlySharedPtr<T> try_get_fast() {
596 return getEntry().try_get_fast();
597 }
598
599 // Quickly ensure the instance exists.
600 static void vivify() {
601 getEntry().vivify();
602 }
603
604 explicit Singleton(
605 std::nullptr_t /* _ */ = nullptr,
606 typename Singleton::TeardownFunc t = nullptr)
607 : Singleton([]() { return new T; }, std::move(t)) {}
608
609 explicit Singleton(
610 typename Singleton::CreateFunc c,
611 typename Singleton::TeardownFunc t = nullptr) {
612 if (c == nullptr) {
613 detail::singletonThrowNullCreator(typeid(T));
614 }
615
616 auto vault = SingletonVault::singleton<VaultTag>();
617 getEntry().registerSingleton(std::move(c), getTeardownFunc(std::move(t)));
618 vault->registerSingleton(&getEntry());
619 }
620
621 /**
622 * Should be instantiated as soon as "doEagerInit[Via]" is called.
623 * Singletons are usually lazy-loaded (built on-demand) but for those which
624 * are known to be needed, to avoid the potential lag for objects that take
625 * long to construct during runtime, there is an option to make sure these
626 * are built up-front.
627 *
628 * Use like:
629 * Singleton<Foo> gFooInstance = Singleton<Foo>(...).shouldEagerInit();
630 *
631 * Or alternately, define the singleton as usual, and say
632 * gFooInstance.shouldEagerInit();
633 *
634 * at some point prior to calling registrationComplete().
635 * Then doEagerInit() or doEagerInitVia(Executor*) can be called.
636 */
637 Singleton& shouldEagerInit() {
638 auto vault = SingletonVault::singleton<VaultTag>();
639 vault->addEagerInitSingleton(&getEntry());
640 return *this;
641 }
642
643 /**
644 * Construct and inject a mock singleton which should be used only from tests.
645 * Unlike regular singletons which are initialized once per process lifetime,
646 * mock singletons live for the duration of a test. This means that one
647 * process running multiple tests can initialize and register the same
648 * singleton multiple times. This functionality should be used only from tests
649 * since it relaxes validation and performance in order to be able to perform
650 * the injection. The returned mock singleton is functionality identical to
651 * regular singletons.
652 */
653 static void make_mock(
654 std::nullptr_t /* c */ = nullptr,
655 typename Singleton<T>::TeardownFunc t = nullptr) {
656 make_mock([]() { return new T; }, t);
657 }
658
659 static void make_mock(
660 CreateFunc c,
661 typename Singleton<T>::TeardownFunc t = nullptr) {
662 if (c == nullptr) {
663 detail::singletonThrowNullCreator(typeid(T));
664 }
665
666 auto& entry = getEntry();
667
668 entry.registerSingletonMock(c, getTeardownFunc(t));
669 }
670
671 private:
672 inline static detail::SingletonHolder<T>& getEntry() {
673 return detail::SingletonHolder<T>::template singleton<Tag, VaultTag>();
674 }
675
676 // Construct TeardownFunc.
677 static typename detail::SingletonHolder<T>::TeardownFunc getTeardownFunc(
678 TeardownFunc t) {
679 if (t == nullptr) {
680 return [](T* v) { delete v; };
681 } else {
682 return t;
683 }
684 }
685};
686
687template <typename T, typename Tag = detail::DefaultTag>
688class LeakySingleton {
689 public:
690 using CreateFunc = std::function<T*()>;
691
692 LeakySingleton() : LeakySingleton([] { return new T(); }) {}
693
694 explicit LeakySingleton(CreateFunc createFunc) {
695 auto& entry = entryInstance();
696 if (entry.state != State::NotRegistered) {
697 detail::singletonWarnLeakyDoubleRegistrationAndAbort(entry.type_);
698 }
699 entry.createFunc = createFunc;
700 entry.state = State::Dead;
701 }
702
703 static T& get() {
704 return instance();
705 }
706
707 static void make_mock(std::nullptr_t /* c */ = nullptr) {
708 make_mock([]() { return new T; });
709 }
710
711 static void make_mock(CreateFunc createFunc) {
712 if (createFunc == nullptr) {
713 detail::singletonThrowNullCreator(typeid(T));
714 }
715
716 auto& entry = entryInstance();
717 if (entry.ptr) {
718 // Make sure existing pointer doesn't get reported as a leak by LSAN.
719 entry.leakedPtrs.push_back(std::exchange(entry.ptr, nullptr));
720 }
721 entry.createFunc = createFunc;
722 entry.state = State::Dead;
723 }
724
725 private:
726 enum class State { NotRegistered, Dead, Living };
727
728 struct Entry {
729 Entry() {}
730 Entry(const Entry&) = delete;
731 Entry& operator=(const Entry&) = delete;
732
733 std::atomic<State> state{State::NotRegistered};
734 T* ptr{nullptr};
735 CreateFunc createFunc;
736 std::mutex mutex;
737 detail::TypeDescriptor type_{typeid(T), typeid(Tag)};
738 std::list<T*> leakedPtrs;
739 };
740
741 static Entry& entryInstance() {
742 return detail::createGlobal<Entry, Tag>();
743 }
744
745 static T& instance() {
746 auto& entry = entryInstance();
747 if (UNLIKELY(entry.state != State::Living)) {
748 createInstance();
749 }
750
751 return *entry.ptr;
752 }
753
754 static void createInstance() {
755 auto& entry = entryInstance();
756
757 std::lock_guard<std::mutex> lg(entry.mutex);
758 if (entry.state == State::Living) {
759 return;
760 }
761
762 if (entry.state == State::NotRegistered) {
763 detail::singletonWarnLeakyInstantiatingNotRegisteredAndAbort(entry.type_);
764 }
765
766 entry.ptr = entry.createFunc();
767 entry.state = State::Living;
768 }
769};
770} // namespace folly
771
772#include <folly/Singleton-inl.h>
773