/*
- * Copyright 2015 Facebook, Inc.
+ * Copyright 2017 Facebook, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
// SingletonVault - a library to manage the creation and destruction
// of interdependent singletons.
//
-// Basic usage of this class is very simple; suppose you have a class
+// Recommended usage of this class: suppose you have a class
// called MyExpensiveService, and you only want to construct one (ie,
// it's a singleton), but you only want to construct it if it is used.
//
// In your .h file:
-// class MyExpensiveService { ... };
+// class MyExpensiveService {
+// // Caution - may return a null ptr during startup and shutdown.
+// static std::shared_ptr<MyExpensiveService> getInstance();
+// ....
+// };
//
// In your .cpp file:
-// namespace { folly::Singleton<MyExpensiveService> the_singleton; }
+// namespace { struct PrivateTag {}; }
+// static folly::Singleton<MyExpensiveService, PrivateTag> the_singleton;
+// std::shared_ptr<MyExpensiveService> MyExpensiveService::getInstance() {
+// return the_singleton.try_get();
+// }
//
-// Code can access it via:
+// Code in other modules can access it via:
//
-// MyExpensiveService* instance = Singleton<MyExpensiveService>::get();
-// or
-// std::weak_ptr<MyExpensiveService> instance =
-// Singleton<MyExpensiveService>::get_weak();
+// auto instance = MyExpensiveService::getInstance();
//
-// You also can directly access it by the variable defining the
-// singleton rather than via get(), and even treat that variable like
-// a smart pointer (dereferencing it or using the -> operator).
+// Advanced usage and notes:
//
-// Please note, however, that all non-weak_ptr interfaces are
-// inherently subject to races with destruction. Use responsibly.
+// You can also access a singleton instance with
+// `Singleton<ObjectType, TagType>::try_get()`. We recommend
+// that you prefer the form `the_singleton.try_get()` because it ensures that
+// `the_singleton` is used and cannot be garbage-collected during linking: this
+// is necessary because the constructor of `the_singleton` is what registers it
+// to the SingletonVault.
//
// The singleton will be created on demand. If the constructor for
// MyExpensiveService actually makes use of *another* Singleton, then
// circular dependency, a runtime error will occur.
//
// You can have multiple singletons of the same underlying type, but
-// each must be given a unique tag. If no tag is specified - default tag is used
+// each must be given a unique tag. If no tag is specified a default tag is
+// used. We recommend that you use a tag from an anonymous namespace private to
+// your implementation file, as this ensures that the singleton is only
+// available via your interface and not also through Singleton<T>::try_get()
//
// namespace {
// struct Tag1 {};
// struct Tag2 {};
-// folly::Singleton<MyExpensiveService> s_default();
-// folly::Singleton<MyExpensiveService, Tag1> s1();
-// folly::Singleton<MyExpensiveService, Tag2> s2();
+// folly::Singleton<MyExpensiveService> s_default;
+// folly::Singleton<MyExpensiveService, Tag1> s1;
+// folly::Singleton<MyExpensiveService, Tag2> s2;
// }
// ...
// MyExpensiveService* svc_default = s_default.get();
// Where create and destroy are functions, Singleton<T>::CreateFunc
// Singleton<T>::TeardownFunc.
//
+// For example, if you need to pass arguments to your class's constructor:
+// class X {
+// public:
+// X(int a1, std::string a2);
+// // ...
+// }
+// Make your singleton like this:
+// folly::Singleton<X> singleton_x([]() { return new X(42, "foo"); });
+//
+// The above examples detail a situation where an expensive singleton is loaded
+// on-demand (thus only if needed). However if there is an expensive singleton
+// that will likely be needed, and initialization takes a potentially long time,
+// e.g. while initializing, parsing some files, talking to remote services,
+// making uses of other singletons, and so on, the initialization of those can
+// be scheduled up front, or "eagerly".
+//
+// In that case the singleton can be declared this way:
+//
+// namespace {
+// auto the_singleton =
+// folly::Singleton<MyExpensiveService>(/* optional create, destroy args */)
+// .shouldEagerInit();
+// }
+//
+// This way the singleton's instance is built at program initialization,
+// if the program opted-in to that feature by calling "doEagerInit" or
+// "doEagerInitVia" during its startup.
+//
// What if you need to destroy all of your singletons? Say, some of
// your singletons manage threads, but you need to fork? Or your unit
// test wants to clean up all global state? Then you can call
// should call reenableInstances.
#pragma once
-#include <folly/Baton.h>
+#include <folly/Demangle.h>
#include <folly/Exception.h>
-#include <folly/Hash.h>
+#include <folly/Executor.h>
#include <folly/Memory.h>
#include <folly/RWSpinLock.h>
-#include <folly/Demangle.h>
-#include <folly/io/async/Request.h>
+#include <folly/Synchronized.h>
+#include <folly/detail/StaticSingletonManager.h>
+#include <folly/experimental/ReadMostlySharedPtr.h>
+#include <folly/hash/Hash.h>
+#include <folly/synchronization/Baton.h>
#include <algorithm>
-#include <vector>
-#include <mutex>
-#include <thread>
+#include <atomic>
#include <condition_variable>
-#include <string>
-#include <unordered_map>
#include <functional>
-#include <typeinfo>
+#include <memory>
+#include <mutex>
+#include <string>
+#include <thread>
#include <typeindex>
+#include <typeinfo>
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
#include <glog/logging.h>
+// use this guard to handleSingleton breaking change in 3rd party code
+#ifndef FOLLY_SINGLETON_TRY_GET
+#define FOLLY_SINGLETON_TRY_GET
+#endif
+
namespace folly {
// For actual usage, please see the Singleton<T> class at the bottom
//
// A vault goes through a few stages of life:
//
-// 1. Registration phase; singletons can be registered, but no
-// singleton can be created.
+// 1. Registration phase; singletons can be registered:
+// a) Strict: no singleton can be created in this stage.
+// b) Relaxed: singleton can be created (the default vault is Relaxed).
// 2. registrationComplete() has been called; singletons can no
// longer be registered, but they can be created.
// 3. A vault can return to stage 1 when destroyInstances is called.
// SingletonHolders.
class SingletonHolderBase {
public:
- virtual ~SingletonHolderBase() {}
+ explicit SingletonHolderBase(TypeDescriptor typeDesc) : type_(typeDesc) {}
+ virtual ~SingletonHolderBase() = default;
- virtual TypeDescriptor type() = 0;
+ TypeDescriptor type() const {
+ return type_;
+ }
virtual bool hasLiveInstance() = 0;
+ virtual void createInstance() = 0;
+ virtual bool creationStarted() = 0;
+ virtual void preDestroyInstance(ReadMostlyMainPtrDeleter<>&) = 0;
virtual void destroyInstance() = 0;
- protected:
- static constexpr std::chrono::seconds kDestroyWaitTime{5};
+ private:
+ TypeDescriptor type_;
};
// An actual instance of a singleton, tracking the instance itself,
inline T* get();
inline std::weak_ptr<T> get_weak();
+ inline std::shared_ptr<T> try_get();
+ inline folly::ReadMostlySharedPtr<T> try_get_fast();
void registerSingleton(CreateFunc c, TeardownFunc t);
void registerSingletonMock(CreateFunc c, TeardownFunc t);
- virtual TypeDescriptor type();
- virtual bool hasLiveInstance();
- virtual void destroyInstance();
+ bool hasLiveInstance() override;
+ void createInstance() override;
+ bool creationStarted() override;
+ void preDestroyInstance(ReadMostlyMainPtrDeleter<>&) override;
+ void destroyInstance() override;
private:
SingletonHolder(TypeDescriptor type, SingletonVault& vault);
- void createInstance();
-
enum class SingletonHolderState {
NotRegistered,
Dead,
Living,
};
- TypeDescriptor type_;
SingletonVault& vault_;
// mutex protects the entire entry during construction/destruction
std::atomic<SingletonHolderState> state_{SingletonHolderState::NotRegistered};
// the thread creating the singleton (only valid while creating an object)
- std::thread::id creating_thread_;
+ std::atomic<std::thread::id> creating_thread_;
// The singleton itself and related functions.
- // holds a shared_ptr to singleton instance, set when state is changed from
- // Dead to Living. Reset when state is changed from Living to Dead.
- std::shared_ptr<T> instance_;
+ // holds a ReadMostlyMainPtr to singleton instance, set when state is changed
+ // from Dead to Living. Reset when state is changed from Living to Dead.
+ folly::ReadMostlyMainPtr<T> instance_;
+ // used to release all ReadMostlyMainPtrs at once
+ folly::ReadMostlySharedPtr<T> instance_copy_;
// weak_ptr to the singleton instance, set when state is changed from Dead
// to Living. We never write to this object after initialization, so it is
// safe to read it from different threads w/o synchronization if we know
// that state is set to Living
std::weak_ptr<T> instance_weak_;
+ // Fast equivalent of instance_weak_
+ folly::ReadMostlyWeakPtr<T> instance_weak_fast_;
// Time we wait on destroy_baton after releasing Singleton shared_ptr.
std::shared_ptr<folly::Baton<>> destroy_baton_;
T* instance_ptr_ = nullptr;
SingletonHolder(SingletonHolder&&) = delete;
};
-}
+} // namespace detail
class SingletonVault {
public:
- enum class Type { Strict, Relaxed };
+ enum class Type {
+ Strict, // Singletons can't be created before registrationComplete()
+ Relaxed, // Singletons can be created before registrationComplete()
+ };
+
+ /**
+ * Clears all singletons in the given vault at ctor and dtor times.
+ * Useful for unit-tests that need to clear the world.
+ *
+ * This need can arise when a unit-test needs to swap out an object used by a
+ * singleton for a test-double, but the singleton needing its dependency to be
+ * swapped has a type or a tag local to some other translation unit and
+ * unavailable in the current translation unit.
+ *
+ * Other, better approaches to this need are "plz 2 refactor" ....
+ */
+ struct ScopedExpunger {
+ SingletonVault* vault;
+ explicit ScopedExpunger(SingletonVault* v) : vault(v) { expunge(); }
+ ~ScopedExpunger() { expunge(); }
+ void expunge() {
+ vault->destroyInstances();
+ vault->reenableInstances();
+ }
+ };
- explicit SingletonVault(Type type = Type::Relaxed) : type_(type) {}
+ static Type defaultVaultType();
+
+ explicit SingletonVault(Type type = defaultVaultType()) : type_(type) {}
// Destructor is only called by unit tests to check destroyInstances.
~SingletonVault();
// registration is not complete. If validations succeeds,
// register a singleton of a given type with the create and teardown
// functions.
- void registerSingleton(detail::SingletonHolderBase* entry) {
- RWSpinLock::ReadHolder rh(&stateMutex_);
-
- stateCheck(SingletonVaultState::Running);
-
- if (UNLIKELY(registrationComplete_)) {
- throw std::logic_error(
- "Registering singleton after registrationComplete().");
- }
-
- RWSpinLock::ReadHolder rhMutex(&mutex_);
- CHECK_THROW(singletons_.find(entry->type()) == singletons_.end(),
- std::logic_error);
+ void registerSingleton(detail::SingletonHolderBase* entry);
- RWSpinLock::UpgradedHolder wh(&mutex_);
- singletons_[entry->type()] = entry;
- }
+ /**
+ * Called by `Singleton<T>.shouldEagerInit()` to ensure the instance
+ * is built when `doEagerInit[Via]` is called; see those methods
+ * for more info.
+ */
+ void addEagerInitSingleton(detail::SingletonHolderBase* entry);
// Mark registration is complete; no more singletons can be
// registered at this point.
- void registrationComplete() {
- RequestContext::getStaticContext();
- std::atexit([](){ SingletonVault::singleton()->destroyInstances(); });
-
- RWSpinLock::WriteHolder wh(&stateMutex_);
+ void registrationComplete();
- stateCheck(SingletonVaultState::Running);
-
- if (type_ == Type::Strict) {
- for (const auto& p: singletons_) {
- if (p.second->hasLiveInstance()) {
- throw std::runtime_error(
- "Singleton created before registration was complete.");
- }
- }
- }
+ /**
+ * Initialize all singletons which were marked as eager-initialized
+ * (using `shouldEagerInit()`). No return value. Propagates exceptions
+ * from constructors / create functions, as is the usual case when calling
+ * for example `Singleton<Foo>::get_weak()`.
+ */
+ void doEagerInit();
- registrationComplete_ = true;
- }
+ /**
+ * Schedule eager singletons' initializations through the given executor.
+ * If baton ptr is not null, its `post` method is called after all
+ * early initialization has completed.
+ *
+ * If exceptions are thrown during initialization, this method will still
+ * `post` the baton to indicate completion. The exception will not propagate
+ * and future attempts to `try_get` or `get_weak` the failed singleton will
+ * retry initialization.
+ *
+ * Sample usage:
+ *
+ * folly::IOThreadPoolExecutor executor(max_concurrency_level);
+ * folly::Baton<> done;
+ * doEagerInitVia(executor, &done);
+ * done.wait(); // or 'try_wait_for', etc.
+ *
+ */
+ void doEagerInitVia(Executor& exe, folly::Baton<>* done = nullptr);
// Destroy all singletons; when complete, the vault can't create
// singletons once again until reenableInstances() is called.
// For testing; how many registered and living singletons we have.
size_t registeredSingletonCount() const {
- RWSpinLock::ReadHolder rh(&mutex_);
-
- return singletons_.size();
+ return singletons_.rlock()->size();
}
+ /**
+ * Flips to true if eager initialization was used, and has completed.
+ * Never set to true if "doEagerInit()" or "doEagerInitVia" never called.
+ */
+ bool eagerInitComplete() const;
+
size_t livingSingletonCount() const {
- RWSpinLock::ReadHolder rh(&mutex_);
+ auto singletons = singletons_.rlock();
size_t ret = 0;
- for (const auto& p : singletons_) {
+ for (const auto& p : *singletons) {
if (p.second->hasLiveInstance()) {
++ret;
}
// tests only.
template <typename VaultTag = detail::DefaultTag>
static SingletonVault* singleton() {
- static SingletonVault* vault = new SingletonVault();
+ /* library-local */ static auto vault =
+ detail::createGlobal<SingletonVault, VaultTag>();
return vault;
}
typedef std::string(*StackTraceGetterPtr)();
static std::atomic<StackTraceGetterPtr>& stackTraceGetter() {
- static std::atomic<StackTraceGetterPtr> stackTraceGetterPtr;
- return stackTraceGetterPtr;
+ /* library-local */ static auto stackTraceGetterPtr = detail::
+ createGlobal<std::atomic<StackTraceGetterPtr>, SingletonVault>();
+ return *stackTraceGetterPtr;
+ }
+
+ void setType(Type type) {
+ type_ = type;
}
private:
template <typename T>
- friend class detail::SingletonHolder;
+ friend struct detail::SingletonHolder;
// The two stages of life for a vault, as mentioned in the class comment.
enum class SingletonVaultState {
Quiescing,
};
+ struct State {
+ SingletonVaultState state{SingletonVaultState::Running};
+ bool registrationComplete{false};
+ };
+
// Each singleton in the vault can be in two states: dead
// (registered but never created), living (CreateFunc returned an instance).
- void stateCheck(SingletonVaultState expected,
- const char* msg="Unexpected singleton state change") {
- if (expected != state_) {
- throw std::logic_error(msg);
+ static void stateCheck(
+ SingletonVaultState expected,
+ const State& state,
+ const char* msg = "Unexpected singleton state change") {
+ if (expected != state.state) {
+ throw std::logic_error(msg);
}
}
typedef std::unordered_map<detail::TypeDescriptor,
detail::SingletonHolderBase*,
detail::TypeDescriptorHasher> SingletonMap;
+ folly::Synchronized<SingletonMap> singletons_;
+ folly::Synchronized<std::unordered_set<detail::SingletonHolderBase*>>
+ eagerInitSingletons_;
+ folly::Synchronized<std::vector<detail::TypeDescriptor>> creationOrder_;
+
+ // Using SharedMutexReadPriority is important here, because we want to make
+ // sure we don't block nested singleton creation happening concurrently with
+ // destroyInstances().
+ folly::Synchronized<State, folly::SharedMutexReadPriority> state_;
- mutable folly::RWSpinLock mutex_;
- SingletonMap singletons_;
- std::vector<detail::TypeDescriptor> creation_order_;
- SingletonVaultState state_{SingletonVaultState::Running};
- bool registrationComplete_{false};
- folly::RWSpinLock stateMutex_;
- Type type_{Type::Relaxed};
+ Type type_;
};
// This is the wrapper class that most users actually interact with.
// It allows for simple access to registering and instantiating
// singletons. Create instances of this class in the global scope of
// type Singleton<T> to register your singleton for later access via
-// Singleton<T>::get().
-template <typename T,
- typename Tag = detail::DefaultTag,
- typename VaultTag = detail::DefaultTag /* for testing */>
+// Singleton<T>::try_get().
+template <
+ typename T,
+ typename Tag = detail::DefaultTag,
+ typename VaultTag = detail::DefaultTag /* for testing */>
class Singleton {
public:
typedef std::function<T*(void)> CreateFunc;
// Generally your program life cycle should be fine with calling
// get() repeatedly rather than saving the reference, and then not
// call get() during process shutdown.
- static T* get() {
- return getEntry().get();
- }
+ FOLLY_DEPRECATED("Replaced by try_get")
+ static T* get() { return getEntry().get(); }
// If, however, you do need to hold a reference to the specific
// singleton, you can try to do so with a weak_ptr. Avoid this when
// possible but the inability to lock the weak pointer can be a
// signal that the vault has been destroyed.
- static std::weak_ptr<T> get_weak() {
- return getEntry().get_weak();
- }
+ FOLLY_DEPRECATED("Replaced by try_get")
+ static std::weak_ptr<T> get_weak() { return getEntry().get_weak(); }
- // Allow the Singleton<t> instance to also retrieve the underlying
- // singleton, if desired.
- T& operator*() { return *get(); }
- T* operator->() { return get(); }
+ // Preferred alternative to get_weak, it returns shared_ptr that can be
+ // stored; a singleton won't be destroyed unless shared_ptr is destroyed.
+ // Avoid holding these shared_ptrs beyond the scope of a function;
+ // don't put them in member variables, always use try_get() instead
+ //
+ // try_get() can return nullptr if the singleton was destroyed, caller is
+ // responsible for handling nullptr return
+ static std::shared_ptr<T> try_get() {
+ return getEntry().try_get();
+ }
- explicit Singleton(std::nullptr_t _ = nullptr,
- Singleton::TeardownFunc t = nullptr) :
- Singleton ([]() { return new T; }, std::move(t)) {
+ static folly::ReadMostlySharedPtr<T> try_get_fast() {
+ return getEntry().try_get_fast();
}
- explicit Singleton(Singleton::CreateFunc c,
- Singleton::TeardownFunc t = nullptr) {
+ explicit Singleton(std::nullptr_t /* _ */ = nullptr,
+ typename Singleton::TeardownFunc t = nullptr)
+ : Singleton([]() { return new T; }, std::move(t)) {}
+
+ explicit Singleton(typename Singleton::CreateFunc c,
+ typename Singleton::TeardownFunc t = nullptr) {
if (c == nullptr) {
throw std::logic_error(
"nullptr_t should be passed if you want T to be default constructed");
vault->registerSingleton(&getEntry());
}
+ /**
+ * Should be instantiated as soon as "doEagerInit[Via]" is called.
+ * Singletons are usually lazy-loaded (built on-demand) but for those which
+ * are known to be needed, to avoid the potential lag for objects that take
+ * long to construct during runtime, there is an option to make sure these
+ * are built up-front.
+ *
+ * Use like:
+ * Singleton<Foo> gFooInstance = Singleton<Foo>(...).shouldEagerInit();
+ *
+ * Or alternately, define the singleton as usual, and say
+ * gFooInstance.shouldEagerInit();
+ *
+ * at some point prior to calling registrationComplete().
+ * Then doEagerInit() or doEagerInitVia(Executor*) can be called.
+ */
+ Singleton& shouldEagerInit() {
+ auto vault = SingletonVault::singleton<VaultTag>();
+ vault->addEagerInitSingleton(&getEntry());
+ return *this;
+ }
+
/**
* Construct and inject a mock singleton which should be used only from tests.
* Unlike regular singletons which are initialized once per process lifetime,
* the injection. The returned mock singleton is functionality identical to
* regular singletons.
*/
- static void make_mock(std::nullptr_t c = nullptr,
+ static void make_mock(std::nullptr_t /* c */ = nullptr,
typename Singleton<T>::TeardownFunc t = nullptr) {
make_mock([]() { return new T; }, t);
}
}
};
-}
+template <typename T, typename Tag = detail::DefaultTag>
+class LeakySingleton {
+ public:
+ using CreateFunc = std::function<T*()>;
+
+ LeakySingleton() : LeakySingleton([] { return new T(); }) {}
+
+ explicit LeakySingleton(CreateFunc createFunc) {
+ auto& entry = entryInstance();
+ if (entry.state != State::NotRegistered) {
+ LOG(FATAL) << "Double registration of singletons of the same "
+ << "underlying type; check for multiple definitions "
+ << "of type folly::LeakySingleton<" + entry.type_.name() + ">";
+ }
+ entry.createFunc = createFunc;
+ entry.state = State::Dead;
+ }
+
+ static T& get() { return instance(); }
+
+ static void make_mock(std::nullptr_t /* c */ = nullptr) {
+ make_mock([]() { return new T; });
+ }
+
+ static void make_mock(CreateFunc createFunc) {
+ auto& entry = entryInstance();
+ if (createFunc == nullptr) {
+ throw std::logic_error(
+ "nullptr_t should be passed if you want T to be default constructed");
+ }
+
+ entry.createFunc = createFunc;
+ entry.state = State::Dead;
+ }
+
+ private:
+ enum class State { NotRegistered, Dead, Living };
+
+ struct Entry {
+ Entry() {}
+ Entry(const Entry&) = delete;
+ Entry& operator=(const Entry&) = delete;
+
+ std::atomic<State> state{State::NotRegistered};
+ T* ptr{nullptr};
+ CreateFunc createFunc;
+ std::mutex mutex;
+ detail::TypeDescriptor type_{typeid(T), typeid(Tag)};
+ };
+
+ static Entry& entryInstance() {
+ /* library-local */ static auto entry = detail::createGlobal<Entry, Tag>();
+ return *entry;
+ }
+
+ static T& instance() {
+ auto& entry = entryInstance();
+ if (UNLIKELY(entry.state != State::Living)) {
+ createInstance();
+ }
+
+ return *entry.ptr;
+ }
+
+ static void createInstance() {
+ auto& entry = entryInstance();
+
+ std::lock_guard<std::mutex> lg(entry.mutex);
+ if (entry.state == State::Living) {
+ return;
+ }
+
+ if (entry.state == State::NotRegistered) {
+ auto ptr = SingletonVault::stackTraceGetter().load();
+ LOG(FATAL) << "Creating instance for unregistered singleton: "
+ << entry.type_.name() << "\n"
+ << "Stacktrace:"
+ << "\n" << (ptr ? (*ptr)() : "(not available)");
+ }
+
+ entry.ptr = entry.createFunc();
+ entry.state = State::Living;
+ }
+};
+} // namespace folly
#include <folly/Singleton-inl.h>
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