/*
- * 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 {};
// 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,
// 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/Executor.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 <atomic>
// SingletonHolders.
class SingletonHolderBase {
public:
+ 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,
void registerSingleton(CreateFunc c, TeardownFunc t);
void registerSingletonMock(CreateFunc c, TeardownFunc t);
- virtual TypeDescriptor type() override;
- virtual bool hasLiveInstance() override;
- virtual void createInstance() override;
- virtual bool creationStarted() override;
- virtual void destroyInstance() override;
+ bool hasLiveInstance() override;
+ void createInstance() override;
+ bool creationStarted() override;
+ void preDestroyInstance(ReadMostlyMainPtrDeleter<>&) override;
+ void destroyInstance() override;
private:
SingletonHolder(TypeDescriptor type, SingletonVault& vault);
Living,
};
- TypeDescriptor type_;
SingletonVault& vault_;
// mutex protects the entire entry during construction/destruction
// 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
SingletonHolder(SingletonHolder&&) = delete;
};
-}
+} // namespace detail
class SingletonVault {
public:
}
};
- 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();
*
* Sample usage:
*
- * wangle::IOThreadPoolExecutor executor(max_concurrency_level);
+ * folly::IOThreadPoolExecutor executor(max_concurrency_level);
* folly::Baton<> done;
* doEagerInitVia(executor, &done);
* done.wait(); // or 'timed_wait', or spin with 'try_wait'
// 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();
}
/**
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:
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::unordered_set<detail::SingletonHolderBase*> eagerInitSingletons_;
- 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.
// singletons. Create instances of this class in the global scope of
// type Singleton<T> to register your singleton for later access via
// Singleton<T>::try_get().
-template <typename T,
- typename Tag = detail::DefaultTag,
- typename VaultTag = detail::DefaultTag /* for testing */>
+template <
+ typename T,
+ typename Tag = detail::DefaultTag,
+ typename VaultTag = detail::DefaultTag /* for testing */>
class Singleton {
public:
typedef std::function<T*(void)> CreateFunc;
}
};
-}
+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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