rippled/modules/beast_core/threads/ThreadLocalValue.h

//------------------------------------------------------------------------------
/*
    This file is part of Beast: https://github.com/vinniefalco/Beast
    Copyright 2013, Vinnie Falco <vinnie.falco@gmail.com>

    Portions of this file are from JUCE.
    Copyright (c) 2013 - Raw Material Software Ltd.
    Please visit http://www.juce.com

    Permission to use, copy, modify, and/or distribute this software for any
    purpose  with  or without fee is hereby granted, provided that the above
    copyright notice and this permission notice appear in all copies.

    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
//==============================================================================

#ifndef BEAST_THREADLOCALVALUE_H_INCLUDED
#define BEAST_THREADLOCALVALUE_H_INCLUDED

// (NB: on win32, native thread-locals aren't possible in a dynamically loaded DLL in XP).
#if ! ((BEAST_MSVC && (BEAST_64BIT || ! defined (BeastPlugin_PluginCode))) \
       || (BEAST_MAC && BEAST_CLANG && defined (MAC_OS_X_VERSION_10_7) \
             && MAC_OS_X_VERSION_MIN_REQUIRED >= MAC_OS_X_VERSION_10_7))
 #define BEAST_NO_COMPILER_THREAD_LOCAL 1
#endif

//==============================================================================
/**
    Provides cross-platform support for thread-local objects.

    This class holds an internal list of objects of the templated type, keeping
    an instance for each thread that requests one. The first time a thread attempts
    to access its value, an object is created and added to the list for that thread.

    Typically, you'll probably want to create a static instance of a ThreadLocalValue
    object, or hold one within a singleton.

    The templated class for your value could be a primitive type, or any class that
    has a default constructor and copy operator.

    When a thread no longer needs to use its value, it can call releaseCurrentThreadStorage()
    to allow the storage to be re-used by another thread. If a thread exits without calling
    this method, the object storage will be left allocated until the ThreadLocalValue object
    is deleted.
*/
template <typename Type>
class ThreadLocalValue : public Uncopyable
{
public:
    /** */
    ThreadLocalValue() noexcept
    {
    }

    /** Destructor.
        When this object is deleted, all the value objects for all threads will be deleted.
    */
    ~ThreadLocalValue()
    {
       #if BEAST_NO_COMPILER_THREAD_LOCAL
        for (ObjectHolder* o = first.value; o != nullptr;)
        {
            ObjectHolder* const next = o->next;
            delete o;
            o = next;
        }
       #endif
    }

    /** Returns a reference to this thread's instance of the value.
        Note that the first time a thread tries to access the value, an instance of the
        value object will be created - so if your value's class has a non-trivial
        constructor, be aware that this method could invoke it.
    */
    Type& operator*() const noexcept                        { return get(); }

    /** Returns a pointer to this thread's instance of the value.
        Note that the first time a thread tries to access the value, an instance of the
        value object will be created - so if your value's class has a non-trivial
        constructor, be aware that this method could invoke it.
    */
    operator Type*() const noexcept                         { return &get(); }

    /** Accesses a method or field of the value object.
        Note that the first time a thread tries to access the value, an instance of the
        value object will be created - so if your value's class has a non-trivial
        constructor, be aware that this method could invoke it.
    */
    Type* operator->() const noexcept                       { return &get(); }

    /** Assigns a new value to the thread-local object. */
    ThreadLocalValue& operator= (const Type& newValue)      { get() = newValue; return *this; }

    /** Returns a reference to this thread's instance of the value.
        Note that the first time a thread tries to access the value, an instance of the
        value object will be created - so if your value's class has a non-trivial
        constructor, be aware that this method could invoke it.
    */
    Type& get() const noexcept
    {
       #if BEAST_NO_COMPILER_THREAD_LOCAL
        const Thread::ThreadID threadId = Thread::getCurrentThreadId();

        for (ObjectHolder* o = first.get(); o != nullptr; o = o->next)
            if (o->threadId == threadId)
                return o->object;

        for (ObjectHolder* o = first.get(); o != nullptr; o = o->next)
        {
            if (o->threadId == nullptr)
            {
                {
                    SpinLock::ScopedLockType sl (lock);

                    if (o->threadId != nullptr)
                        continue;

                    o->threadId = threadId;
                }

                o->object = Type();
                return o->object;
            }
        }

        ObjectHolder* const newObject = new ObjectHolder (threadId);

        do
        {
            newObject->next = first.get();
        }
        while (! first.compareAndSetBool (newObject, newObject->next));

        return newObject->object;
       #elif BEAST_MAC
        static __thread Type object;
        return object;
       #elif BEAST_MSVC
        static __declspec(thread) Type object;
        return object;
       #endif
    }

    /** Called by a thread before it terminates, to allow this class to release
        any storage associated with the thread.
    */
    void releaseCurrentThreadStorage()
    {
       #if BEAST_NO_COMPILER_THREAD_LOCAL
        const Thread::ThreadID threadId = Thread::getCurrentThreadId();

        for (ObjectHolder* o = first.get(); o != nullptr; o = o->next)
        {
            if (o->threadId == threadId)
            {
                SpinLock::ScopedLockType sl (lock);
                o->threadId = nullptr;
            }
        }
       #endif
    }

private:
    //==============================================================================
   #if BEAST_NO_COMPILER_THREAD_LOCAL
    struct ObjectHolder : public Uncopyable
    {
        ObjectHolder (const Thread::ThreadID& tid)
            : threadId (tid), object()
        {}

        Thread::ThreadID threadId;
        ObjectHolder* next;
        Type object;
    };

    mutable Atomic<ObjectHolder*> first;
    SpinLock lock;
   #endif
};


#endif   // BEAST_THREADLOCALVALUE_H_INCLUDED