qthreadstorage.cpp

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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
 
#include "qthreadstorage.h"
 
#include "qthread.h"
#include "qthread_p.h"
#include "qmutex.h"
 
#include <string.h>
 
QT_BEGIN_NAMESPACE
 
// #define THREADSTORAGE_DEBUG
#ifdef THREADSTORAGE_DEBUG
#  define DEBUG_MSG qtsDebug
 
#  include <stdio.h>
#  include <stdarg.h>
void qtsDebug(const char *fmt, ...)
{
    va_list va;
    va_start(va, fmt);
 
    fprintf(stderr, "QThreadStorage: ");
    vfprintf(stderr, fmt, va);
    fprintf(stderr, "\n");
 
    va_end(va);
}
#else
#  define DEBUG_MSG if (false)qDebug
#endif
 
Q_CONSTINIT static QBasicMutex destructorsMutex;
typedef QList<void (*)(void *)> DestructorMap;
Q_GLOBAL_STATIC(DestructorMap, destructors)
 
QThreadStorageData::QThreadStorageData(void (*func)(void *))
{
    QMutexLocker locker(&destructorsMutex);
    DestructorMap *destr = destructors();
    if (!destr) {
        /*
         the destructors vector has already been destroyed, yet a new
         QThreadStorage is being allocated. this can only happen during global
         destruction, at which point we assume that there is only one thread.
         in order to keep QThreadStorage working, we need somewhere to store
         the data, best place we have in this situation is at the tail of the
         current thread's tls vector. the destructor is ignored, since we have
         no where to store it, and no way to actually call it.
         */
        QThreadData *data = QThreadData::current();
        id = data->tls.size();
        DEBUG_MSG("QThreadStorageData: Allocated id %d, destructor %p cannot be stored", id, func);
        return;
    }
    for (id = 0; id < destr->size(); id++) {
        if (destr->at(id) == nullptr)
            break;
    }
    if (id == destr->size()) {
        destr->append(func);
    } else {
        (*destr)[id] = func;
    }
    DEBUG_MSG("QThreadStorageData: Allocated id %d, destructor %p", id, func);
}
 
QThreadStorageData::~QThreadStorageData()
{
    DEBUG_MSG("QThreadStorageData: Released id %d", id);
    QMutexLocker locker(&destructorsMutex);
    if (destructors())
        (*destructors())[id] = nullptr;
}
 
void **QThreadStorageData::get() const
{
    QThreadData *data = QThreadData::current();
    if (!data) {
        qWarning("QThreadStorage::get: QThreadStorage can only be used with threads started with QThread");
        return nullptr;
    }
    QList<void *> &tls = data->tls;
    if (tls.size() <= id)
        tls.resize(id + 1);
    void **v = &tls[id];
 
    DEBUG_MSG("QThreadStorageData: Returning storage %d, data %p, for thread %p",
          id,
          *v,
          data->thread.loadRelaxed());
 
    return *v ? v : nullptr;
}
 
void **QThreadStorageData::set(void *p)
{
    QThreadData *data = QThreadData::current();
    if (!data) {
        qWarning("QThreadStorage::set: QThreadStorage can only be used with threads started with QThread");
        return nullptr;
    }
    QList<void *> &tls = data->tls;
    if (tls.size() <= id)
        tls.resize(id + 1);
 
    void *&value = tls[id];
    // delete any previous data
    if (value != nullptr) {
        DEBUG_MSG("QThreadStorageData: Deleting previous storage %d, data %p, for thread %p",
                id,
                value,
                data->thread.loadRelaxed());
 
        QMutexLocker locker(&destructorsMutex);
        DestructorMap *destr = destructors();
        void (*destructor)(void *) = destr ? destr->value(id) : nullptr;
        locker.unlock();
 
        void *q = value;
        value = nullptr;
 
        if (destructor)
            destructor(q);
    }
 
    // store new data
    value = p;
    DEBUG_MSG("QThreadStorageData: Set storage %d for thread %p to %p", id, data->thread.loadRelaxed(), p);
    return &value;
}
 
void QThreadStorageData::finish(void **p)
{
    QList<void *> *tls = reinterpret_cast<QList<void *> *>(p);
    if (!tls || tls->isEmpty() || !destructors())
        return; // nothing to do
 
    DEBUG_MSG("QThreadStorageData: Destroying storage for thread %p", QThread::currentThread());
    while (!tls->isEmpty()) {
        void *&value = tls->last();
        void *q = value;
        value = nullptr;
        int i = tls->size() - 1;
        tls->resize(i);
 
        if (!q) {
            // data already deleted
            continue;
        }
 
        QMutexLocker locker(&destructorsMutex);
        void (*destructor)(void *) = destructors()->value(i);
        locker.unlock();
 
        if (!destructor) {
            if (QThread::currentThread())
                qWarning("QThreadStorage: Thread %p exited after QThreadStorage %d destroyed",
                         QThread::currentThread(), i);
            continue;
        }
        destructor(q); //crash here might mean the thread exited after qthreadstorage was destroyed
 
        if (tls->size() > i) {
            //re reset the tls in case it has been recreated by its own destructor.
            (*tls)[i] = nullptr;
        }
    }
    tls->clear();
}
 
QT_END_NAMESPACE