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
// Qt-Security score:significant reason:default
 
#include "qthreadstorage.h"
#include "qthreadstorage_p.h"
 
#include "private/qcoreapplication_p.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();
    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();
    QList<void *> &tls = data->tls;
    if (tls.size() <= id)
        tls.resize(id + 1);
 
    void* *ptr = &tls[id];
    if (*ptr == p) // pointer-based setLocalData(localData()) → must be no-op
        return ptr;
 
    // delete any previous data
    if (*ptr != nullptr) {
        DEBUG_MSG("QThreadStorageData: Deleting previous storage %d, data %p, for thread %p",
                id,
                *ptr,
                data->thread.loadRelaxed());
 
        QMutexLocker locker(&destructorsMutex);
        DestructorMap *destr = destructors();
        void (*destructor)(void *) = destr ? destr->value(id) : nullptr;
        locker.unlock();
 
        if (destructor) {
            void *q = std::exchange(*ptr, nullptr);
            destructor(q);
            // `destructor` may have re-entered and grown `tls`; re-seat `ptr`:
            ptr = &tls[id];
        }
    }
 
    // store new data
    *ptr = p;
    DEBUG_MSG("QThreadStorageData: Set storage %d for thread %p to %p", id, data->thread.loadRelaxed(), p);
    return ptr;
}
 
void QThreadStorageData::clear()
{
    // Skip if this thread doesn't have a QThreadData (anymore): there's
    // nothing to clear, then, and set() would otherwise re-create it, here,
    // during shutdown, with nothing left to destroy it again.
    if (QThreadData::currentThreadData())
        set(nullptr);
}
 
void QThreadStoragePrivate::init()
{
    // Make sure the Q_GLOBAL_STATIC is initialized, ensuring consistent
    // destruction order.
    destructors();
}
 
void QThreadStoragePrivate::finish(QList<void *> *tls, bool suppressWarnings)
{
    if (tls->isEmpty() || !destructors())
        return; // nothing to do
    if (!QCoreApplication::instanceExists())
        suppressWarnings = true;
 
    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 (!suppressWarnings)
                qWarning("QThreadStorage: entry %d destroyed before end of thread %p",
                         i, QThread::currentThread());
            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();
}
 
/*!
    \class QThreadStorage
    \inmodule QtCore
    \brief The QThreadStorage class provides per-thread data storage.
 
    \threadsafe
 
    \ingroup thread
 
    QThreadStorage is a template class where the template parameter \a T
    specifies the type of data stored per-thread.
 
    The setLocalData() function stores a single thread-specific value
    for the calling thread. The data can be accessed later using
    localData().
 
    The hasLocalData() function allows the programmer to determine if
    data has previously been set using the setLocalData() function.
    This is also useful for lazy initialization.
 
    If T is a pointer type, QThreadStorage takes ownership of the data
    (which must be created on the heap with \c new) and deletes it when
    the thread exits, either normally or via termination.
 
    For example, the following code uses QThreadStorage to store a
    single cache for each thread that calls the cacheObject() and
    removeFromCache() functions. The cache is automatically
    deleted when the calling thread exits.
 
    \snippet threads/threads.cpp 7
    \snippet threads/threads.cpp 8
    \snippet threads/threads.cpp 9
 
    \section1 Caveats
 
    \list
 
    \target qthreadstorage-shutdown-caveat
    \li Calling hasLocalData(), localData() or setLocalData() during thread or
    program shutdown may re-create the thread-local storage for the current
    thread with nothing left to destroy it afterwards. The clearLocalData()
    function is the only one that is unconditionally safe to call during
    shutdown.
 
    \li The QThreadStorage destructor does not delete per-thread data.
    QThreadStorage only deletes per-thread data when the thread exits
    or when setLocalData() is called multiple times.
 
    \li QThreadStorage can be used to store data for the \c main()
    thread. QThreadStorage deletes all data set for the \c main()
    thread when QApplication is destroyed, regardless of whether or
    not the \c main() thread has actually finished.
 
    \endlist
 
    \sa QThread
*/
 
/*!
    \fn template <class T> QThreadStorage<T>::QThreadStorage()
 
    Constructs a new per-thread data storage object.
*/
 
/*!
    \fn template <class T> QThreadStorage<T>::~QThreadStorage()
 
    Destroys the per-thread data storage object.
 
    Note: The per-thread data stored is not deleted. Any data left
    in QThreadStorage is leaked. Make sure that all threads using
    QThreadStorage have exited before deleting the QThreadStorage.
 
    \sa hasLocalData()
*/
 
/*!
    \fn template <class T> bool QThreadStorage<T>::hasLocalData() const
 
    If T is a pointer type, returns \c true if the calling thread has
    non-zero data available.
 
    If T is a value type, returns whether the data has already been
    constructed by calling setLocalData or localData.
 
    \sa localData()
*/
 
/*!
    \fn template <class T> T &QThreadStorage<T>::localData()
 
    Returns a reference to the data that was set by the calling
    thread.
 
    If no data has been set, this will create a default constructed
    instance of type T.
 
    \sa hasLocalData()
*/
 
/*!
    \fn template <class T> const T QThreadStorage<T>::localData() const
    \overload
 
    Returns a copy of the data that was set by the calling thread.
 
    \sa hasLocalData()
*/
 
/*!
    \fn template <class T> void QThreadStorage<T>::setLocalData(T data)
 
    Sets the local data for the calling thread to \a data. It can be
    accessed later using the localData() functions.
 
    If T is a pointer type, QThreadStorage takes ownership of the data
    and deletes it automatically either when the thread exits (either
    normally or via termination) or when setLocalData() is called again.
 
    \sa localData(), hasLocalData(), clearLocalData()
*/
 
/*!
    \fn template <class T> void QThreadStorage<T>::clearLocalData()
    \since 6.13
 
    Resets the local data (if any) for the calling thread.
 
    If the thread had no localData(), does nothing. After the call,
    hasLocalData() is \c{false} (unless the destructor of the previous value
    stored a new one).
 
    \note If \c{T} is a pointer type, this is mostly the same as
    \c{setLocalData(nullptr)}. For non-pointers, this functionality was not
    accessible before Qt 6.13.
 
    This function differs from setLocalData() in that it will not create
    thread-local storage when it doesn't exist for the current thread (yet, or
    anymore). This makes calling the function safe in shutdown code, where
    thread-local storage may already have been destroyed, and setLocalData()
    would re-create it with nothing left to destroy it afterwards.
 
    \sa setLocalData(), hasLocalData(), {qthreadstorage-shutdown-caveat}{Caveats}
*/
 
QT_END_NAMESPACE