QMYSQLResult::cleanup: unable to free statement handle - qt

Question

I'm trying to use a thread to bring data from a MySQL database, but after a call to:
QThread* thread = new QThread;
Beacon *beacon = new Beacon(m_db, begin, end);
beacon->moveToThread(thread);
connect(beacon, &Beacon::values, this, &Tm::insertRow);
connect(beacon, &Beacon::finished, thread, &QThread::quit);
connect(beacon, &Beacon::finished, beacon, &Beacon::deleteLater);
connect(thread, &QThread::finished, thread, &QThread::deleteLater);
connect(thread, &QThread::started, beacon, &Beacon::executeQuerySmooth);
connect(beacon, &Beacon::finished, this, &Tm::finished);
thread->start();
I get:
QMYSQLResult::cleanup: unable to free statement handle
And the next time a try to use the connection I get:
QSqlError("2006", "QMYSQL: Unable to execute query", "MySQL server has gone away")
What might be happening?

Answer 1

You can only use the connection from a thread that created it (reference), or in general from one thread only.
There are two approaches you can take:
Maintain the database connection in a dedicated thread, and use it from there. The use of shortlived threads is a premature pessimization anyway, so it was a bad approach to begin with.
Move the database connection between worker threads from a thread pool. Use QtConcurrent::run to run a functor on a thread from the thread pool.
Note: Beacon::Beacon(...) constructor can't be using the database, it can only store a reference/pointer to it!
See this answer for more about postCall.
template <typename T>
void postCall(QThread * thread, T && functor) {
QObject source;
QObject::connect(&source, &QObject::destroyed,
QEventDispatcher::instance(thread), std::forward(functor));
}
Workers From The Thread Pool
class Tm : public QObject {
Q_OBJECT
QMutex m_dbMutex;
QSqlDatabase m_db;
Q_SIGNAL void openFailed();
Q_SIGNAL void openSucceeded();
public:
void openConnection() {
QMutexLocker lock(&m_dbMutex);
m_db.addDatabase("QSQLITE");
... // set database's properties
m_db.moveToThread(0); // we don't know what thread it will be used in
lock.unlock();
QtConcurrent::run([this]{
QMutexLocker lock(&m_dbMutex);
m_db.moveToThread(QThread::currentThread());
bool rc = m_db.open();
if (rc) {
m_db.moveToThread(0);
emit openSucceeded();
} else {
m_db.moveToThread(this->thread());
emit openFailed();
}
});
}
void beaconate() {
...
QSharedPointer<Beacon> beacon(new Beacon(&m_db, begin, end));
connect(beacon, &Beacon::values, this, &Tm::insertRow);
connect(beacon, &Beacon::finished, this, &Tm::finished);
beacon->setMoveToThread(0);
QtConcurrent::run([beacon]{
beacon->moveToThread(QThread::currentThread());
QMutexLocker lock(&m_dbMutex);
m_db.moveToThread(QThread::currentThread());
QEventLoop loop; // only if Beacon needs one
connect(beacon, &Beacon::finished, &loop, &QEventLoop::quit);
beacon->executeQuerySmooth();
loop.exec();
m_db.moveToThread(0);
});
}
~Tm() {
QMutexLocker lock(&m_dbMutex);
m_db.moveToThread(thread());
}
...
};
Dedicated Thread
class Thread : public QThread
{ using QThread::run; public: ~Thread() { quit(); wait(); } };
class Tm : public QObject {
Q_OBJECT
QSqlDatabase m_db;
Thread m_dbThread; // must be declared after m_db, so that it's destructed prior to m_db
Q_SIGNAL void openFailed();
Q_SIGNAL void openSucceeded();
public:
Tm(QObject * parent = 0) : QObject(parent) {
m_dbThread.start();
}
void openConnection() {
m_db.addDatabase("QSQLITE");
... // set database's properties
m_db.moveToThread(&m_dbThread);
postCall(&m_dbThread, [this]{
if (! m_db.open()) {
m_db.moveToThread(this->thread());
emit openFailed();
} else
emit openSucceeded();
});
}
void beaconate() {
...
auto beacon = new Beacon(&m_db, begin, end);
beacon.moveToThread(&m_dbThread);
connect(beacon, &Beacon::values, this, &Tm::insertRow);
connect(beacon, &Beacon::finished, beacon, &Beacon::deleteLater);
connect(beacon, &Beacon::finished, this, &Tm::finished);
postCall(&m_dbThread, [beacon]{ beacon->executeQuerySmooth(); });
}
~Tm() {
// Destructing objects living in other threads is undefined behavior
postCall(&m_dbThread, [this]{
if (m_db.thread() == QThread::currentThread())
m_db.moveToThread(thread());
});
}
...
};