QMYSQLResult：：清理:无法释放语句句柄

Question

我试图使用线程从MySQL数据库中获取数据，但在调用：

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();

我得到：

QMYSQLResult::cleanup: unable to free statement handle

下一次尝试使用我得到的连接：

QSqlError("2006", "QMYSQL: Unable to execute query", "MySQL server has gone away")

可能会发生什么？

Answer 1

您只能使用创建它的线程(参考文献)的连接，或者通常只能从一个线程使用连接。

你可以采取两种方法：

1. 在一个专用线程中维护数据库连接，并从那里使用它。无论如何，使用短命线程是一种过早的悲观，所以这是一种糟糕的方法。
2. 将工作线程之间的数据库连接从线程池中移动。使用QtConcurrent::run在线程池中的线程上运行函子。

注意：Beacon::Beacon(...) 构造函数不能使用数据库，它只能存储对它的引用/指针！

有关这个答案的更多信息，请参见postCall。

template <typename T>
void postCall(QThread * thread, T && functor) {
   QObject source;
   QObject::connect(&source, &QObject::destroyed,
                    QEventDispatcher::instance(thread), std::forward(functor));
}

线程池的工人

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());
  }
  ...
};

专用螺纹

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());
    });
  }
  ...
};