qt simultaneous MySQL queries in different threads = crash - c++

Question

I have a multithreaded Qt5 app written in C++ where each thread is writing to the same database.
I share the same database QSqlDatabase variable across threads, but each thread creates its own query. I am regularly getting crashes and I've captured this info at the time of a crash:
Driver error [QMYSQL3: Unable to store statement results]
Database error [Commands out of sync; you can't run this command now]
Type [2]
Number [2014]
First of all, can I make simultaneous MySQL queries in multiple threads? If so, do I need to protect SQL calls with a mutex? Or do I need to post more info....
UPDATE: I have a seperate DBManager thread which handles opening/closing the DB, and simple database writes. I did this because my DB often goes offline, and I don't want other threads to hang for up to 2 minutes while a db open fails. No I have more threads which do reports, and must retrieve substantial amounts of data from the db.
As noted below, sharing the db handle across threads is not permitted. So now perhaps this is more of a design question - what is the best way to handle this situation? I don't want each thread that does DB access to attempt it's own open and wait for 2 minutes in case the DB is offline

Answer 1

Read the documentation for the SQL module, in particular the threads section that states each connection can only be used within the thread that created it.
So adding a mutex is not good enough, you will have to either create a new connection object in each thread, or pass the required data to/from a dedicated thread that performs all DB queries.

Answer 2

I generally handle this problem by providing a factory method on my database manager class, something that looks similar to this (semi-pseudocode):
static QHash<QThread, QSqlDatabase> DatabaseManager::s_instances;
static QMutex DatabaseManager::s_databaseMutex;
QSqlDatabase DatabaseManager::database() {
QMutexLocker locker(s_databaseMutex);
QThread *thread = QThread::currentThread();
// if we have a connection for this thread, return it
if (s_instances.contains(thread))
return s_instances[thread];
}
// otherwise, create a new connection for this thread
QSqlDatabase connection =
QSqlDatabase::cloneDatabase(existingConnection, uniqueNameForNewInstanceOnThisThread);
// open the database connection
// initialize the database connection
s_instances.insert(thread, connection);
return connection;
}
As long as you make sure you create an initial connection (you can modify the factory to include that, or just create the initial connection in your DatabaseManager constructor), you should be able to replace most existing cases where you're using a QSqlDatabase with:
QSqlQuery query(DatabaseManager::database());
query.exec(<some sql>);
without worrying about which thread you're calling from. Hope that helps!

Answer 3

This is just mbroadst's excellent answer with minor changes. This code should be complete and directly compilable:
database_manager.h:
#ifndef DATABASEMANAGER_H
#define DATABASEMANAGER_H
#include <QMutex>
#include <QHash>
#include <QSqlDatabase>
class QThread;
class DatabaseManager
{
public:
static QSqlDatabase database(const QString& connectionName = QLatin1String(QSqlDatabase::defaultConnection));
private:
static QMutex s_databaseMutex;
static QHash<QThread*, QSqlDatabase> s_instances;
};
#endif // DATABASEMANAGER_H
database_manager.cpp:
#include "database_manager.h"
#include <QSqlDatabase>
#include <QMutexLocker>
#include <QThread>
#include <stdexcept>
QMutex DatabaseManager::s_databaseMutex;
QHash<QThread*, QHash<QString, QSqlDatabase>> DatabaseManager::s_instances;
QSqlDatabase DatabaseManager::database(const QString& connectionName)
{
QMutexLocker locker(&s_databaseMutex);
QThread *thread = QThread::currentThread();
// if we have a connection for this thread, return it
auto it_thread = s_instances.find(thread);
if (it_thread != s_instances.end()) {
auto it_conn = it_thread.value().find(connectionName);
if (it_conn != it_thread.value().end()) {
return it_conn.value();
}
}
// otherwise, create a new connection for this thread
QSqlDatabase connection = QSqlDatabase::cloneDatabase(
QSqlDatabase::database(connectionName),
QString("%1_%2").arg(connectionName).arg((int)thread));
// open the database connection
// initialize the database connection
if (!connection.open()) {
throw std::runtime_error("Unable to open the new database connection.");
}
s_instances[thread][connectionName] = connection;
return connection;
}
Usage: Instead of QSqlDatabase::database(), use DatabaseManager::database()
Edited 2016-07-01: fixed bug with multiple connection names