MySql: my_thread_global_end(): 1 threads didn't exit

[mentalmushroom]

When QSqlDatabase::removeDatabase() is called from a different thread than QSqlDatabase::addDatabase() was called from, I get the following error message:

Error in my_thread_global_end(): 1 threads didn't exit

It is easy to reproduce with the following code sample even without MySql database. But it seems to happen on Ubuntu and NOT on Windows.

#include <QtCore>
#include <QtSql>
 
class Client: public QObject
{
    Q_OBJECT
 
public:
    Client();
    ~Client();
 
public slots:
    void start();
 
signals:
    void done();
}; // Client
 
Client::Client()
{
    QSqlDatabase::addDatabase("QMYSQL", "my_connection");
}
 
Client::~Client()
{
}
 
void Client::start()
{
    // we could do some work here, but for the test simply remove the connection
    QSqlDatabase::removeDatabase("my_connection");
    emit done();
}
 
 
int main(int argc, char* argv[])
{
    QCoreApplication app(argc, argv);
 
    QThread t;
    Client client;
    client.moveToThread(&t);
    QObject::connect(&t, SIGNAL(started()), &client, SLOT(start()));
    QObject::connect(&client, SIGNAL(done()), &t, SLOT(quit()));
    QObject::connect(&t, SIGNAL(finished()), &app, SLOT(quit()));
    t.start();
 
    return app.exec();
}
 
#include "main.moc"

#include <QtCore>
#include <QtSql>

class Client: public QObject
{
    Q_OBJECT

public:
    Client();
    ~Client();

public slots:
    void start();

signals:
    void done();
}; // Client

Client::Client()
{
    QSqlDatabase::addDatabase("QMYSQL", "my_connection");
}

Client::~Client()
{
}

void Client::start()
{
    // we could do some work here, but for the test simply remove the connection
    QSqlDatabase::removeDatabase("my_connection");
    emit done();
}


int main(int argc, char* argv[])
{
    QCoreApplication app(argc, argv);

    QThread t;
    Client client;
    client.moveToThread(&t);
    QObject::connect(&t, SIGNAL(started()), &client, SLOT(start()));
    QObject::connect(&client, SIGNAL(done()), &t, SLOT(quit()));
    QObject::connect(&t, SIGNAL(finished()), &app, SLOT(quit()));
    t.start();

    return app.exec();
}

#include "main.moc"

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What am I doing wrong?

[wysota]

What am I doing wrong?

You are removing the database from a wrong thread. QSqlDatabase (and friends) objects are not to be used from within different threads.

Threads and the sql module

[mentalmushroom]

Ok, good to know. But I noticed the error happens when I add and remove the connection from the same thread either, but less regular.

The issue can be noticed with the following code sample:

#include <QtCore>
#include <QtSql>
 
class Client: public QRunnable
{
public:
	virtual void run();	
};
 
void Client::run()
{
    {
		QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL", QString::number((unsigned long long)QThread::currentThreadId()));
 
		db.setHostName("localhost");
		db.setDatabaseName("test");
		db.setUserName("me");
		db.setPassword("mypass");
 
		db.open();
 
		//QSqlQuery q(strQuery);
    }
 
    QSqlDatabase::removeDatabase(QString::number((unsigned long long)QThread::currentThreadId()));
 
}
 
int main(int argc, char *argv[])
{
	QCoreApplication a(argc, argv);
 
	QThreadPool pool;
	pool.setMaxThreadCount(100);
 
	for (int i = 0; i < 100; ++i)
	{
		Client *client = new Client;
		bool started = pool.tryStart(client);
		Q_ASSERT(started);
	}
 
	pool.waitForDone();
 
	//return a.exec();
	return 0;
}

#include <QtCore>
#include <QtSql>

class Client: public QRunnable
{
public:
	virtual void run();	
};

void Client::run()
{
    {
		QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL", QString::number((unsigned long long)QThread::currentThreadId()));

		db.setHostName("localhost");
		db.setDatabaseName("test");
		db.setUserName("me");
		db.setPassword("mypass");

		db.open();

		//QSqlQuery q(strQuery);
    }

    QSqlDatabase::removeDatabase(QString::number((unsigned long long)QThread::currentThreadId()));

}

int main(int argc, char *argv[])
{
	QCoreApplication a(argc, argv);

	QThreadPool pool;
	pool.setMaxThreadCount(100);

	for (int i = 0; i < 100; ++i)
	{
		Client *client = new Client;
		bool started = pool.tryStart(client);
		Q_ASSERT(started);
	}

	pool.waitForDone();

	//return a.exec();
	return 0;
}

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Here goes the output of two successive runs:

mushroom@ubuntu:~/projects/mysqltest/mysqlthread/debug$ ./mysqlthread
Error in my_thread_global_end(): 48 threads didn't exit

mushroom@ubuntu:~/projects/mysqltest/mysqlthread/debug$ ./mysqlthread
Error in my_thread_global_end(): 33 threads didn't exit
Error in my_thread_global_end(): 85 threads didn't exit

[wysota]

Close the database first before removing it.

[mentalmushroom]

I've updated the code, so it is closed:

#include <QtCore>
#include <QtSql>
 
class Client: public QRunnable
{
public:
	virtual void run();	
};
 
void Client::run()
{
    {
		QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL", QString::number((unsigned long long)QThread::currentThreadId()));
 
		db.setHostName("localhost");
		db.setDatabaseName("test");
		db.setUserName("me");
		db.setPassword("mypass");
 
		db.open();
 
		db.close(); // ! close the database before removing it !
    }
 
    QSqlDatabase::removeDatabase(QString::number((unsigned long long)QThread::currentThreadId()));
 
}
 
int main(int argc, char *argv[])
{
	QCoreApplication a(argc, argv);
 
	QThreadPool pool;
	pool.setMaxThreadCount(100);
 
	for (int i = 0; i < 100; ++i)
	{
		Client *client = new Client;
		bool started = pool.tryStart(client);
		Q_ASSERT(started);
	}
 
	pool.waitForDone();
 
	return 0;
}

#include <QtCore>
#include <QtSql>

class Client: public QRunnable
{
public:
	virtual void run();	
};

void Client::run()
{
    {
		QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL", QString::number((unsigned long long)QThread::currentThreadId()));

		db.setHostName("localhost");
		db.setDatabaseName("test");
		db.setUserName("me");
		db.setPassword("mypass");

		db.open();

		db.close(); // ! close the database before removing it !
    }

    QSqlDatabase::removeDatabase(QString::number((unsigned long long)QThread::currentThreadId()));

}

int main(int argc, char *argv[])
{
	QCoreApplication a(argc, argv);

	QThreadPool pool;
	pool.setMaxThreadCount(100);

	for (int i = 0; i < 100; ++i)
	{
		Client *client = new Client;
		bool started = pool.tryStart(client);
		Q_ASSERT(started);
	}

	pool.waitForDone();

	return 0;
}

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The error still persists:

mushroom@ubuntu:~/projects/mysqltest/mysqlthread/debug$ ./mysqlthread
Error in my_thread_global_end(): 51 threads didn't exit
Error in my_thread_global_end(): 65 threads didn't exit

[gfernandes]

Hi,

I have the same issue. Did you resolve your issue? If yes, could you please share your solution?

Thank you

[mentalmushroom]

No, I didn't, but so far I can't see any obvious issues, except that warning message. However, I've reported it to the bug tracker. Vote for it, if it worries you.

[danadam]

From what I found out, there may be two causes for that:

1. QSqlDatabase::addDatabase() and QSqlDatabase::removeDatabase() are not entirely thread-safe when using MySQL driver (maybe also when using other drivers, I didn't check them)
2. MySQL driver doesn't clean up properly after itself when call to QSqlDatabase::open() fails.

See my response to your bug for more details.