How to deploy QSQL plugins

[Mark81]

On my dev machines I can run applications that uses SQL connections:

$ ls -l Qt/6.2.0/gcc_64/plugins/sqldrivers/libqsql*
-rwxr-xr-x 1 mark mark 1363192 set 24 18:30 Qt/6.2.0/gcc_64/plugins/sqldrivers/libqsqlite.so
-rwxr-xr-x 1 mark mark   86608 set 24 18:30 Qt/6.2.0/gcc_64/plugins/sqldrivers/libqsqlmysql.so
-rwxr-xr-x 1 mark mark  128176 set 24 18:30 Qt/6.2.0/gcc_64/plugins/sqldrivers/libqsqlodbc.so
-rwxr-xr-x 1 mark mark   98800 set 24 18:30 Qt/6.2.0/gcc_64/plugins/sqldrivers/libqsqlpsql.so

Now I want to deploy the application on a remote target. Both machines run Ubuntu 20.04.
In the target machine I have:

• Qt libraries in /home/user/qt6 (LD_LIBRARY_PATH is set to this directory)
• app binary in /home/user/bin

Reading the docs I tried to:

• place the above files in /home/user/bin (application directory)
• export QT_PLUGIN_PATH=/home/user/bin/

But in both cases I get:

$ ./app
QSqlDatabase: QMYSQL driver not loaded
QSqlDatabase: available drivers:

Here the test code that runs fine on the dev machine:

#include <QCoreApplication>
#include <QSqlDatabase>
#include <QSqlQuery>
#include <QSqlError>
#include <QObject>

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

    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
    db.setHostName("192.168.1.28");
    db.setPort(3306);
    db.setDatabaseName("<database>");
    db.setUserName("<username>");
    db.setPassword("<password>");

    qDebug() << db.open();

    return a.exec();
}

Is there anything else I need to deploy on the target machine?

[jsulm]

@Mark81 said in How to deploy QSQL plugins:

Reading the docs I tried to:

place the above files in /home/user/bin (application directory)
export QT_PLUGIN_PATH=/home/user/bin/

You should take a look at https://doc.qt.io/qt-6/linux-deployment.html
Qt aoolication expect specific directory structure (plug-ins go to plugins subfolder for example).

[Mark81]

@jsulm
Ok, I if understood correcly this is the key:

When looking for plugins, the application searches in a plugins subdirectory inside the directory of the application executable.

So this is the new directory structure:

$ tree .
.
└── bin
    ├── plugins
    │   ├── libqsqlite.so
    │   ├── libqsqlmysql.so
    │   ├── libqsqlodbc.so
    │   └── libqsqlpsql.so
    └── app

Same error.
Also:

An alternative to putting the plugins in the plugins subdirectory is to add a custom search path when you start your application using QApplication::addLibraryPath() or QApplication::setLibraryPaths().

QCoreApplication a(argc, argv);
QCoreApplication::setLibraryPaths(QStringList() << "/home/user/bin/plugins");

leads to the same error.

Where is my mistake now?

[Mark81]

I also checked the dependencies of the mysql plugin are there:

$ ldd libqsqlmysql.so 
	linux-vdso.so.1 (0x00007ffd755cb000)
	libmysqlclient.so.21 => /lib/x86_64-linux-gnu/libmysqlclient.so.21 (0x00007fb188a16000)
	libQt6Sql.so.6 => /home/user/qt6/libQt6Sql.so.6 (0x00007fb1889c9000)
	libQt6Core.so.6 => /home/user/qt6/libQt6Core.so.6 (0x00007fb18839b000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fb188378000)
	libstdc++.so.6 => /lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007fb188196000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fb188047000)
	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007fb18802a000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fb187e38000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fb187e32000)
	libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007fb187e16000)
	libssl.so.1.1 => /lib/x86_64-linux-gnu/libssl.so.1.1 (0x00007fb187d83000)
	libcrypto.so.1.1 => /lib/x86_64-linux-gnu/libcrypto.so.1.1 (0x00007fb187aad000)
	libresolv.so.2 => /lib/x86_64-linux-gnu/libresolv.so.2 (0x00007fb187a8f000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fb189175000)
	libicui18n.so.56 => /home/user/qt6/libicui18n.so.56 (0x00007fb1875f6000)
	libicuuc.so.56 => /home/user/qt6/libicuuc.so.56 (0x00007fb18723e000)
	libicudata.so.56 => /home/user/qt6/libicudata.so.56 (0x00007fb18585b000)
	libglib-2.0.so.0 => /lib/x86_64-linux-gnu/libglib-2.0.so.0 (0x00007fb185732000)
	libgthread-2.0.so.0 => /lib/x86_64-linux-gnu/libgthread-2.0.so.0 (0x00007fb18572d000)
	librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007fb185720000)
	libpcre.so.3 => /lib/x86_64-linux-gnu/libpcre.so.3 (0x00007fb1856ad000)

[jsulm]

@Mark81 said in How to deploy QSQL plugins:

QCoreApplication::setLibraryPaths(QStringList() << "/home/user/bin/plugins");

I think this line should be above the other one.

See also https://doc.qt.io/qt-6/windows-deployment.html#application-dependencies
"Similar to the platform plugin, each type of plugin must be located within a specific subdirectory (such as printsupport, imageformats or sqldrivers) within your distribution directory"
So, it should be YOUR_APP_FOLDER/plugins/sqldrivers (basically same directory structure as in your Qt installation).

[Mark81]

@jsulm said in How to deploy QSQL plugins:

Aaaarghhh, it's too fragmented the documentation :-) It would be better if they reported all the information in the same place!
Anyway:

$ tree .
.
└── bin
    ├── plugins
    │   └── sqldrivers
    │       ├── libqsqlite.so
    │       ├── libqsqlmysql.so
    │       ├── libqsqlodbc.so
    │       └── libqsqlpsql.so
    └── app

Leads to the very same error.
Furthermore:

QPluginLoader loader;
loader.setFileName("/home/user/bin/plugins/sqldrivers/libqsqlmysql.so");
qDebug() << loader.load();
qDebug() << loader.errorString();

outputs:

true
"Unknown error"

BUT:

QCoreApplication::setLibraryPaths(QStringList() << "/home/user/bin/plugins");

did the trick!
I still don't understand:

1. why QPluginLoader told me it loaded the plugin correctly while after a while the application said the plugin were not loaded
2. why I have to specify with setLibraryPaths the expected path for plugins (app path/plugins) ?

[jsulm]

@Mark81 said in How to deploy QSQL plugins:

why QPluginLoader told me it loaded the plugin correctly while after a while the application said the plugin were not loaded

Because you used absolute path to the plug-in.
I'm not sure why the plug-ins are not found, but you can also provide https://doc.qt.io/qt-6/qt-conf.html file where you specify all needed paths.