QtService

A platform independent library to easily create system services and use some of their features

Looking for a maintainer! If anyone is interested in continuing development of the library, please write me here: Skycoder42#30

Features

• Allows you to create system (and user) services for various systems
  • Provides a single Interface to implement only once without having to care to much about the platform
  • Actual platform specifc stuff is provided via plugins, making it easy to support other backends as well
  • Most backends support native logging (i.e. journald, syslog, winevents, ...)
  • Supports socket-activation (for systemd & launchd)
  • Interfaces can be used to handle general service events
• Easy testing/debugging by simply switching the service backend via the start arguments
• Provides very basic service control to, e.g. get the status of a service or start/stop it
• Provides generic callbacks to allow realtively easy access to platform specific features
• Comes with a "Terminal frontend mode" which allows to create a CLI interface that communicates with the actual service without any extra coding needed
  • Terminals will connect to the running service. If none is running, they can start it (on some platforms)
  • Terminals send their command line arguments to the running service
  • A connected terminal can send input to the master (via stdin) and receive output (stdout)
  • Terminals can disconnect by beeing closed, leaving the service running (even the one that started the master)
  • The Service gets a "Terminal" handle for each connected terminal, allowing parallel connections. Each terminal implements QIODevice, making communication easy
  • For all this no extra code is needed from your side
• Supported service backends:
  • Systemd (For most modern Linux Distros)
  • Windows Services
  • Launchd (Tested on MacOs, but should theoretically work on other unixes as well)
  • Android Services
  • "Standard Services" - A platform independed dummy backend that runs services as a normal proccess - useful for testing, debugging or as fallback.
• QtCreator template to easily create a service project, with configuration file templates for each platform

Note: What this framework does explicitly not handle is "service installation" - as that is extremly different on all platforms and cannot be easily generalized. Performing the service installation, registration or whatever the platform needs is up to the developer. However, all the files that are needed as metadata for a service on each platform are provided via the QtCreator project template, which makes it significantly easier to deploy your service, as all thats left is to correctly "apply" those.

Details on what exactly is supported by each backend can be found in the doxygen documentation under the "Related Pages" tab.

Download/Installation

1. Package Managers: The library is available via:
   • Arch-Linux: AUR-Repository: qt5-service
   • MacOs:
     • Tap: brew tap Skycoder42/qt-modules
     • Package: qtservice
     • IMPORTANT: Due to limitations of homebrew, you must run source /usr/local/opt/qtservice/bashrc.sh before you can use the module.
2. Simply add my repository to your Qt MaintenanceTool (Image-based How-To here: Add custom repository):
   1. Start the MaintenanceTool from the commandline using /path/to/MaintenanceTool --addTempRepository <url> with one of the following urls (GUI-Method is currently broken, see QTIFW-1156) - This must be done every time you start the tool:
      • On Linux: https://install.skycoder42.de/qtmodules/linux_x64
      • On Windows: https://install.skycoder42.de/qtmodules/windows_x86
      • On Mac: https://install.skycoder42.de/qtmodules/mac_x64
   2. A new entry appears under all supported Qt Versions (e.g. Qt > Qt 5.11 > Skycoder42 Qt modules)
   3. You can install either all of my modules, or select the one you need: Qt Service
   4. Continue the setup and thats it! you can now use the module for all of your installed Kits for that Qt Version
3. Download the compiled modules from the release page. Note: You will have to add the correct ones yourself and may need to adjust some paths to fit your installation! In addition to that, you will have to download the modules this one depends on as well.
4. Build it yourself! Note: This requires all build an runtime dependencies to be available. If you don't have/need cmake, you can ignore the related warnings. To automatically build and install into your Qt installation, run:
   1. Install and prepare qdep
   2. Download the sources. Either use git clone or download from the releases. If you choose the second option, you have to manually create a folder named .git in the projects root directory, otherwise the build will fail.
   3. qmake
   4. make (If you want the tests/examples/etc. run make all)
   5. Optional step: make doxygen to generate the documentation
   6. make install

Usage

The C++ part of the Service is relatively simple. All you have to do is implement the QtService::Service interface and start it in your main. The most important methods of the service are start and stop - which are called when the service was started or stopped...

class TestService : public QtService::Service
{
	Q_OBJECT

public:
	explicit TestService(int &argc, char **argv) : //the reference here is important!
		Service{argc, argv}
	{}

protected:
	CommandMode onStart() override {
		qDebug() << "Service was started";
		return Synchronous; //service is now assumed started
	}

	CommandMode onStop(int &exitCode) override {
		qDebug() << "Stop received";
		// do some complicated stopping stuff asynchronously that takes some time...
		QTimer::singleShot(3000, this, [this](){
			emit stopped(EXIT_SUCCESS);
		});
		return Asynchronous; // service is still stopping until "stopped" has been emitted
	}
};

int main(int argc, char *argv[]) {
	// IMPORTANT: do NOT create a QCoreApplication here - this is done internally by the backends!
	// also, do nothing else in the main besides setting the serices properties! Any setup etc. must all be
	// done in the onStart method!!!
	TestService service{argc, argv};
	return service.exec();
}

Rules of usage:
You should always follow the following rules when using/creating a service. You can also find this list in the QtService::Service class documentation:

• Do not create a QCoreApplication yourself - this is done internally
• Use the same constructor signature to pass the main arguments. It is very important that the argc argument is passed as reference! Passing as value will crash your application
• Do nothing else in the main besides setting the serices properties that need to be set early! Any setup etc. must all be done in the Service::onStart method!!! (The properties that need to be set early all have a hint in their documentation)
• Never call QCoreApplication::quit (or QCoreApplication::exit)! Use Service::quit instead
• Do not rely on QCoreApplication::aboutToQuit, as this may not be emitted at all, or on some arbitrary point. Put all cleanup code in the Service::onStop method
• Be careful with Service::preStart - only use it when you have no other choice

Actually running the service depends on your choosen backend. When testing with the standard backend, you can simply run the service without any parameters, or the backend explicitly specified:

/path/to/service --backend standard

To run with a system backend, you need to register the executable somehow in the service system of your operating system. The trick here is to register it with the backend as parameter. So for example, on windows you would register the service as \path\to\service --backend windows. For more details check the documentation or create a service project from the template - it will create service files that already do this for you.

Terminals

To allow terminals, you must set the QtService::Service::terminalActive property to true and implement the QtService::Service::terminalConnected and optionally the QtService::Service::verifyCommand methods. The service is started the same way as usual. Internally, you will get the terminals as soon as they have connected and can then process their arguments and communicate with them.

To start a terminal client, simply prepend --terminal as well as the backend the service is running on to the command line. It is recommended to create a small wrapper script for each platform that takes care of these parameters. On linux, for example, you would create a shell script like below that simply starts the executable with the additional parameters:

#!/bin/sh
# use as "service-cli <arguments>" (assuming this script is named service-cli)
exec /path/to/service --backend systemd --terminal "$@"

(The platform argument is recommended to not depend on any windowing system)

Service Control

The QtService::ServiceControl allows you to control services by sending commands to them and retrieving the status. However, what exactly is possible greatly varies for each platform. Always use QtService::ServiceControl::supportFlags to figure out what you can actually do on the current platform. You can also check the doxygen documentation to get an overview over all the backends and their features.

Trouble shooting

Sometimes, a service just won't start, without any apparent reason. This can be very hard to debug, as you cannot debug a service with traditional means. The best tricks I came by this problems are:

1. Enable as much debugging as possible, by setting the QT_LOGGING_RULES environment variable to qt.service.*.debug=true. This will enable debug logging for the service internals, which might help. To access the system logs, refer to your service managers documentation.
2. Sometimes, logs are empty or the service crashes before starting. This often indicates, that the service plugins cannot be found. In that case, make sure that:
   1. If you deployed your application, check that the servicebackends plugin folder exists and that the plugin you want to use is in there (for example, qwindows[d].dll for the windows backend).
   2. Check or generate the qt.conf file. It should contain an enty named Plugins that points to the directory that contains the servicebackends folder.
   3. If that still does not help, you can try to manually specify the plugin folder via an environment variable. Simply set QT_PLUGIN_PATH to the directory that contains the servicebackends folder. Use an absolute path if possible.

Important: Some service managers (like windows) do not allow to set environment variables for services from the outside. In such cases, you must set the variables in your main, before loading the service. For example, to set QT_PLUGIN_PATH, you would do:

int main(int argc, char **argv) {
    const auto appDir = QFileInfo{QString::fromUtf8(argv[0])}.dir();
    qputenv("QT_PLUGIN_PATH", appDir.absolutePath().toUtf8());
    
    QtService::Service service{argc, argv};
    return service.exec();
}

Documentation

The documentation is available on github pages. It was created using doxygen. The HTML-documentation and Qt-Help files are shipped together with the module for both the custom repository and the package on the release page. Please note that doxygen docs do not perfectly integrate with QtCreator/QtAssistant.

References

The Service backend code for the windows plugin was created by using the code from https://github.com/qtproject/qt-solutions as a basis.