Evaluate the use of the Systemd DBus API using the go-systemd binding package

[ema-pe]

Currently, the DFaaS agent restarts the HAProxy service by executing a command defined in the configuration file:

https://github.com/unimib-datAI/dfaas/blob/034794c87d7bfe638c098c40f09811cb0c35048b/dfaasagent.env#L23C38-L23C80

The command is executed using the os/exec package, which creates a temporary bash shell and checks the exit code:

dfaas/dfaasagent/agent/hacfgupd/hacfgupd.go

Lines 42 to 66 in 034794c

	// UpdateHAConfig updates the HAProxy config file
	func (updater *Updater) UpdateHAConfig(content interface{}) error {
	f, err := os.Create(updater.HAConfigFilePath)
	if err != nil {
	return errors.Wrap(err, "Error while opening the HAProxy configuration file for writing")
	}
	defer f.Close()
	err = updater.template.Execute(f, content)
	if err != nil {
	return errors.Wrap(err, "Error while applying the HAProxy configuration template to the data")
	}
	cmd := exec.Command("bash", "-c", updater.CmdOnUpdated)
	if updateStdoutDebugLogging {
	cmd.Stdout = os.Stdout
	}
	cmd.Stderr = os.Stderr
	err = cmd.Run()
	if err != nil {
	return errors.Wrap(err, "Error while executing the HAProxy configuration update command (command: \""+updater.CmdOnUpdated+"\")")
	}
	return nil
	}

This implementation works, but can be improved: we can call systemd API via DBus and this opens a lot of possibilities (for example, we can restart, reload the service or introspect it). For the Go language there is go-systemd, a module that exposes the DBus systemd API to the language (
https://pkg.go.dev/github.com/coreos/go-systemd/v22/dbus). We should evaluate whether using the DBus API is better for the agent than running the systemd command directly, since it introduces a new dependency (and the DBus server must be running).