ListUnitsByNames returns an error

[rebeccaskinner]

What Happens:

Calling the ListUnitsByNames function on *dbus.Conn results in an unexpected error: Unknown method 'ListUnitsByNames' or interface 'org.freedesktop.systemd1.Manager'

What I expected to happen:

A slice of UnitStatus and no error, or a more descriptive error message.

How to reproduce:

Sample Code:

package main
import (
	"fmt"
	"github.com/coreos/go-systemd/dbus"
)

func main() {
	conn, err := dbus.New()
	if err != nil {
		fmt.Println("error getting connection: ", err)
	}
	res, err := conn.ListUnitsByNames([]string{"acpid.service"})
	if err != nil {
		fmt.Println("error with ListUnitsByNames: ", err)
		return
	}
	fmt.Printf("got %d results\n", len(res))
}

Test system:

uname -a; lsb_release -a
Linux rebecca-vboxdev 4.4.0-62-generic #83-Ubuntu SMP Wed Jan 18 14:10:15 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.2 LTS
Release:	16.04
Codename:	xenial

[lucab]

Which systemd version is this on? ListUnitsByNames has been added only recently in systemd/systemd#3182, which should be v232+ v230+.

[dongsupark]

@lucab I think systemd needs to be v230 or higher for ListUnitsByNames, while systemd on Ubuntu xenial is v229.
So it's recommended to introduce fallback like this.

[rebeccaskinner]

Okay yeah, I'm on 229. I've refactored to work around needing the call. I wonder if it's worthwhile to append a since: <version> line to the method comment descriptions? Happy to take some time to make a PR if that's something that you all think would be valuable.