Type handling issue when using gonetworkmanager

[rdecarreau]

I am using NetworkManager 1.46 on BalenaOS.
I am using the gonetworkmanager library which in-turn uses godbus to communicate to NetworkManager from my application.

I attempt this to manually set priority (hopefully to force NetworkManager to change to a network interface of my choosing).

	// Get the connection
	connection, err := activeConnection.GetPropertyConnection()
	if err != nil {
		return errors.Join(
			fmt.Errorf("error getting connection for interface %s", interfaceName),
			err,
		)
	}

	existingSettings, err := connection.GetSettings()
	if err != nil {
		return errors.Join(
			fmt.Errorf("error getting existing settings for interface %s", interfaceName),
			err,
		)
	}

	existingSettings["connection"] = map[string]interface{}{
		"autoconnect-priority": priority,
	}

	// Try UpdateUnsaved first for better error handling
	err = connection.UpdateUnsaved(existingSettings)
	if err != nil {
		return errors.Join(
			fmt.Errorf("error updating priority for interface %s", interfaceName),
			err,
		)
	}

Upon saving the connection I get an error:

"Failed to update network priorities","error":"failed to set WiFi priority: error updating priority for interface wlan0\nipv6.addresses: can't set property of type 'a(ayuay)' from value of type 'aav'"}

No amount of formatting of values and types seems to make it happy enough to accept my settings, even though I just read them from GetSettings.

See issue Wifx/gonetworkmanager#13 for similar other problems, I could not find a follow-up here.

[guelfey]

This is an unfortunate side-effect from an early choice when this library was created: structs are not "symmetric" when being encoded and decoded. For most types, the decode can use reflection to directly create a variable of the correct type (e.g. create a Go string for a D-Bus "STRING" https://pkg.go.dev/github.com/godbus/dbus/v5#hdr-Conversion_Rules); for structs, it's however not really possible to create a matching struct type at runtime, so these are decoded as []any. Passing such values directly back to the encoder, however, encodes them as an array of variants, which is not the same time. My expectation was that normally the caller "knows" the expected structure and can create a matching Go struct definition, then use https://pkg.go.dev/github.com/godbus/dbus/v5#Store to convert from the []any to the Go struct. Of course that's not really possible if the struct appears nested in a Variant.

I don't really see a way how this could be fixed in this library without changing the behavior of decoding structs (e.g. using a separate type for them, which would be treated differently by the encoder), which however would be a breaking API change. In the meantime Wifx/gonetworkmanager#13 mentions a workaround for this concrete issue