SetGenericSystemLoopbackIPs() needs a "don't care" option

[chrismarget-j]

Similar to #230.

When marshaling JSON for /api/blueprints/%s/systems/%s/loopback/%d, we need to distinguish between these two cases:

• I don't care about one of the addresses
• I intende to clear one of the addresses

This seems to be close:

type foo struct {
	Name string
	IP   *net.IP
}

func (o foo) MarshalJSON() ([]byte, error) {
	type raw struct {
		Name string  `json:"name"`
		IP   *string `json:"ip_address"`
	}

	r := raw{Name: o.Name}

	if o.IP != nil {
		if len(*o.IP) == 0 {
			// non-nil net.IP with empty length == clear the value from the API
			r.IP = toPtr("") // send an empty string
		} else {
			// nil net.IP == don't care
			r.IP = toPtr(o.IP.String()) // send null
		}
	}

	return json.Marshal(&r)
}

func TestFoo(t *testing.T) {
	type testCase struct {
		v foo
		e string
	}

	testCases := map[string]testCase{
		"nil": {
			v: foo{Name: "nil", IP: nil},
			e: `{"name":"nil","ip_address":null}`,
		},
		"empty": {
			v: foo{Name: "empty", IP: &net.IP{}},
			e: `{"name":"empty","ip_address":""}`,
		},
		"with_value": {
			v: foo{Name: "with_value", IP: &net.IP{10, 0, 0, 5}},
			e: `{"name":"with_value","ip_address":"10.0.0.5"}`,
		},
	}

	for tName, tCase := range testCases {
		tName, tCase := tName, tCase
		t.Run(tName, func(t *testing.T) {
			t.Parallel()
			b, err := json.Marshal(&tCase.v)
			require.NoError(t, err)

			t.Log(tName, string(b))
		})
	}
}