Pull request: dhcpd: fix interface ipv6 check

Merge in DNS/adguard-home from 2355-dhcpcheck-ipv6 to master

Updates #2335.

Squashed commit of the following:

commit 5ce1cc7bc244ba5dd4a065d47dec8884fa3d45e7
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Nov 25 14:03:24 2020 +0300

    dhcpd: fix loop exit condition

commit 32b4b946bfa30159326dc295fa1a2607b78172af
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Nov 25 13:26:50 2020 +0300

    dhcpd: fix interface ipv6 check

internal/dhcpd/v6_test.go

@@ -6,6 +6,7 @@ import (
 	"net"
 	"testing"
 
+	"github.com/AdguardTeam/AdGuardHome/internal/agherr"
 	"github.com/insomniacslk/dhcp/dhcpv6"
 	"github.com/insomniacslk/dhcp/iana"
 	"github.com/stretchr/testify/assert"
@@ -223,3 +224,57 @@ func TestV6GetDynamicLease(t *testing.T) {
 	assert.True(t, ip6InRange(net.ParseIP("2001::2"), net.ParseIP("2001::2")))
 	assert.True(t, ip6InRange(net.ParseIP("2001::2"), net.ParseIP("2001::3")))
 }
+
+type fakeIface struct {
+	addrs []net.Addr
+	err   error
+}
+
+// Addrs implements the netIface interface for *fakeIface.
+func (iface *fakeIface) Addrs() (addrs []net.Addr, err error) {
+	if iface.err != nil {
+		return nil, iface.err
+	}
+
+	return iface.addrs, nil
+}
+
+func TestIfaceIPv6Addrs(t *testing.T) {
+	ip := net.IP{1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 3, 4, 5, 6}
+	ip4 := net.IP{1, 2, 3, 4}
+	addr := &net.IPNet{IP: ip}
+	errTest := agherr.Error("test error")
+
+	testCases := []struct {
+		name    string
+		iface   netIface
+		want    []net.IP
+		wantErr error
+	}{{
+		name:    "success",
+		iface:   &fakeIface{addrs: []net.Addr{addr}, err: nil},
+		want:    []net.IP{ip},
+		wantErr: nil,
+	}, {
+		name: "success_with_ipv4",
+		iface: &fakeIface{
+			addrs: []net.Addr{addr, &net.IPNet{IP: ip4}},
+			err:   nil,
+		},
+		want:    []net.IP{ip},
+		wantErr: nil,
+	}, {
+		name:    "error",
+		iface:   &fakeIface{addrs: []net.Addr{addr}, err: errTest},
+		want:    nil,
+		wantErr: errTest,
+	}}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			got, gotErr := ifaceIPv6Addrs(tc.iface)
+			assert.Equal(t, tc.want, got)
+			assert.Equal(t, tc.wantErr, gotErr)
+		})
+	}
+}