wgengine/wgcfg: restore Config.ListenPort for cmd/lopower

It was removed for being unnecessary in 69cdc30c6d41aab2 in 2021
but now we need it for lopower.

Change-Id: I62aca850ad820a83475c8ef7655a08981f23a27f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/wgcfg/config.go

@@ -21,6 +21,7 @@ type Config struct {
 	NodeID     tailcfg.StableNodeID
 	PrivateKey key.NodePrivate
 	Addresses  []netip.Prefix
+	ListenPort uint16 // not used by Tailscale's conn.Bind implementation
 	MTU        uint16
 	DNS        []netip.Addr
 	Peers      []Peer

wgengine/wgcfg/parser.go

@@ -124,7 +124,13 @@ func (cfg *Config) handleDeviceLine(k, value mem.RO, valueBytes []byte) error {
 		if err != nil {
 			return err
 		}
-	case k.EqualString("listen_port") || k.EqualString("fwmark"):
+	case k.EqualString("listen_port"):
+		port, err := mem.ParseUint(value, 10, 16)
+		if err != nil {
+			return fmt.Errorf("failed to parse listen_port: %w", err)
+		}
+		cfg.ListenPort = uint16(port)
+	case k.EqualString("fwmark"):
 	// ignore
 	default:
 		return fmt.Errorf("unexpected IpcGetOperation key: %q", k.StringCopy())

wgengine/wgcfg/wgcfg_clone.go

@@ -39,6 +39,7 @@ var _ConfigCloneNeedsRegeneration = Config(struct {
 	NodeID         tailcfg.StableNodeID
 	PrivateKey     key.NodePrivate
 	Addresses      []netip.Prefix
+	ListenPort     uint16
 	MTU            uint16
 	DNS            []netip.Addr
 	Peers          []Peer

wgengine/wgcfg/writer.go

@@ -42,6 +42,9 @@ func (cfg *Config) ToUAPI(logf logger.Logf, w io.Writer, prev *Config) error {
 	if !prev.PrivateKey.Equal(cfg.PrivateKey) {
 		set("private_key", cfg.PrivateKey.UntypedHexString())
 	}
+	if prev.ListenPort != cfg.ListenPort {
+		setUint16("listen_port", cfg.ListenPort)
+	}
 
 	old := make(map[key.NodePublic]Peer)
 	for _, p := range prev.Peers {