cmd/tailscale/cli: clean up how optional commands get registered

Both @agottardo and I tripped over this today.

Updates #cleanup

Change-Id: I64380a03bfc952b9887b1512dbcadf26499ff1cd
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/advertise.go

@@ -21,7 +21,11 @@
 
 // TODO(naman): This flag may move to set.go or serve_v2.go after the WIPCode
 // envknob is not needed.
-var advertiseCmd = &ffcli.Command{
+func advertiseCmd() *ffcli.Command {
+	if !envknob.UseWIPCode() {
+		return nil
+	}
+	return &ffcli.Command{
 		Name:       "advertise",
 		ShortUsage: "tailscale advertise --services=<services>",
 		ShortHelp:  "Advertise this node as a destination for a service",
@@ -31,13 +35,7 @@
 			fs.StringVar(&advertiseArgs.services, "services", "", "comma-separated services to advertise; each must start with \"svc:\" (e.g. \"svc:idp,svc:nas,svc:database\")")
 			return fs
 		})(),
-}
-
-func maybeAdvertiseCmd() []*ffcli.Command {
-	if !envknob.UseWIPCode() {
-		return nil
 	}
-	return []*ffcli.Command{advertiseCmd}
 }
 
 func runAdvertise(ctx context.Context, args []string) error {

cmd/tailscale/cli/cli.go

@@ -25,6 +25,7 @@
 	"tailscale.com/cmd/tailscale/cli/ffcomplete"
 	"tailscale.com/envknob"
 	"tailscale.com/paths"
+	"tailscale.com/util/slicesx"
 	"tailscale.com/version/distro"
 )
 
@@ -182,7 +183,7 @@ func newRootCmd() *ffcli.Command {
 This CLI is still under active development. Commands and flags will
 change in the future.
 `),
-		Subcommands: append([]*ffcli.Command{
+		Subcommands: nonNilCmds(
 			upCmd,
 			downCmd,
 			setCmd,
@@ -214,7 +215,8 @@ func newRootCmd() *ffcli.Command {
 			debugCmd,
 			driveCmd,
 			idTokenCmd,
-		}, maybeAdvertiseCmd()...),
+			advertiseCmd(),
+		),
 		FlagSet: rootfs,
 		Exec: func(ctx context.Context, args []string) error {
 			if len(args) > 0 {
@@ -239,6 +241,10 @@ func newRootCmd() *ffcli.Command {
 	return rootCmd
 }
 
+func nonNilCmds(cmds ...*ffcli.Command) []*ffcli.Command {
+	return slicesx.Filter(cmds[:0], cmds, func(c *ffcli.Command) bool { return c != nil })
+}
+
 func fatalf(format string, a ...any) {
 	if Fatalf != nil {
 		Fatalf(format, a...)