cmd/omitsize: fix the --features flag

When you say --features=foo,bar, that was supposed to mean
to only show features "foo" and "bar" in the table.

But it was also being used as the set of all features that are
omittable, which was wrong, leading to misleading numbers
when --features was non-empty.

Updates #12614

Change-Id: Idad2fa67fb49c39454032e84a3dede967890fdf5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/omitsize/omitsize.go

@@ -31,12 +31,16 @@ func main() {
 	flag.Parse()
 
 	var all []string
-	if *features == "" {
-		for k := range featuretags.Features {
-			if k.IsOmittable() {
-				all = append(all, k.OmitTag())
-			}
+	var allOmittable []string
+
+	for k := range featuretags.Features {
+		if k.IsOmittable() {
+			allOmittable = append(allOmittable, k.OmitTag())
 		}
+	}
+
+	if *features == "" {
+		all = slices.Clone(allOmittable)
 	} else {
 		for v := range strings.SplitSeq(*features, ",") {
 			if !strings.HasPrefix(v, "ts_omit_") {
@@ -49,15 +53,15 @@ func main() {
 	slices.Sort(all)
 	all = slices.Compact(all)
 
-	baseD := measure("tailscaled")
-	baseC := measure("tailscale")
-	baseBoth := measure("tailscaled", "ts_include_cli")
-
-	minD := measure("tailscaled", all...)
-	minC := measure("tailscale", all...)
-	minBoth := measure("tailscaled", append(slices.Clone(all), "ts_include_cli")...)
+	minD := measure("tailscaled", allOmittable...)
+	minC := measure("tailscale", allOmittable...)
+	minBoth := measure("tailscaled", append(slices.Clone(allOmittable), "ts_include_cli")...)
 
 	if *showRemovals {
+		baseD := measure("tailscaled")
+		baseC := measure("tailscale")
+		baseBoth := measure("tailscaled", "ts_include_cli")
+
 		fmt.Printf("Starting with everything and removing a feature...\n\n")
 
 		fmt.Printf("%9s %9s %9s\n", "tailscaled", "tailscale", "combined (linux/amd64)")
@@ -80,7 +84,7 @@ func main() {
 	fmt.Printf("%9s %9s %9s\n", "tailscaled", "tailscale", "combined (linux/amd64)")
 	fmt.Printf("%9d %9d %9d omitting everything\n", minD, minC, minBoth)
 	for _, t := range all {
-		tags := allExcept(all, t)
+		tags := allExcept(allOmittable, t)
 		sizeD := measure("tailscaled", tags...)
 		sizeC := measure("tailscale", tags...)
 		sizeBoth := measure("tailscaled", append(tags, "ts_include_cli")...)