clarify SelectedPackageTypes meaning and improve its usage (#30142)

- document SelectedPackageTypes meaning
- simplify isPackageTypeSelected

dev-tools/mage/crossbuild.go

@@ -43,7 +43,8 @@ const defaultCrossBuildTarget = "golangCrossBuild"
 // See NewPlatformList for details about platform filtering expressions.
 var Platforms = BuildPlatforms.Defaults()
 
-// SelectedPackageTypes is the list of package types
+// SelectedPackageTypes is the list of package types. If empty, all packages types
+// are considered to be selected (see isPackageTypeSelected).
 var SelectedPackageTypes []PackageType
 
 func init() {
@@ -65,7 +66,7 @@ func init() {
 	}
 }
 
-// CrossBuildOption defines a option to the CrossBuild target.
+// CrossBuildOption defines an option to the CrossBuild target.
 type CrossBuildOption func(params *crossBuildParams)
 
 // ImageSelectorFunc returns the name of the builder image.

dev-tools/mage/pkg.go

@@ -112,17 +112,19 @@ func Package() error {
 	return nil
 }
 
+// isPackageTypeSelected returns true if SelectedPackageTypes is empty or if
+// pkgType is present on SelectedPackageTypes. It returns false otherwise.
 func isPackageTypeSelected(pkgType PackageType) bool {
-	if SelectedPackageTypes != nil {
-		selected := false
-		for _, t := range SelectedPackageTypes {
-			if t == pkgType {
-				selected = true
-			}
+	if len(SelectedPackageTypes) == 0 {
+		return true
+	}
+
+	for _, t := range SelectedPackageTypes {
+		if t == pkgType {
+			return true
 		}
-		return selected
 	}
-	return true
+	return false
 }
 
 type packageBuilder struct {