util/syspolicy/setting: add package that contains types for the next syspolicy PRs

Package setting contains types for defining and representing policy settings.
It facilitates the registration of setting definitions using Register and RegisterDefinition,
and the retrieval of registered setting definitions via Definitions and DefinitionOf.
This package is intended for use primarily within the syspolicy package hierarchy,
and added in a preparation for the next PRs.

Updates #12687

Signed-off-by: Nick Khyl <nickk@tailscale.com>

util/syspolicy/syspolicy.go

@@ -7,6 +7,8 @@ package syspolicy
 import (
 	"errors"
 	"time"
+
+	"tailscale.com/util/syspolicy/setting"
 )
 
 func GetString(key Key, defaultValue string) (string, error) {
@@ -45,78 +47,20 @@ func GetStringArray(key Key, defaultValue []string) ([]string, error) {
 	return v, err
 }
 
-// PreferenceOption is a policy that governs whether a boolean variable
-// is forcibly assigned an administrator-defined value, or allowed to receive
-// a user-defined value.
-type PreferenceOption int
-
-const (
-	showChoiceByPolicy PreferenceOption = iota
-	neverByPolicy
-	alwaysByPolicy
-)
-
-// Show returns if the UI option that controls the choice administered by this
-// policy should be shown. Currently this is true if and only if the policy is
-// showChoiceByPolicy.
-func (p PreferenceOption) Show() bool {
-	return p == showChoiceByPolicy
-}
-
-// ShouldEnable checks if the choice administered by this policy should be
-// enabled. If the administrator has chosen a setting, the administrator's
-// setting is returned, otherwise userChoice is returned.
-func (p PreferenceOption) ShouldEnable(userChoice bool) bool {
-	switch p {
-	case neverByPolicy:
-		return false
-	case alwaysByPolicy:
-		return true
-	default:
-		return userChoice
-	}
-}
-
-// WillOverride checks if the choice administered by the policy is different
-// from the user's choice.
-func (p PreferenceOption) WillOverride(userChoice bool) bool {
-	return p.ShouldEnable(userChoice) != userChoice
-}
-
 // GetPreferenceOption loads a policy from the registry that can be
 // managed by an enterprise policy management system and allows administrative
 // overrides of users' choices in a way that we do not want tailcontrol to have
 // the authority to set. It describes user-decides/always/never options, where
 // "always" and "never" remove the user's ability to make a selection. If not
 // present or set to a different value, "user-decides" is the default.
-func GetPreferenceOption(name Key) (PreferenceOption, error) {
-	opt, err := GetString(name, "user-decides")
+func GetPreferenceOption(name Key) (setting.PreferenceOption, error) {
+	s, err := GetString(name, "user-decides")
 	if err != nil {
-		return showChoiceByPolicy, err
+		return setting.ShowChoiceByPolicy, err
 	}
-	switch opt {
-	case "always":
-		return alwaysByPolicy, nil
-	case "never":
-		return neverByPolicy, nil
-	default:
-		return showChoiceByPolicy, nil
-	}
-}
-
-// Visibility is a policy that controls whether or not a particular
-// component of a user interface is to be shown.
-type Visibility byte
-
-const (
-	visibleByPolicy Visibility = 'v'
-	hiddenByPolicy  Visibility = 'h'
-)
-
-// Show reports whether the UI option administered by this policy should be shown.
-// Currently this is true if and only if the policy is visibleByPolicy.
-func (p Visibility) Show() bool {
-	return p == visibleByPolicy
+	var opt setting.PreferenceOption
+	err = opt.UnmarshalText([]byte(s))
+	return opt, err
 }
 
 // GetVisibility loads a policy from the registry that can be managed
@@ -124,17 +68,14 @@ func (p Visibility) Show() bool {
 // for UI elements. The registry value should be a string set to "show" (return
 // true) or "hide" (return true). If not present or set to a different value,
 // "show" (return false) is the default.
-func GetVisibility(name Key) (Visibility, error) {
-	opt, err := GetString(name, "show")
+func GetVisibility(name Key) (setting.Visibility, error) {
+	s, err := GetString(name, "show")
 	if err != nil {
-		return visibleByPolicy, err
-	}
-	switch opt {
-	case "hide":
-		return hiddenByPolicy, nil
-	default:
-		return visibleByPolicy, nil
+		return setting.VisibleByPolicy, err
 	}
+	var visibility setting.Visibility
+	visibility.UnmarshalText([]byte(s))
+	return visibility, nil
 }
 
 // GetDuration loads a policy from the registry that can be managed