ipn: apply tailnet-wide default for auto-updates (#10508)

When auto-update setting in local Prefs is unset, apply the tailnet
default value from control. This only happens once, when we apply the
default (or when the user manually overrides it), tailnet default no
longer affects the node.

Updates #16244

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

ipn/prefs.go

@@ -21,6 +21,7 @@ import (
 	"tailscale.com/net/netaddr"
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/tailcfg"
+	"tailscale.com/types/opt"
 	"tailscale.com/types/persist"
 	"tailscale.com/types/preftype"
 	"tailscale.com/types/views"
@@ -237,7 +238,17 @@ type AutoUpdatePrefs struct {
 	// Apply specifies whether background auto-updates are enabled. When
 	// enabled, tailscaled will apply available updates in the background.
 	// Check must also be set when Apply is set.
-	Apply bool
+	Apply opt.Bool
+}
+
+func (au1 AutoUpdatePrefs) Equals(au2 AutoUpdatePrefs) bool {
+	// This could almost be as easy as `au1.Apply == au2.Apply`, except that
+	// opt.Bool("") and opt.Bool("unset") should be treated as equal.
+	apply1, ok1 := au1.Apply.Get()
+	apply2, ok2 := au2.Apply.Get()
+	return au1.Check == au2.Check &&
+		apply1 == apply2 &&
+		ok1 == ok2
 }
 
 // AppConnectorPrefs are the app connector settings for the node agent.
@@ -533,14 +544,14 @@ func (p *Prefs) Equals(p2 *Prefs) bool {
 		compareStrings(p.AdvertiseTags, p2.AdvertiseTags) &&
 		p.Persist.Equals(p2.Persist) &&
 		p.ProfileName == p2.ProfileName &&
-		p.AutoUpdate == p2.AutoUpdate &&
+		p.AutoUpdate.Equals(p2.AutoUpdate) &&
 		p.AppConnector == p2.AppConnector &&
 		p.PostureChecking == p2.PostureChecking &&
 		p.NetfilterKind == p2.NetfilterKind
 }
 
 func (au AutoUpdatePrefs) Pretty() string {
-	if au.Apply {
+	if au.Apply.EqualBool(true) {
 		return "update=on "
 	}
 	if au.Check {
@@ -600,7 +611,7 @@ func NewPrefs() *Prefs {
 		NetfilterMode:    preftype.NetfilterOn,
 		AutoUpdate: AutoUpdatePrefs{
 			Check: true,
-			Apply: false,
+			Apply: opt.Bool("unset"),
 		},
 	}
 }