clientupdate: return true for CanAutoUpdate for macsys (#12746)

While `clientupdate.Updater` won't be able to apply updates on macsys,
we use `clientupdate.CanAutoUpdate` to gate the EditPrefs endpoint in
localAPI. We should allow the GUI client to set AutoUpdate.Apply on
macsys for it to properly get reported to the control plane. This also
allows the tailnet-wide default for auto-updates to propagate to macsys
clients.

Updates https://github.com/tailscale/corp/issues/21339

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

ipn/ipnlocal/autoupdate.go

@@ -11,6 +11,7 @@ import (
 
 	"tailscale.com/clientupdate"
 	"tailscale.com/ipn"
+	"tailscale.com/version"
 )
 
 func (b *LocalBackend) stopOfflineAutoUpdate() {
@@ -30,6 +31,10 @@ func (b *LocalBackend) maybeStartOfflineAutoUpdate(prefs ipn.PrefsView) {
 	if !clientupdate.CanAutoUpdate() {
 		return
 	}
+	// On macsys, auto-updates are managed by Sparkle.
+	if version.IsMacSysExt() {
+		return
+	}
 
 	if b.offlineAutoUpdateCancel != nil {
 		// Already running.

ipn/ipnlocal/c2n.go

@@ -355,7 +355,7 @@ func (b *LocalBackend) newC2NUpdateResponse() tailcfg.C2NUpdateResponse {
 	prefs := b.Prefs().AutoUpdate()
 	return tailcfg.C2NUpdateResponse{
 		Enabled:   envknob.AllowsRemoteUpdate() || prefs.Apply.EqualBool(true),
-		Supported: clientupdate.CanAutoUpdate(),
+		Supported: clientupdate.CanAutoUpdate() && !version.IsMacSysExt(),
 	}
 }