clientupdate: cache CanAutoUpdate, avoid log spam when false

I noticed logs on one of my machines where it can't auto-update with
scary log spam about "failed to apply tailnet-wide default for
auto-updates".

This avoids trying to do the EditPrefs if we know it's just going to
fail anyway.

Updates #282

Change-Id: Ib7db3b122185faa70efe08b60ebd05a6094eed8c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

clientupdate/clientupdate.go

@@ -28,6 +28,7 @@ import (
 	"strings"
 
 	"tailscale.com/hostinfo"
+	"tailscale.com/types/lazy"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/cmpver"
 	"tailscale.com/version"
@@ -249,9 +250,13 @@ func (up *Updater) getUpdateFunction() (fn updateFunction, canAutoUpdate bool) {
 	return nil, false
 }
 
+var canAutoUpdateCache lazy.SyncValue[bool]
+
 // CanAutoUpdate reports whether auto-updating via the clientupdate package
 // is supported for the current os/distro.
-func CanAutoUpdate() bool {
+func CanAutoUpdate() bool { return canAutoUpdateCache.Get(canAutoUpdateUncached) }
+
+func canAutoUpdateUncached() bool {
 	if version.IsMacSysExt() {
 		// Macsys uses Sparkle for auto-updates, which doesn't have an update
 		// function in this package.

ipn/ipnlocal/local.go

@@ -3479,18 +3479,20 @@ func (b *LocalBackend) onTailnetDefaultAutoUpdate(au bool) {
 		// can still manually enable auto-updates on this node.
 		return
 	}
-	b.logf("using tailnet default auto-update setting: %v", au)
-	prefsClone := prefs.AsStruct()
-	prefsClone.AutoUpdate.Apply = opt.NewBool(au)
-	_, err := b.editPrefsLockedOnEntry(&ipn.MaskedPrefs{
-		Prefs: *prefsClone,
-		AutoUpdateSet: ipn.AutoUpdatePrefsMask{
-			ApplySet: true,
-		},
-	}, unlock)
-	if err != nil {
-		b.logf("failed to apply tailnet-wide default for auto-updates (%v): %v", au, err)
-		return
+	if clientupdate.CanAutoUpdate() {
+		b.logf("using tailnet default auto-update setting: %v", au)
+		prefsClone := prefs.AsStruct()
+		prefsClone.AutoUpdate.Apply = opt.NewBool(au)
+		_, err := b.editPrefsLockedOnEntry(&ipn.MaskedPrefs{
+			Prefs: *prefsClone,
+			AutoUpdateSet: ipn.AutoUpdatePrefsMask{
+				ApplySet: true,
+			},
+		}, unlock)
+		if err != nil {
+			b.logf("failed to apply tailnet-wide default for auto-updates (%v): %v", au, err)
+			return
+		}
 	}
 }