priv: a failed root probe is retried, not cached for the life of the daemon

The Mac mini's daemon started while the machine was still booting, sudo -n true
took longer than the 3 s timeout, and sync.Once kept the answer: no root, so the
next self-update could not replace the binary.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/priv/priv.go

@@ -13,12 +13,24 @@ import (
 )
 
 var (
-	once   sync.Once
-	mode   string // "root" | "sudo" | "doas" | ""
-	prefix []string
+	mu      sync.Mutex
+	mode    string // "root" | "sudo" | "doas" | ""
+	prefix  []string
+	probed  time.Time
+	reprobe = 30 * time.Second
 )
 
+// detect settles how root is reached. Finding a way is remembered for good; finding none
+// is only believed for a while, because the probe can fail for reasons that pass — sudo
+// taking longer than the timeout on a machine that is still booting left a daemon
+// convinced it had no root until its next restart.
 func detect() {
+	mu.Lock()
+	defer mu.Unlock()
+	if mode != "" || (!probed.IsZero() && time.Since(probed) < reprobe) {
+		return
+	}
+	probed = time.Now()
 	if os.Geteuid() == 0 {
 		mode = "root"
 		return
@@ -38,7 +50,7 @@ func detect() {
 }
 
 // Mode reports how root is reached: "root", "sudo", "doas", or "" when it can't be.
-func Mode() string { once.Do(detect); return mode }
+func Mode() string { detect(); mu.Lock(); defer mu.Unlock(); return mode }
 
 // Available reports whether commands can be run as root without a prompt.
 func Available() bool { return Mode() != "" }
@@ -46,7 +58,9 @@ func Available() bool { return Mode() != "" }
 // Wrap returns args prefixed so they run as root. Without privilege it returns args
 // unchanged; callers gate on Available() where that matters.
 func Wrap(args ...string) []string {
-	once.Do(detect)
+	detect()
+	mu.Lock()
+	defer mu.Unlock()
 	return append(append([]string{}, prefix...), args...)
 }