internal/router: make router.Cleanup idempotent

On routers where we want to wait for NTP by checking nvram key. Before
waiting, we clean up the router to ensure it's restored to original
state. However, router.Cleanup is not idempotent, causing dnsmasq
restarted. On tomato/ddwrt, restarting have no delay, and spawning new
dnsmasq process immediately. On merlin, somehow it takes time to spawn
new dnsmasq process, causing ctrld wrongly think there's no one
listening on port 53.

Fixing this by ensuring router.Cleanup is idempotent. While at it, also
adding "ntp_done" to nvram key, which is now using on latest ddwrt.

cmd/ctrld/os_linux.go

@@ -24,6 +24,8 @@ import (
 	"github.com/Control-D-Inc/ctrld/internal/resolvconffile"
 )
 
+const resolvConfBackupFailedMsg = "open /etc/resolv.pre-ctrld-backup.conf: read-only file system"
+
 // allocate loopback ip
 // sudo ip a add 127.0.0.2/24 dev lo
 func allocateIP(ip string) error {
@@ -73,6 +75,14 @@ func setDNS(iface *net.Interface, nameservers []string) error {
 				trySystemdResolve = true
 				break
 			}
+			// This error happens on read-only file system, which causes ctrld failed to create backup
+			// for /etc/resolv.conf file. It is ok, because the DNS is still set anyway, and restore
+			// DNS will fallback to use DHCP if there's no backup /etc/resolv.conf file.
+			// The error format is controlled by us, so checking for error string is fine.
+			// See: ../../internal/dns/direct.go:L278
+			if r.Mode() == "direct" && strings.Contains(err.Error(), resolvConfBackupFailedMsg) {
+				return nil
+			}
 			return err
 		}
 		currentNS := currentDNS(iface)

internal/router/ddwrt/ddwrt.go

@@ -87,12 +87,14 @@ func (d *Ddwrt) Cleanup() error {
 	if d.cfg.FirstListener().IsDirectDnsListener() {
 		return nil
 	}
-	if val, _ := nvram.Run("get", nvram.CtrldSetupKey); val == "1" {
+	if val, _ := nvram.Run("get", nvram.CtrldSetupKey); val != "1" {
-		nvramKvMap["dnsmasq_options"] = ""
+		return nil // was restored, nothing to do.
-		// Restore old configs.
+	}
-		if err := nvram.Restore(nvramKvMap, nvram.CtrldSetupKey); err != nil {
+
-			return err
+	nvramKvMap["dnsmasq_options"] = ""
-		}
+	// Restore old configs.
+	if err := nvram.Restore(nvramKvMap, nvram.CtrldSetupKey); err != nil {
+		return err
 	}
 
 	// Restart dnsmasq service.

internal/router/merlin/merlin.go

@@ -52,6 +52,13 @@ func (m *Merlin) Setup() error {
 	if m.cfg.FirstListener().IsDirectDnsListener() {
 		return nil
 	}
+	// Already setup.
+	if val, _ := nvram.Run("get", nvram.CtrldSetupKey); val == "1" {
+		return nil
+	}
+	if _, err := nvram.Run("set", nvram.CtrldSetupKey+"=1"); err != nil {
+		return err
+	}
 	buf, err := os.ReadFile(dnsmasq.MerlinPostConfPath)
 	// Already setup.
 	if bytes.Contains(buf, []byte(dnsmasq.MerlinPostConfMarker)) {
@@ -92,11 +99,13 @@ func (m *Merlin) Cleanup() error {
 	if m.cfg.FirstListener().IsDirectDnsListener() {
 		return nil
 	}
-	if val, _ := nvram.Run("get", nvram.CtrldSetupKey); val == "1" {
+	if val, _ := nvram.Run("get", nvram.CtrldSetupKey); val != "1" {
-		// Restore old configs.
+		return nil // was restored, nothing to do.
-		if err := nvram.Restore(nvramKvMap, nvram.CtrldSetupKey); err != nil {
+	}
-			return err
+
-		}
+	// Restore old configs.
+	if err := nvram.Restore(nvramKvMap, nvram.CtrldSetupKey); err != nil {
+		return err
 	}
 
 	buf, err := os.ReadFile(dnsmasq.MerlinPostConfPath)

internal/router/ntp/ntp.go

@@ -18,12 +18,15 @@ func WaitNvram() error {
 	// Wait until `ntp_ready=1` set.
 	b := backoff.NewBackoff("ntp.Wait", func(format string, args ...any) {}, 10*time.Second)
 	for {
-		out, err := nvram.Run("get", "ntp_ready")
+		// ddwrt use "ntp_done": https://github.com/mirror/dd-wrt/blob/a08c693527ab3204bf7bebd408a7c9a83b6ede47/src/router/rc/ntp.c#L100
-		if err != nil {
+		for _, key := range []string{"ntp_ready", "ntp_done"} {
-			return fmt.Errorf("PreStart: nvram: %w", err)
+			out, err := nvram.Run("get", key)
-		}
+			if err != nil {
-		if out == "1" {
+				return fmt.Errorf("PreStart: nvram: %w", err)
-			return nil
+			}
+			if out == "1" {
+				return nil
+			}
 		}
 		b.BackOff(context.Background(), errors.New("ntp not ready"))
 	}

internal/router/tomato/tomato.go

@@ -89,12 +89,14 @@ func (f *FreshTomato) Cleanup() error {
 	if f.cfg.FirstListener().IsDirectDnsListener() {
 		return nil
 	}
-	if val, _ := nvram.Run("get", nvram.CtrldSetupKey); val == "1" {
+	if val, _ := nvram.Run("get", nvram.CtrldSetupKey); val != "1" {
-		nvramKvMap["dnsmasq_custom"] = ""
+		return nil // was restored, nothing to do.
-		// Restore old configs.
+	}
-		if err := nvram.Restore(nvramKvMap, nvram.CtrldSetupKey); err != nil {
+
-			return err
+	nvramKvMap["dnsmasq_custom"] = ""
-		}
+	// Restore old configs.
+	if err := nvram.Restore(nvramKvMap, nvram.CtrldSetupKey); err != nil {
+		return err
 	}
 
 	// Restart dnscrypt-proxy service.