control/controlknobs: make Knobs.AsDebugJSON automatic, not require maintenance

The AsDebugJSON method (used only for a LocalAPI debug call) always
needed to be updated whenever a new controlknob was added. We had a
test for it, which was nice, but it was a tedious step we don't need
to do. Use reflect instead.

Updates #14788

Change-Id: If59cd776920f3ce7c748f86ed2eddd9323039a0b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlknobs/controlknobs.go

@@ -6,6 +6,8 @@
 package controlknobs
 
 import (
+	"fmt"
+	"reflect"
 	"sync/atomic"
 
 	"tailscale.com/syncs"
@@ -174,26 +176,19 @@ func (k *Knobs) AsDebugJSON() map[string]any {
 	if k == nil {
 		return nil
 	}
-	return map[string]any{
-		"DisableUPnP":                          k.DisableUPnP.Load(),
-		"KeepFullWGConfig":                     k.KeepFullWGConfig.Load(),
-		"RandomizeClientPort":                  k.RandomizeClientPort.Load(),
-		"OneCGNAT":                             k.OneCGNAT.Load(),
-		"ForceBackgroundSTUN":                  k.ForceBackgroundSTUN.Load(),
-		"DisableDeltaUpdates":                  k.DisableDeltaUpdates.Load(),
-		"PeerMTUEnable":                        k.PeerMTUEnable.Load(),
-		"DisableDNSForwarderTCPRetries":        k.DisableDNSForwarderTCPRetries.Load(),
-		"SilentDisco":                          k.SilentDisco.Load(),
-		"LinuxForceIPTables":                   k.LinuxForceIPTables.Load(),
-		"LinuxForceNfTables":                   k.LinuxForceNfTables.Load(),
-		"SeamlessKeyRenewal":                   k.SeamlessKeyRenewal.Load(),
-		"ProbeUDPLifetime":                     k.ProbeUDPLifetime.Load(),
-		"AppCStoreRoutes":                      k.AppCStoreRoutes.Load(),
-		"UserDialUseRoutes":                    k.UserDialUseRoutes.Load(),
-		"DisableSplitDNSWhenNoCustomResolvers": k.DisableSplitDNSWhenNoCustomResolvers.Load(),
-		"DisableLocalDNSOverrideViaNRPT":       k.DisableLocalDNSOverrideViaNRPT.Load(),
-		"DisableCryptorouting":                 k.DisableCryptorouting.Load(),
-		"DisableCaptivePortalDetection":        k.DisableCaptivePortalDetection.Load(),
-		"DisableSkipStatusQueue":               k.DisableSkipStatusQueue.Load(),
+	ret := map[string]any{}
+	rt := reflect.TypeFor[Knobs]()
+	rv := reflect.ValueOf(k).Elem() // of *k
+	for i := 0; i < rt.NumField(); i++ {
+		name := rt.Field(i).Name
+		switch v := rv.Field(i).Addr().Interface().(type) {
+		case *atomic.Bool:
+			ret[name] = v.Load()
+		case *syncs.AtomicValue[opt.Bool]:
+			ret[name] = v.Load()
+		default:
+			panic(fmt.Sprintf("unknown field type %T for %v", v, name))
+		}
 	}
+	return ret
 }

control/controlknobs/controlknobs_test.go

@@ -6,6 +6,8 @@ package controlknobs
 import (
 	"reflect"
 	"testing"
+
+	"tailscale.com/types/logger"
 )
 
 func TestAsDebugJSON(t *testing.T) {
@@ -18,4 +20,5 @@ func TestAsDebugJSON(t *testing.T) {
 	if want := reflect.TypeFor[Knobs]().NumField(); len(got) != want {
 		t.Errorf("AsDebugJSON map has %d fields; want %v", len(got), want)
 	}
+	t.Logf("Got: %v", logger.AsJSON(got))
 }