cmd/utils: disallow --lightserv in light mode (ethereum#15514)

* Disallow --lightserv in light mode

* Reformatted

* cmd/utils: reduce nesting levels a bit

cmd/utils/flags.go

@@ -929,15 +929,41 @@ func setEthash(ctx *cli.Context, cfg *eth.Config) {
 	}
 }
 
-func checkExclusive(ctx *cli.Context, flags ...cli.Flag) {
+// checkExclusive verifies that only a single isntance of the provided flags was
+// set by the user. Each flag might optionally be followed by a string type to
+// specialize it further.
+func checkExclusive(ctx *cli.Context, args ...interface{}) {
 	set := make([]string, 0, 1)
-	for _, flag := range flags {
+	for i := 0; i < len(args); i++ {
+		// Make sure the next argument is a flag and skip if not set
+		flag, ok := args[i].(cli.Flag)
+		if !ok {
+			panic(fmt.Sprintf("invalid argument, not cli.Flag type: %T", args[i]))
+		}
+		// Check if next arg extends current and expand its name if so
+		name := flag.GetName()
+
+		if i+1 < len(args) {
+			switch option := args[i+1].(type) {
+			case string:
+				// Extended flag, expand the name and shift the arguments
+				if ctx.GlobalString(flag.GetName()) == option {
+					name += "=" + option
+				}
+				i++
+
+			case cli.Flag:
+			default:
+				panic(fmt.Sprintf("invalid argument, not cli.Flag or string extension: %T", args[i+1]))
+			}
+		}
+		// Mark the flag if it's set
 		if ctx.GlobalIsSet(flag.GetName()) {
-			set = append(set, "--"+flag.GetName())
+			set = append(set, "--"+name)
 		}
 	}
 	if len(set) > 1 {
-		Fatalf("flags %v can't be used at the same time", strings.Join(set, ", "))
+		Fatalf("Flags %v can't be used at the same time", strings.Join(set, ", "))
 	}
 }
 
@@ -956,6 +982,8 @@ func SetEthConfig(ctx *cli.Context, stack *node.Node, cfg *eth.Config) {
 	// Avoid conflicting network flags
 	checkExclusive(ctx, DeveloperFlag, TestnetFlag, RinkebyFlag)
 	checkExclusive(ctx, FastSyncFlag, LightModeFlag, SyncModeFlag)
+	checkExclusive(ctx, LightServFlag, LightModeFlag)
+	checkExclusive(ctx, LightServFlag, SyncModeFlag, "light")
 
 	ks := stack.AccountManager().Backends(keystore.KeyStoreType)[0].(*keystore.KeyStore)
 	setEtherbase(ctx, ks, cfg)