cmd/headscale: fix JSON output of value-typed API objects

Generated API types have a pointer-receiver MarshalJSON that omits unset
optional fields. The CLI marshalled them by value, so stdlib reflection
called Opt*.MarshalJSON directly and failed with "unexpected end of JSON
input", breaking every -o json/-o yaml command. Make values addressable
first so the generated marshaler runs.

cmd/headscale/cli/utils.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"reflect"
 	"slices"
 
 	"github.com/juanfont/headscale/hscontrol"
@@ -58,9 +59,37 @@ func newHeadscaleServerWithConfig() (*hscontrol.Headscale, error) {
 	return app, nil
 }
 
+// addressableForJSON makes value-typed generated API structs (and slice
+// elements) addressable so their pointer-receiver MarshalJSON is used. That
+// method omits unset optional fields; stdlib reflection over a value instead
+// calls Opt*.MarshalJSON directly, which returns empty for an unset field and
+// fails with "unexpected end of JSON input".
+func addressableForJSON(v any) any {
+	rv := reflect.ValueOf(v)
+
+	switch rv.Kind() { //nolint:exhaustive // default handles all other kinds
+	case reflect.Slice, reflect.Array:
+		out := make([]any, rv.Len())
+		for i := range out {
+			out[i] = addressableForJSON(rv.Index(i).Interface())
+		}
+
+		return out
+	case reflect.Struct:
+		p := reflect.New(rv.Type())
+		p.Elem().Set(rv)
+
+		return p.Interface()
+	default:
+		return v
+	}
+}
+
 // formatOutput serialises result into the requested format. For the
 // default (empty) format the human-readable override string is returned.
 func formatOutput(result any, override string, outputFormat string) (string, error) {
+	result = addressableForJSON(result)
+
 	switch outputFormat {
 	case outputFormatJSON:
 		b, err := json.MarshalIndent(result, "", "\t")

cmd/headscale/cli/utils_test.go

@@ -0,0 +1,48 @@
+package cli
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+
+	apiv1 "github.com/juanfont/headscale/gen/api/v1"
+)
+
+// TestFormatOutputValueWithUnsetOptional guards against a regression where the
+// generated API types, passed by value with unset optional fields, fail to
+// marshal because stdlib reflection calls Opt*.MarshalJSON directly.
+func TestFormatOutputValueWithUnsetOptional(t *testing.T) {
+	// DisplayName, Email, CreatedAt and PictureURL are intentionally unset.
+	user := apiv1.User{
+		ID:   apiv1.NewOptUint64(1),
+		Name: apiv1.NewOptString("alice"),
+	}
+
+	for _, format := range []string{outputFormatJSON, outputFormatJSONLine, outputFormatYAML} {
+		out, err := formatOutput(user, "", format)
+		if err != nil {
+			t.Fatalf("formatOutput(%s) on value type: %v", format, err)
+		}
+
+		if !strings.Contains(out, "alice") {
+			t.Errorf("formatOutput(%s) missing name: %q", format, out)
+		}
+	}
+
+	// A slice of value types must work too (e.g. "users list").
+	listOut, err := formatOutput([]apiv1.User{user}, "", outputFormatJSON)
+	if err != nil {
+		t.Fatalf("formatOutput(json) on slice of value type: %v", err)
+	}
+
+	var decoded []map[string]any
+
+	err = json.Unmarshal([]byte(listOut), &decoded)
+	if err != nil {
+		t.Fatalf("list output is not valid JSON: %v\n%s", err, listOut)
+	}
+
+	if len(decoded) != 1 || decoded[0]["name"] != "alice" {
+		t.Errorf("unexpected list output: %s", listOut)
+	}
+}