util/eventbus: allow test expectations reporting only an error (#17146)

Extend the Expect method of a Watcher to allow filter functions that report
only an error value, and which "pass" when the reported error is nil.

Updates #15160

Change-Id: I582d804554bd1066a9e499c1f3992d068c9e8148
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

util/eventbus/eventbustest/eventbustest.go

@@ -79,6 +79,11 @@ func Type[T any]() func(T) { return func(T) {} }
 //	// The if error != nil, the test helper will return that error immediately.
 //	func(e ExpectedType) (bool, error)
 //
+//	// Tests for event type and whatever is defined in the body.
+//	// If a non-nil error is reported, the test helper will return that error
+//	// immediately; otherwise the expectation is considered to be met.
+//	func(e ExpectedType) error
+//
 // If the list of events must match exactly with no extra events,
 // use [ExpectExactly].
 func Expect(tw *Watcher, filters ...any) error {
@@ -179,15 +184,22 @@ func eventFilter(f any) filter {
 			return []reflect.Value{reflect.ValueOf(true), reflect.Zero(reflect.TypeFor[error]())}
 		}
 	case 1:
-		if ft.Out(0) != reflect.TypeFor[bool]() {
-			panic(fmt.Sprintf("result is %T, want bool", ft.Out(0)))
-		}
-		fixup = func(vals []reflect.Value) []reflect.Value {
-			return append(vals, reflect.Zero(reflect.TypeFor[error]()))
+		switch ft.Out(0) {
+		case reflect.TypeFor[bool]():
+			fixup = func(vals []reflect.Value) []reflect.Value {
+				return append(vals, reflect.Zero(reflect.TypeFor[error]()))
+			}
+		case reflect.TypeFor[error]():
+			fixup = func(vals []reflect.Value) []reflect.Value {
+				pass := vals[0].IsZero()
+				return append([]reflect.Value{reflect.ValueOf(pass)}, vals...)
+			}
+		default:
+			panic(fmt.Sprintf("result is %v, want bool or error", ft.Out(0)))
 		}
 	case 2:
 		if ft.Out(0) != reflect.TypeFor[bool]() || ft.Out(1) != reflect.TypeFor[error]() {
-			panic(fmt.Sprintf("results are %T, %T; want bool, error", ft.Out(0), ft.Out(1)))
+			panic(fmt.Sprintf("results are %v, %v; want bool, error", ft.Out(0), ft.Out(1)))
 		}
 		fixup = func(vals []reflect.Value) []reflect.Value { return vals }
 	default:

util/eventbus/eventbustest/eventbustest_test.go

@@ -54,6 +54,27 @@ func TestExpectFilter(t *testing.T) {
 			},
 			wantErr: false,
 		},
+		{
+			name:   "filter-with-nil-error",
+			events: []int{1, 2, 3},
+			expectFunc: func(event EventFoo) error {
+				if event.Value > 10 {
+					return fmt.Errorf("value > 10: %d", event.Value)
+				}
+				return nil
+			},
+		},
+		{
+			name:   "filter-with-non-nil-error",
+			events: []int{100, 200, 300},
+			expectFunc: func(event EventFoo) error {
+				if event.Value > 10 {
+					return fmt.Errorf("value > 10: %d", event.Value)
+				}
+				return nil
+			},
+			wantErr: true,
+		},
 		{
 			name:   "first event has to be func",
 			events: []int{24, 42},