Go SDK: reject a task function whose last result type is not error

go-sdk/airflow/dag_test.go

@@ -316,6 +316,11 @@ func TestTaskPanicsOnBadFunction(t *testing.T) {
 			fn:   func(context.Context) error { return nil },
 			want: "parameter 0 is context.Context, but the first parameter must be airflow.Context",
 		},
+		{
+			name: "error result of a concrete type",
+			fn:   func(Context) (int, *taskError) { return 0, nil },
+			want: "must declare its last result as error, not *airflow.taskError",
+		},
 		{
 			// The name of a method expression keeps a dot, as a function literal's name does.
 			// This row checks that the signature check rejects the method expression for its

go-sdk/airflow/task_handler_test.go

@@ -85,6 +85,10 @@ func panicMessage(t *testing.T, f func()) (msg string) {
 	return ""
 }
 
+type taskError struct{}
+
+func (*taskError) Error() string { return "task error" }
+
 func TestTaskHandlerPanicsOnBadHandler(t *testing.T) {
 	var unassigned func(Context) error
 
@@ -115,6 +119,11 @@ func TestTaskHandlerPanicsOnBadHandler(t *testing.T) {
 			fn:   func(Context) int { return 0 },
 			want: "last return value to return error but found int",
 		},
+		{
+			name: "error result of a concrete type",
+			fn:   func(Context) (int, *taskError) { return 0, nil },
+			want: "must declare its last result as error, not *airflow.taskError",
+		},
 		{
 			name: "parameter that cannot hold an argument",
 			fn:   func(Context, chan int) error { return nil },

go-sdk/internal/bundle/task.go

@@ -195,11 +195,22 @@ func (f *taskFunction) validateFn(
 			fnType.Out(0).Kind(),
 		)
 	}
-	if !isError(fnType.Out(fnType.NumOut() - 1)) {
+	last := fnType.Out(fnType.NumOut() - 1)
+	if !isError(last) {
 		return fmt.Errorf(
 			"expected task function %s last return value to return error but found %v",
 			f.fullName,
-			fnType.Out(fnType.NumOut()-1).Kind(),
+			last.Kind(),
+		)
+	}
+	// The last result must be error itself, not just a type that implements error. A nil *MyErr,
+	// for example, becomes a non-nil error when call converts it. The task would then fail even
+	// though the function returned nil.
+	if last != errorType {
+		return fmt.Errorf(
+			"task function %s must declare its last result as error, not %s",
+			f.fullName,
+			last,
 		)
 	}
 

go-sdk/internal/bundle/task_test.go

@@ -49,6 +49,19 @@ func TestTaskSuite(t *testing.T) {
 	suite.Run(t, &TaskSuite{})
 }
 
+type taskError struct{}
+
+func (*taskError) Error() string { return "task error" }
+
+type errno int
+
+func (errno) Error() string { return "errno" }
+
+type codedError interface {
+	error
+	Code() int
+}
+
 func (s *TaskSuite) TestReturnValidation() {
 	cases := map[string]struct {
 		fn          any
@@ -66,6 +79,18 @@ func (s *TaskSuite) TestReturnValidation() {
 			func(contexttest.Context) (c chan int) { return },
 			`func\d+ last return value to return error but found chan`,
 		},
+		"pointer-error-ret": {
+			func(contexttest.Context) (int, *taskError) { return 0, nil },
+			`func\d+ must declare its last result as error, not \*bundle\.taskError$`,
+		},
+		"value-error-ret": {
+			func(contexttest.Context) errno { return 0 },
+			`func\d+ must declare its last result as error, not bundle\.errno$`,
+		},
+		"interface-error-ret": {
+			func(contexttest.Context) (int, codedError) { return 0, nil },
+			`func\d+ must declare its last result as error, not bundle\.codedError$`,
+		},
 	}
 
 	for name, tt := range cases {