Interceptor onError handling broken

[fischermario]

Package

dio

Version

5.4.2

Operating-System

Android

Adapter

Default Dio

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Dart Version

3.3.2

Steps to Reproduce

The onError handler of a new class that extends the "Interceptor" class will not allow subsequent interceptors to successfully catch exceptions.

Please consider the following example which is derived from this example:

class DioCustomInterceptor extends Interceptor {
  @override
  void onError(
    DioException err,
    ErrorInterceptorHandler handler,
  ) async {
    debugPrint('onError');

    super.onError(err, handler);
  }
}

// ...

Dio dio = Dio(BaseOptions(
  receiveTimeout: const Duration(milliseconds: 2000),
  connectTimeout: const Duration(milliseconds: 2000),
  sendTimeout: const Duration(milliseconds: 2000),
  receiveDataWhenStatusError: true,
));
dio.interceptors.addAll([
  DioCustomInterceptor(),
  InterceptorsWrapper(
    onError: (err, handler) {
      debugPrint('CAUGHT error: $err');
      handler.resolve(
        Response(
          requestOptions: err.requestOptions,
          statusCode: 500,
        ),
      );
    },
  ),
]);
Response resp = await dio.get('https://httpbin.org/delay/10');
debugPrint(resp.statusCode.toString());

Expected Result

I/flutter (xx): onError
I/flutter (xx): CAUGHT error: DioException [receive timeout]: The request took longer than 0:00:02.000000 to receive data. It was aborted. To get rid of this exception, try raising the RequestOptions.receiveTimeout above the duration of 0:00:02.000000 or improve the response time of the server.
I/flutter (xx): 500

Actual Result

The debugger halts the execution after the "onError" output. Forcing the debugger to continue yields the following error:

E/flutter (xx): [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: Instance of 'InterceptorState<DioException>'
E/flutter (xx): #0      DioMixin.request (package:dio/src/dio_mixin.dart:350:36)
E/flutter (xx): #1      DioMixin.get (package:dio/src/dio_mixin.dart:64:12)
...
E/flutter (xx): 
I/flutter (xx): CAUGHT error: DioException [receive timeout]: The request took longer than 0:00:02.000000 to receive data. It was aborted. To get rid of this exception, try raising the RequestOptions.receiveTimeout above the duration of 0:00:02.000000 or improve the response time of the server.
I/flutter (xx): 500

This means the onError handler of the second interceptor is acutally executed, but it is not able not catch the exception properly.
If we now replace

class DioCustomInterceptor extends Interceptor {

with

class DioCustomInterceptor extends QueuedInterceptor {

everything works as expected.
Using inline InterceptorsWrapper instead of a custom class also prevents this problem from appearing.

I guess that there is a simple explanation for this behavior, but I could not find it in the docs.

For example the dio_cache_interceptor package is implemented by extending the "Interceptor" class which triggers the described behavior that breaks my project.

[kuhnroyal]

The cause is #2139

In your example you can just remove the async keyword, or actually return an uncompleted Future.
Not sure how/if that works for other interceptors...

[AlexV525]

Generally we should avoid async on void, as the lint rule: https://dart.dev/tools/linter-rules/avoid_void_async

We can figure out what we can do here for better experiences.

[AlexV525]

It actually breaks all interceptors written in void async or Future<void> async.