Dart http: "Bad state: Can't finalize a finalized Request" when retrying a http.Request after fetching a new access token

Question

I'm currently trying to access a Web API in Flutter that requires a JWT access token for authorization. The access token expires after a certain amount of time.

A new access token can be requested with a separate refresh token. Right now this access token refresh is performed as soon as a request returns a 401 response. After that, the failed request should be retried with the new access token.

I'm having trouble with this last step. It seems like a http.BaseRequest can only be sent once. How would I retry the http request with the new token?

---

As suggested in the dart http readme, I created a subclass of http.BaseClient to add the authorization behavior. Here is a simplified version:

import 'dart:async';

import 'package:http/http.dart' as http;

class AuthorizedClient extends http.BaseClient {
  AuthorizedClient(this._authService) : _inner = http.Client();

  final http.Client _inner;
  final AuthService _authService;

  Future<http.StreamedResponse> send(http.BaseRequest request) async {
    final token = await _authService.getAccessToken();
    request.headers['Authorization'] = 'Bearer $token';

    final response = await _inner.send(request);

    if (response.statusCode == 401) {
      final newToken = await _authService.refreshAccessToken();
      request.headers['Authorization'] = 'Bearer $newToken';

      // throws error: Bad state: Can't finalize a finalized Request
      final retryResponse = await _inner.send(request);

      return retryResponse;
    }

    return response;
  }
}

abstract class AuthService {
  Future<String> getAccessToken();
  Future<String> refreshAccessToken();
}

Answer 1

Here is what I came up with so far, based on Richard Heap's answer: To resend a request, we have to copy it.

So far I was not able to come up for a solution for stream requests!

http.BaseRequest _copyRequest(http.BaseRequest request) {
  http.BaseRequest requestCopy;

  if(request is http.Request) {
    requestCopy = http.Request(request.method, request.url)
      ..encoding = request.encoding
      ..bodyBytes = request.bodyBytes;
  }
  else if(request is http.MultipartRequest) {
    requestCopy = http.MultipartRequest(request.method, request.url)
      ..fields.addAll(request.fields)
      ..files.addAll(request.files);
  }
  else if(request is http.StreamedRequest) {
    throw Exception('copying streamed requests is not supported');
  }
  else {
    throw Exception('request type is unknown, cannot copy');
  }

  requestCopy
    ..persistentConnection = request.persistentConnection
    ..followRedirects = request.followRedirects
    ..maxRedirects = request.maxRedirects
    ..headers.addAll(request.headers);

  return requestCopy;
}