Dart 1.20.1: malformed type cannot resolve class 'Future'

[evanweible-wf]

I discovered this issue via the w_transport package when upgrading from Dart 1.19.1 to 1.20.1. All tests pass on 1.19.1, and then without any changes other than upgrading, there are a few tests that fail with the following error:

'package:w_transport/src/http/browser/requests.dart': malformed type: line 55 pos 1: cannot resolve class 'Future' from 'CommonPlainTextRequest&BrowserRequestMixin'

      ^

      dart:html                                                      HttpRequest.abort
      package:w_transport/src/http/browser/request_mixin.dart 32:15  CommonPlainTextRequest&BrowserRequestMixin.abortRequest
      package:w_transport/src/http/common/request.dart 332:5         CommonRequest.abort
      http/common_request/suite.dart 402:15                          _runCommonRequestSuiteFor.<fn>.<fn>.<async>
      ===== asynchronous gap ===========================
      dart:html                                                      _EventStream.listen
      package:w_transport/src/http/browser/request_mixin.dart 90:22  CommonPlainTextRequest&BrowserRequestMixin.sendRequestAndFetchResponse.<async>
      ===== asynchronous gap ===========================
      dart:async                                                     Future.Future.microtask
      package:w_transport/src/http/browser/request_mixin.dart        CommonPlainTextRequest&BrowserRequestMixin.sendRequestAndFetchResponse
      package:w_transport/src/http/common/request.dart 682:7         CommonRequest._send.<async>
      dart:async                                                     _SyncCompleter.complete
      package:w_transport/src/http/browser/request_mixin.dart 39:3   CommonPlainTextRequest&BrowserRequestMixin.openRequest.<async>
      ===== asynchronous gap ===========================
      dart:async                                                     _asyncThenWrapperHelper
      package:w_transport/src/http/common/request.dart               CommonRequest._send
      package:w_transport/src/http/common/request.dart 476:7         CommonRequest.get
      http/common_request/suite.dart 395:30                          _runCommonRequestSuiteFor.<fn>.<fn>.<async>
      ===== asynchronous gap ===========================
      dart:async                                                     Future.Future.microtask
      http/common_request/suite.dart                                 _runCommonRequestSuiteFor.<fn>.<fn>

---

To start, it helps to understand the structure of the w_transport code involved in this failure.

The main entry point (w_transport/w_transport.dart) exposes some abstract request classes that use factory functions to construct the correct implementations based on the configured platform (browser vs VM). I'll use Request as the example here.

abstract class Request extends BaseRequest {
  factory Request() => ...;
}

https://github.com/Workiva/w_transport/blob/master/lib/src/http/requests.dart#L197-L220

There are then two platform-specific entry points for browser and VM. The consumer has to choose one to import and configure so that the platform-independent code knows how to construct the correct concrete implementations at runtime.

Most of the logic is shared between request types and between platforms, so there's an internal common class to house it all.

abstract class CommonRequest implements BaseRequest {
  // Shared concrete logic goes here
  ...

  // Abstract methods here allow platform-specific implementations to be mixed in later
  ...
}

https://github.com/Workiva/w_transport/blob/master/lib/src/http/common/request.dart#L38

To fill in the gaps specific to each platform, I use a mixin.

abstract class BrowserRequestMixin implements BaseRequest, CommonRequest {
  // Provide concrete implementations of the abstract methods defined in CommonRequest
}

https://github.com/Workiva/w_transport/blob/master/lib/src/http/browser/request_mixin.dart#L27

Finally, a concrete version of the request is created by extending the common request class and mixing in a platform-specific one.

class BrowserRequest extends CommonRequest with BrowserRequestMixin {}

https://github.com/Workiva/w_transport/blob/master/lib/src/http/browser/requests.dart#L26

To bring it full circle.. if a user of w_transport had configured the platform to be "browser", then constructing a new instance of the abstract Request class would result in that factory using a browser-specific factory that constructs an instance of BrowserRequest.

---

Now back to the failure that was seemingly introduced due to a change in Dart 1.20. The file where I define all of the concrete browser request classes (https://github.com/Workiva/w_transport/blob/master/lib/src/http/browser/requests.dart#L26) is relatively small - it just imports other classes and extends/mixes them to create the desired result. Because of this, there is no dart:async import in that file (there's no logic that depends on it).

I mention this because the stack trace from the failure above points to this file, but it points to the very last line (which is blank), and if I add import 'dart:async'; to that file, the failure no longer occurs. And then the analyzer complains that the import is unused.

This also seems to be related to the HttpRequest class from dart:html somehow. The only tests that fail are those that are exercising one of two things:

• Calling .abort() on an instance of dart:html.HttpRequest
• Listening to the onAbort stream on an instance of dart:html.HttpRequest

The exact same pattern is followed for the VM implementations (using dart:io), but those are still working as expected.

---

I'll keep trying to come up with a reduced test case, but so far it seems to be the result of the way I'm using abstract classes and mixins to work around platform-specific dependencies (like dart:html) and I haven't been able to simplify that much without losing the failure.

In the mean time, if anyone has some ideas or suspicions about what might be happening here, please let me know!

[kevmoo]

Does it work when you compile to js?

[evanweible-wf]

@kevmoo Yes, looks like it does. Tests pass in Chrome, Firefox, and Safari. So I guess that might narrow it down to a Dartium issue.

[kevmoo]

@evanweible-wf If we checkout your repository, is there a minimal repo we can run to validate?

[evanweible-wf]

@kevmoo here you go:

https://github.com/ekweible/dart_1.20.1_w_transport_future_bug_repro

If you clone that, you can see the failure by running the test in either content-shell or dartium:

$ pub run test -p content-shell
$ pub run test -p dartium

Running in any other browser, the test will pass:

$ pub run test -p chrome

[kevmoo]

@lrhn @floitschG Would you guys take a look? – also confirmed on 1.21.0-dev.4.0

[lrhn]

The class being mentioned is CommonPlainTextRequest&BrowserRequestMixin.abortRequest which is a class created using a mixin in w_transport/lib/src/http/browser/requests.dart as CommonPlainTextRequest with BrowserRequestMixin. That library doesn't import dart:async. It seems like the VM is messing up the type lookup because it can't find the Future class there. The error is reported at line 55 which is the end of the file, so it suggests a problem with synthetic code, not actual code. Now, it shouldn't be looking there at all, it should be doing static lookup relative to where the mixin is declared, and dart:async is imported in that library (request_mixin.dart in the same dir).

I'm not sure why this only happens in dartium/content-shell. One hint is that the stack trace starts with:

dart:html                                                      HttpRequest.abort

but I have no idea why it matters.

I think it's safe to say that it's a VM issue of some sort.

[crelier]

Can someone point me to a x64 debug build of content_shell? Or even better, to a recipe for building it myself?

[crelier]

Not having a debug build of content_shell, I used the latest version of dart_bootstrap to compile the test case. Tracing type finalization shows that 'Future' is being finalized for class 'BrowserRequestMixin', and not for class 'CommonPlainTextRequest&BrowserRequestMixin', which would cause the problem.

I related bug was fixed on September 19. Could it be that content_shell/dartium are using an older dart VM?

[crelier]

Interestingly, if I bypass pub and use content_shell directly with an index.html pointing to w_transport_future_bug_test.dart, I get "00:00 +1: All tests passed!"

[kevmoo]

@crelier success w/out pub might be due to the code exiting before the async error is thrown...

[kevmoo]

@crelier any updates here?

[a-siva]

Debugging on this problem has been slow because of various issues with building a debug version of content shell. I believe Regis managed to get a debug build after various tries only this afternoon.

[crelier]

Yes, I learned the hard way that our older version of dartium/content_shell cannot be built out of the box anymore without manually fixing depot_tools. Another complication is that the issue does not occur if content_shell is ran by itself, but only when invoked by the test runner, which requires modification in order to launch a debug build of content_shell that can be inspected under gdb. It is a very long story, but in short, no update yet.

[crelier]

After the fix, the test sometimes succeeds and sometimes fails in a way unrelated to the issue of this report (timeout?):

$ pub run test -p content-shell 00:03 +1: All tests passed!

$ pub run test -p content-shell 00:03 +0 -1: test/w_transport_future_bug_test.dart: w_transport Future not resolved repro
RequestException: GET 0 http://httpstat.us/200 [object XMLHttpRequestProgressEvent] package:w_transport/src/http/browser/request_mixin.dart 85:43 CommonPlainTextRequest&BrowserRequestMixin.sendRequestAndFetchResponse..onError. dart:async _SyncCompleter.complete package:w_transport/src/http/browser/request_mixin.dart 142:5 CommonPlainTextRequest&BrowserRequestMixin._createResponse. ===== asynchronous gap =========================== dart:async _Completer.completeError package:w_transport/src/http/browser/request_mixin.dart 85:11 CommonPlainTextRequest&BrowserRequestMixin.sendRequestAndFetchResponse..onError. dart:async _SyncCompleter.complete package:w_transport/src/http/browser/request_mixin.dart 142:5 CommonPlainTextRequest&BrowserRequestMixin._createResponse. ===== asynchronous gap =========================== dart:html _EventStream.listen package:w_transport/src/http/browser/request_mixin.dart 90:22 CommonPlainTextRequest&BrowserRequestMixin.sendRequestAndFetchResponse. ===== asynchronous gap =========================== dart:async Future.Future.microtask package:w_transport/src/http/browser/request_mixin.dart CommonPlainTextRequest&BrowserRequestMixin.sendRequestAndFetchResponse package:w_transport/src/http/common/request.dart 682:7 CommonRequest._send. dart:async _SyncCompleter.complete package:w_transport/src/http/browser/request_mixin.dart 39:3 CommonPlainTextRequest&BrowserRequestMixin.openRequest. ===== asynchronous gap =========================== dart:async _asyncThenWrapperHelper package:w_transport/src/http/common/request.dart CommonRequest._send package:w_transport/src/http/common/request.dart 476:7 CommonRequest.get w_transport_future_bug_test.dart 14:34 main.. ===== asynchronous gap =========================== dart:async Future.Future.microtask w_transport_future_bug_test.dart main.

00:03 +0 -1: Some tests failed.