Error: "connection not upgraded to websocket"

[mindplay-dk]

I've got a straight forward setup like this:

main(List<String> args) async {
  if (HotReloader.isHotReloadable) {
    print('Hot reloading enabled');
    final reloader = HotReloader();
    await reloader.addPackagePath(Uri(scheme: 'package', path: 'server_test/'));
    await reloader.go();
  }

  // ... launch a web server (using shelf) ...
}

When I launch my server.dart script, I get this output:

C:\workspace\dart\server-test>dart --enable-vm-service bin/server.dart
Observatory listening on http://127.0.0.1:8181/2vuzbj7xrus=/
Hot reloading enabled
Listening for file changes at C:\workspace\dart\server-test\lib\
Serving at http://localhost:8080

But the moment I change one of the files, it crashes with this stack trace:

Paths C:\workspace\dart\server-test\lib\src\images.dart changed!
Reloading the application...
Unhandled exception:
WebSocketException: Connection to 'http://localhost:8181/ws#' was not upgraded to websocket
#0      _WebSocketImpl.connect.<anonymous closure>.error (dart:_http/websocket_impl.dart:1052:9)
#1      _WebSocketImpl.connect.<anonymous closure> (dart:_http/websocket_impl.dart:1061:14)
#2      _RootZone.runUnary (dart:async/zone.dart:1379:54)
#3      _FutureListener.handleValue (dart:async/future_impl.dart:137:18)
#4      Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:678:45)
#5      Future._propagateToListeners (dart:async/future_impl.dart:707:32)
#6      Future._completeWithValue (dart:async/future_impl.dart:522:5)
#7      Future.wait.<anonymous closure> (dart:async/future.dart:400:22)
#8      _RootZone.runUnary (dart:async/zone.dart:1379:54)
#9      _FutureListener.handleValue (dart:async/future_impl.dart:137:18)
#10     Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:678:45)
#11     Future._propagateToListeners (dart:async/future_impl.dart:707:32)
#12     Future._completeWithValue (dart:async/future_impl.dart:522:5)
#13     Future._asyncComplete.<anonymous closure> (dart:async/future_impl.dart:552:7)
#14     _microtaskLoop (dart:async/schedule_microtask.dart:41:21)
#15     _startMicrotaskLoop (dart:async/schedule_microtask.dart:50:5)
#16     _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:116:13)
#17     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:173:5)

I'm on 64-bit Windows 10 with the following Dart version:

2.6.1 (Mon Nov 11 13:12:24 2019 +0100) on "windows_x64"

I noticed a mismatch in socket URLs between the startup message:

Observatory listening on http://127.0.0.1:8181/2vuzbj7xrus=/

And the error message:

Connection to 'http://localhost:8181/ws#' was not upgraded to websocket

The websocket URL displayed at startup contains a random key that changes with every launch, so a constant is probably not enough to make this work.

Likely related to #5, though the error message in that issue is different. (?)

Let me know if there's anything I can do to help resolve the issue.

[mindplay-dk]

Okay, a bit of progress - I was able to reference this library for a way to get the correct URL.

It looks like this:

import 'dart:developer' as dev;
import 'package:path/path.dart' as path;

main(List<String> args) async {
  if (HotReloader.isHotReloadable) {
    var info = await dev.Service.getInfo();
    var uri = info.serverUri;
    uri = uri.replace(path: path.join(uri.path, 'ws'));
    if (uri.scheme == 'https') {
      uri = uri.replace(scheme: 'wss');
    } else {
      uri = uri.replace(scheme: 'ws');
    }

    print('Hot reloading enabled');
    final reloader = HotReloader(vmServiceUrl: uri.toString());
    await reloader.addPackagePath(Uri(scheme: 'package', path: 'server_test/'));
    await reloader.go();
  }

  // ...
}

Something similar should probably replace the constant URL in the library?

(Let me know if you'd like a PR for that change.)

This gets me connected and running - I now see messages like these:

Paths C:\workspace\dart\server-test\lib\src\images.dart changed!
Reloading the application...

So far, so good.

It doesn't work though - changes made to that script don't have any effect.

I'm using shelf, and the thing I was hoping to reload is a Middleware inserted into a Pipeline at start-up. So this is effectively a function instance that persists in an object property somewhere.

I don't know the limitations or the precise workings of hot reloading, but would I need to do something else to get this working? (e.g. tear down and rebuild my HttpServer instance with new instances of the middleware and router etc.?)

[mindplay-dk]

I've also tried the basic example that ships with the package, and this doesn't work either.

For example, changing the count value, or changing the increment expression to count += 2 doesn't have any effect other than seeing the "paths changed" and "reloading" messages on the console.

[mindplay-dk]

For the record, someone on the Dart Gitter channel gave it a shot on Linux - he also had to copy the code I posted (to get the websocket path) to get it working, so that bit needs a PR either way.

He then confirmed that the example did work for him.

So the issue is likely Windows-specific.

We also found that the vm_service_lib package has been superseded by the vm_service package and should be upgraded - I tried switching that dependency, which was easy enough, just needs a simple update of the import paths, but it didn't address this issue.

[tejainece]

Please submit a PR with the fix.

[mindplay-dk]

I will open a separate issue to track the Windows problem.

[jahan-addison]

Thank you so much @mindplay-dk, your snippet worked for me on a macOS setup where I was having this issue as well.