Altered routes break between 8.1 and 8.2

Why get cute with routes and misuse _format ?

You could have delivered the json on a url with your own controller like /content-hub/{entity}/{id}

I don't know if I would blame the API changing if I was misusing it to begin with.

I'd consider rewriting the solution to unblock the issue rather than point the finger at core.

Why get cute with routes and misuse _format ?

They're not misusing _format. They just want to expose this additional custom format. See https://www.drupal.org/docs/8/api/serialization-api/serialization-api-ov....

This makes sense: rather than defining new routes, new controllers, and so on, they just want to reuse the existing REST resource for nodes and have it support one additional format.

This is poor architecture, don't blame the API changing for your misuse.

This much is true, the complexity of this code is incredible. It's merely trying to be selective about which routes it alters: only GET routes that use this specific format. The code is terribly convoluted, poorly maintainable. And that is poor architecture indeed.

The utter lack of documentation is also astonishing. It still has the exact same documentation as \Drupal\rest\Routing\ResourceRoutes, where this was clearly copied from!

The error in 2820096-error-1.txt is not an error in your code, it is happening in core's \Drupal\rest\RequestHandler::handle(). The line that is throwing the error was last modified in #2308745: Remove rest.settings.yml, use rest_resource config entities. Only internal controller details were modified there.

I honestly don't understand how this code could even cause the error that it clearly is causing. I think some other code must also be interacting with it in some other way.

Before

    $plugin = $route_match->getRouteObject()->getDefault('_plugin');
    $method = strtolower($request->getMethod());

    $resource = $this->container
      ->get('plugin.manager.rest')
      ->createInstance($plugin);

After

    $resource_config_id = $route_match->getRouteObject()->getDefault('_rest_resource_config');
    /** @var \Drupal\rest\RestResourceConfigInterface $resource_config */
    $resource_config = $this->resourceStorage->load($resource_config_id);
    $resource = $resource_config->getResourcePlugin();

Note that it used to read _plugin from the route object, now it reads _rest_resource_config. That must be where the problem lies: some other code must be modifying the route defaults.

No worries :)

Going forward, I strongly encourage you to add defensive tests when you're altering certain things. And honestly, even the most basic integration tests would have caught this. But since https://www.drupal.org/node/2759027/qa indicates the acquia_contenthub is not running automated tests, this went unnoticed. (It automatically runs against Drupal 8 "HEAD", which was 8.2.x, and now is 8.3.x..)

Oh, and more importantly: did you manage to fix it already?