When configured to use \yii\web\JsonResponseFormatter and yii\filters\HttpCache 304 responses include a response body

[darrylkuhn]

When the app is configured to configured return JSON formatted payloads using \yii\web\JsonResponseFormatter and configured to perform caching with yii\filters\HttpCache the response includes a body of null when the cache is hit. However per rfc7232

A 304 response cannot contain a message-body; it is always terminated by the first empty line after the header fields.

This would be a problem for 204 No Content responses as well however \yii\web\Response::prepare() explicitly sets the content of the response to '' when the status is 204. It seems the same ought to be done for 304s as well. If this change is welcome I'd be happy to open a PR.

What steps will reproduce the problem?

1. Configure your response formatter to support json

Yii::$app->response->formatters[\yii\web\Response::FORMAT_JSON] = \yii\web\JsonResponseFormatter::class;

2. Configure your controller to implement yii\filters\HttpCache

public function behaviors()
{
    $behaviors = parent::behaviors();

    $behaviors[] = [
        'class' => 'yii\filters\HttpCache',
        'only' => ['view'],
        'etagSeed' => function ($action, $params) {
            return 'static-etag';
        },
    ];

    return $behaviors;
}

3. Make a request using the etag

curl -v --location --request GET 'http://localhost/example/1' --header 'If-None-Match: "ywR/RvOEhT8oO8ayzGOORj2GzbM"' --header 'Accept: application/json' --header 'Content-Type: application/json'

What is the expected result?

The expected result is a response that with a 304 status and no response body.

What do you get instead?

A response which includes the response body null. This is indicated by curl error message:

* Excess found in a non pipelined read: excess = 4 url = /example/1 (zero-length body)

Additional info

Q	A
Yii version	2.0.16
PHP version	7.1.17
Operating system	alpine3.4

[rad8329]

I'll handle this one