Symfony run code after response was sent

In the onTerminate callback you get an instance of PostResponseEvent as first parameter. You can get the Request as well as the Response from that object. Then you should be able to decide if you want to run the actual termination code.

Also you can store custom data in the attributes bag of the Request. See this link: Symfony and HTTP Fundamentals

The Request class also has a public attributes property, which holds special data related to how the application works internally. For the Symfony Framework, the attributes holds the values returned by the matched route, like _controller, id (if you have an {id} wildcard), and even the name of the matched route (_route). The attributes property exists entirely to be a place where you can prepare and store context-specific information about the request.

Your code could look something like this:

// ...

class TestListener implements EventSubscriberInterface
{
    // ...

    public function onTerminate(PostResponseEvent $event)
    {
        $request = $event->getRequest();
        if ($request->attributes->get('_route') == 'some_route_name') {
            // do stuff
        }
    }

    // ...
}

Edit:

The kernel.terminate event is designed to run after the response is sent. But the symfony documentation is saying the following (taken from here):

Internally, the HttpKernel makes use of the fastcgi_finish_request PHP function. This means that at the moment, only the PHP FPM server API is able to send a response to the client while the server's PHP process still performs some tasks. With all other server APIs, listeners to kernel.terminate are still executed, but the response is not sent to the client until they are all completed.

Edit 2:

To use the solution from here, you could either directly edit the web/app.php file to add it there (but this is some kind of "hacking core" imo, even though it would be easier to use than the following). Or you could do it like this:

1. Add a listener to kernel.request event with a high priority and start output buffering (ob_start).
2. Add a listener to kernel.response and add the header values to the response.
3. Add another listener with highest priority to kernel.terminate and do the flushing (ob_flush, flush).
4. Run your code in a separate listener with lower priority to kernel.terminate

I did not try it, but it should actually work.