I'm not sure which implementation you're referring to; in stevejones's example, incoming data remains buffered by the OS until the client specifically requests one response's worth in read_response(). If the OS's buffer ever gets full, it will signal the server's OS to stop sending data; if it continues to get packets it will simply drop them (thus minimizing resource usage).
It's of course totally possible for one to make a client that reads responses regardless of whether it send a request, but that's rather silly as giving up flow control like that immediately opens your application up to a DoS attack. (Of course just because it's silly doesn't mean no-one does it!)
Ah, I see, you're right, this assumes 'blocking' code.
I wonder if that's really how browers work or if they employ an array of open connections that are periodically polled for responses to outstanding requests.
Obviously, responding to something that wasn't requested is a bad idea.
Flow control only works for large responses (which is good, because that is at least one resource that you can protect), makes you wonder what you could do with multiple answers small enough to fit in the same window and if that would allow you to identify HTTP implementations that have taken 'asynchronous' one step too far.
Surely reading the responses opens the browser up to active exploit whilst simply buffering unrequested "responses" allows the possibility of denial of service by filling the buffer and causing actively requested packets to be refused.
So, presumably if someone requests anything from your site you can keep bombarding their browser with unrequested content that will get queued. As jacquesm indicates having an array of connections with reserved queues would avoid this blocking requested content.
In the OS, the buffers are per TCP connection. Only packets destined for the full buffer are dropped. TCP connections can't be opened by a remote attacker unless the browser is actively listening for them.
So effectively all that can happen is a website can DoS the connection to itself.
(Yes, an attacker can try to initiate many many TCP connections. This uses much fewer resources than an actual HTTP session would but can still be an effective DoS attack. This is known as a SYN flood.)
It's of course totally possible for one to make a client that reads responses regardless of whether it send a request, but that's rather silly as giving up flow control like that immediately opens your application up to a DoS attack. (Of course just because it's silly doesn't mean no-one does it!)