Add `offset` and `count` to Path Send

[Archmonger]

This discussion exists to talk about whether the Path Send spec can be reasonably modified to support offset and count, in a similar manner to those respective variables within zerocopysend.

As a summary of the original discussion...

• zerocopysend was effectively dead on arrival due to integration challenges with existing webservers.
• pathsend can be considered as an alternative that is more compatible with a variety of webservers.
• pathsend currently does not support offset and count, which makes it impossible to use with HTTP range requests.
• andrewgodwin agreed that offset and count can be added to pathsend if it can be implemented in a backwards compatible manner
• I suggested if backwards compatibility is too challenging, then the extension namespace can be cluttered with http.response.pathsend2

[gi0baro]

On the backward compatibility thing, I personally think is fine to add new features on top of an existing extension, given there's a way for the application to understand if those features are supported.

One idea might be to add this information to the scope, eg:

"scope": {
    "extensions": {
        "http.response.pathsend": {
            "ranges": True
        },
    },
}

in such way the application knows if the server supports the additional keys in the messages and use them; the lack of that boolean means no support is given. I'd also personally prefer a single key in the message for the range, as the application should already know the total size of the file to produce valid range headers, eg:

{
    "type": "http.response.pathsend",
    "path": "foobar.txt",
    "range": (0, 4096)   # send only the first 4KB
}

[andrewgodwin]

That's a good idea - extensions entries were meant to be dicts exactly so we could definitely add signal in there.