Move early hints into responseEarlyHints field

[thomasjm]

This is a continuation of #523, developing the idea that we can put the early hints into a new field of the Response object called responseEarlyHints.

AFAICT this is not a breaking change, since the Response data constructors are not exposed and the StatusHeaders type is only exposed in an internal module.

[thomasjm]

Hmm, I just realized this is not going to actually provide a performance improvement, because parseStatusHeaders won't return until it actually reads the final status + headers, at which time the response is presumably ready to go anyway.

This limits the usefulness of this when it comes to implementing performant reverse proxying, but at least it fixes the behavior on #521.

It seems like it will require more of an API change in order to first return early hints headers, and then at a later time return the actual status + headers + response.

[thomasjm]

I just added the cabal version bump and changelog update.

I'm still not sure how the early hint headers can be exposed in a way that the user can access them immediately after they arrive, as when you're trying to e.g. write a reverse proxy. When you look at the key functions of http-client such as responseOpen:

responseOpen :: Request -> Manager -> IO (Response BodyReader)

it seems like it would have to change to something like this:

-- An extra callback that receives any early hint headers
responseOpen' :: Request -> Manager -> ([Header] -> IO ()) -> IO (Response BodyReader)

OR, less invasively, how about being able to specify a [Header] -> IO () callback on the Request object? It already has a couple IO callbacks.

[snoyberg]

I think adding a callback to the Request object makes perfect sense in a case like this. Do you want to include that in this PR before I merge?

[thomasjm]

I might as well include it, it looks like I'll have to plumb the callback through most of this.

[thomasjm]

Okay, I've added the callback.

[thomasjm]

Okay, the callback signature is [Header] -> IO () now and I added a test of multiple 103 header sections.

[snoyberg]

Merged and released, thanks!