Add gun:trailers/3 for HTTP/1.1 and HTTP/2

[the-mikedavis]

This is a work-in-progress for adding gun:trailers/3. So far I adapted what data/7 is doing in HTTP/1 and HTTP/2 - I still need to look at adding test cases.

Closes #356

[essen]

This will need to be made to work with HTTP/3 as well. It likely already works if you do the same as HTTP/2. Tests will confirm. Worst case, using trailers with HTTP/3 should trigger an error (there's plenty of things not implemented with HTTP/3 so if tests show it doesn't work it's fine to error out for now).

[the-mikedavis]

For HTTP/3, a case that errors is probably the easiest (for me at least 😅) for now. It looks like we need to access/update the #http3_state.http3_machine for the QPACK encoding. (I'm pretty familiar with HTTP/2 but not HTTP/3 - maybe this is easier than I am thinking.) gun_http3:data/7 uses cow_http3:data/1 but I think we want to use cow_http3:headers/1 on the headers block from cow_http3_machine:prepare_trailers/3 instead.

[essen]

Right if it's not the same as HTTP/2 let's error out for now in gun_http3.

[the-mikedavis]

Sorry for the really long delay 😅. I ran into (what I think is?) a bug in cowlib for HTTP/2. I sent a PR over there with the fix, ninenines/cowlib#149.

[the-mikedavis]

Ultimately I ended up not using this change since I'm targetting S3 and it has a weird content-encoding: aws-chunked which is kind of HTTP/1.1 chunked transfer but not really. You have to send the content-length and include the extra bytes for framing in it, and then also set the true content-length in x-amz-decoded-content-length. https://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-streaming.html / https://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-streaming-trailers.html

I think gun:trailers/3 makes sense to have regardless, but it looks like I won't be using it myself directly.

(Surely someone must know why S3 does this weird content encoding thing. I'll go complain 🙂)

Update: it's a purely historical artifact, they invented aws-chunked because HTTP/1.1 chunked transfer+trailers was very immature in HTTP clients at the time. There's really no technical reason you couldn't use standard HTTP/1.1 though, if they allowed it on their end.

[essen]

Yes I think it will be good to have in the next release, whenever it happens. I am working on a large H3/WT update for Cowboy that will also be done in Gun and will include other PRs with that.