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Trailers

Did you think it was a bit odd that we had to write 0\r\n\r\n (two registered nurses) to signal the end of the chunked encoding?

There's actually a good reason for it – you can have additional headers at the end of chunked encoding, called (of course) Trailers. They work the same way that headers do with one catch: you have to specify the names of the trailers in a Trailer header. For example:

HTTP/1.1 200 OK
Content-Type: text/plain
Transfer-Encoding: chunked
Trailer: Lane, Prime, TJ

1E
I could go for a cup of coffee
C
But not Java
12
Never go full Java
0
Lane: goober
Prime: chill-guy
TJ: 1-indexer

They go between the two \r\n sequences in our original chunked encoding example.

Why Trailers?

Trailers are often used to send information about the message body that can't be known until the message body is fully sent. For example, the hash of the message body... in fact, let's do that.

Assignment

Add a new method to your response package that does what you'd expect based on your knowledge of trailers. I used func (w *Writer) WriteTrailers(h headers.Headers) error.

Remember, trailers are formatted just like headers.
In your proxy handler (the httpbingo.org one):
- Ensure that a request to /httpbin/range/4096 on your server will proxy https://httpbingo.org/range/4096 instead of the streaming endpoint we used before. We'll be using that endpoint for this assignment.
- Announce X-Content-SHA256 and X-Content-Length as trailers in the Trailer header.
- Remove the Content-Length header.
- Keep track of the full response body as you read it in chunks from the upstream server.
- Once you're done writing the chunks, calculate the hash of the full response body using sha256.Sum256.
- Add the following headers:
  - X-Content-SHA256: <hash>
  - X-Content-Length: <length of raw body in bytes>
Run your server, and debug your code with curl --raw.

HTTP header names are case-insensitive by specification. Some servers and clients may canonicalize header names in certain ways (e.g., X-Content-Sha256), so don't be surprised if you notice discrepancies. It's best not to rely on any particular casing when working with headers.

Run and submit the CLI tests with your server running.