Forbid changing aggregation job parameters

[mendess]

This is important for implementing the async draft later

[cjpatton]

Looks pretty good, just one blocking comment: Is it feasible to hash the AggregationJobInitReq before decoding?

[cjpatton]

Other than the decode-then-hash issue, I think this is good to go. I have one more suggestion there.

[cjpatton]

Hashing with HashedAggregationJobReq::decode_with_param() is a little awkward because you have to keep track of the start and end of the message in the cursor. Also, and this is very nit picky: HashedAggregationJobReq is not a DAP protocol message, so arguably doesn't belong in the messages module.

I think I would have done this differently:

impl DecodeFromDapHttpBody for HashedAggregationJobReq {
    fn decode_from_http_body(bytes: Bytes, meta: &DapRequestMeta) -> Result<Self, DapAbort> {
        // Check that media type matches.
        meta.get_checked_media_type(DapMediaType::AggregationJobInitReq)?;
        // Decode the body
        let agg_job_req = AggregationJobReq::get_decoded_with_param(&meta.version, bytes.as_ref())
            .map_err(|e| DapAbort::from_codec_error(e, meta.task_id))?;
        Ok(Self{
             agg_job_req:
             hash: sha256(bytes.as_ref()),
        })
    }
}

Note that this also enforces that there is nothing left to read in bytes.