feat: fetch only the root block

[bajtos]

SPARK is about retrieval testing not load testing.

Currently, more than 90% of retrievals fail on timeout or server errors. There is little value in checking whether we can download the entire CAR file; it's more important to know whether SPs are advertising FIL+ deals to IPNI and accepting HTTP-based retrieval requests.

SPARK is also currently unable to fetch and verify CAR files larger than 200MB, which further reduces the value of fetching full CAR files.

Let's fetch the root block for the time being.

• This should significantly reduce the load on "good" SPs.
• It provides enough data to help "bad" SPs fix their setup.

/cc @rvagg @hannahhoward @willscott @gruns do you have any thoughts on this?

See also:

• Reduce load on SPs space-meridian/roadmap#52

[rvagg]

Seems reasonable to me. Depending on whether you think it adds value you could alternatively fetch either entity or all and use lassie's max-blocks feature to halt the retrieval when you get a certain distance into it. Then you could test whether they're able to respond with DAGs or not. But, it'll look like a cancellation on the server side, and you'll probably have to do some manual checking on your side because Lassie (I think) will return it as an error because it's a premature halt.

I could imagine that ipfs/specs#445 might be an interesting thing to add into the mix if we ever got that done for SPs; a shallow fetch that avoids the costly raw blocks but shows they can respond with non-trivial data.