Retry Zcash sprout and sapling parameters download

[oxarbitrage]

Motivation

We want to retry a few times the Sprout and Sapling parameter download if they fail for whatever reason.

Fixes #3239

Solution

Retry the downloads if they fail. Number of retries is defined by a constant(currently = 3).

The code can be maybe improved a bit to make it shorter but i think it should work as it is too.

Review

Anyone can review, the code is pretty simple.

Reviewer Checklist

• Code implements Specs and Designs
• Tests for Expected Behaviour
• Tests for Errors

[codecov]

Codecov Report

Merging #3306 (1f8b977) into main (5f772eb) will decrease coverage by 0.01%.
The diff coverage is 25.00%.

❗ Current head 1f8b977 differs from pull request most recent head 5fb2fe7. Consider uploading reports for the commit 5fb2fe7 to get more accurate results

@@            Coverage Diff             @@
##             main    #3306      +/-   ##
==========================================
- Coverage   77.30%   77.29%   -0.02%     
==========================================
  Files         265      265              
  Lines       31347    31380      +33     
==========================================
+ Hits        24234    24254      +20     
- Misses       7113     7126      +13     

[teor2345]

Logging the error is a blocker for merging this PR, because I've used it a lot to diagnose download errors.

Did you also want advice about simplifying the code?

These changes are optional:

Groth16Parameters::new is getting really long. Can you split out the individual sapling and sprout downloads into separate methods?

If we do that change, we can simplify the loop to just handle the error case, because the success case doesn't need any extra code:

while let Err(error) = Groth16Parameters::download_sapling_parameters() {
    sapling_retries += 1;
    if sapling_retries >= PARAMETER_DOWNLOAD_MAX_RETRIES {
        panic!(
            "error downloading Sapling parameter files after {} retries. {:?} {}",
            PARAMETER_DOWNLOAD_MAX_RETRIES,
            error,
            Groth16Parameters::failure_hint(),
        );
    } else {
        tracing::info!(?error, "error downloading Zcash Sapling parameters, retrying");
    }
}

[teor2345]

Looks good, but we should check that the paths are the ones we expect, otherwise loading might fail. (This is a workaround for the librustzcash API, we should make it easier to use in future.)

See PR oxarbitrage#162 for a commit that adds the asserts back in.

[teor2345]

The macOS failure is a peer set timeout, it should be fixed by PR #3292 or #3294.

[teor2345]

Looks good, thanks!