Download sprout parameters in zcash_proofs

[teor2345]

Motivation

Zebra wants to use zcash_proofs to download both Sprout and Sapling parameters.

Changes

Add sprout:

• Add a download_sprout_parameters function.
• Stream sprout and sapling downloads to disk, rather than loading the whole file into RAM.

Functionality changes:

• If the parameter files already exist, check them instead of starting a download.
• Allow the caller to specify a download timeout.
• Check file sizes as well as file hashes (useful for debugging).
• Remove downloaded files on error (but leave existing files alone) .

Interface changes:

• Add download_sapling_parameters and deprecate download_parameters. This avoids confusion between sprout and sapling downloads, while maintaining backward compatibility.
• Make the Sprout and Sapling parameter file names public. This makes it easier for callers to supply the correct paths to load_parameters.

Bug fixes:

• Add a missing feature dependency: download_params -> directories. This makes sure download_parameters will work if it's the only feature specified.

Tweaks:

• Refactor file loads to use the same verifying function as downloads.

[conradoplg]

@teor2345 asked me to take a look. It seems good.

I think it would improve usability to delete downloaded files on errors, so that the user won't need to manually delete them. But it's not a big deal.

[teor2345]

I think it would improve usability to delete downloaded files on errors, so that the user won't need to manually delete them. But it's not a big deal.

Sounds like a good idea, I'll see if I can make it happen today.

I was concerned we could accidentally delete files we haven't created. But now that we're handling existing and downloaded files separately, we can just do the deletes for downloaded files.

[teor2345]

Chaining the Sprout requests is failing, probably because the second connection times out.

This is fixed now.

Tested using:

RUSTFLAGS="--cfg always_download" cargo run --release --features=download-params --example download-sprout-and-sapling-params

[teor2345]

I also checked the code with clippy using:

cargo clippy --all-features --no-deps --lib --examples

[teor2345]

@nuttycom @str4d I rebased on master, fixed a clippy warning, and updated this PR.

[defuse]

utACK with comments/questions

[nuttycom]

utACK

[teor2345]

Thanks for the review, everything should be rebased and fixed now.

I ran cargo test locally, and everything passed.

[teor2345]

I forgot to test with and without features, I pushed some minor fixes.

[codecov]

Codecov Report

Merging #459 (c0f20b3) into main (8101b1a) will decrease coverage by 0.39%.
The diff coverage is 26.00%.

@@            Coverage Diff             @@
##             main     #459      +/-   ##
==========================================
- Coverage   51.51%   51.11%   -0.40%     
==========================================
  Files          91       94       +3     
  Lines        8500     8897     +397     
==========================================
+ Hits         4379     4548     +169     
- Misses       4121     4349     +228     

Impacted Files	Coverage Δ
zcash_proofs/src/lib.rs	23.33% <21.73%> (-11.45%)	⬇️
zcash_proofs/src/hashreader.rs	43.75% <75.00%> (-2.41%)	⬇️
zcash_client_backend/src/keys.rs	54.70% <0.00%> (-16.73%)	⬇️
zcash_primitives/src/sapling/note_encryption.rs	64.08% <0.00%> (-9.36%)	⬇️
zcash_client_sqlite/src/chain.rs	89.44% <0.00%> (-4.58%)	⬇️
components/zcash_address/src/encoding.rs	91.37% <0.00%> (-3.45%)	⬇️
zcash_client_sqlite/src/wallet/transact.rs	87.44% <0.00%> (-3.22%)	⬇️
zcash_client_backend/src/data_api/wallet.rs	18.06% <0.00%> (-3.17%)	⬇️
zcash_history/src/tree.rs	55.94% <0.00%> (-3.09%)	⬇️
... and 57 more

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[daira]

The use of constants that are not always compiled-in is blocking.

Fixed in 40d9115

[daira]

utACK