change(mempool): Evict transactions from the mempool using the ZIP-317 conventional fee

[teor2345]

Motivation

We want to implement the ZIP-317 conventional fee calculation, and use it to evict transactions from the mempool.

This is a security fix independent of the getblocktemplate work.
(But we also use the conventional fee calculation when selecting transactions for new blocks.)

Close #5335.

Specifications

https://zips.z.cash/zip-0317#fee-calculation
https://zips.z.cash/zip-0317#mempool-size-limiting

This change can be deployed immediately:
https://zips.z.cash/zip-0317#deployment

Designs

Do the fee calculation in the specified order, using similar names for everything:
https://zips.z.cash/zip-0317#fee-calculation

Cache the result as a field on UnminedTx, because it only changes when the transaction data changes. This way, it is available during block construction and transaction relaying.

This is a standard rule, not a consensus rule, so variations in the implementation are acceptable.

Solution

• Add a ZIP-317 conventional fee module
• Add ZIP-317 constants
• Calculate the ZIP-317 conventional fee
• Use the new conventional fee for mempool size limiting
• Update existing tests

Related changes:

• Just return a usize from zcash_serialized_size(), removing the unused Result<_, io::Error>
  • This makes the new code a lot simpler

Review

I'd like two people to review this code, including @dconnolly because she helped write the spec.

Here is the corresponding code in zcash_primitives (it doesn't have Orchard support yet):
https://github.com/zcash/librustzcash/blob/main/zcash_primitives/src/transaction/fees/zip317.rs#L117

Do we need any extra tests?

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
• How do you know it works? Does it have tests?

Follow Up Work

The changes to transaction relaying and block production depend on enough wallets updating to pay these fees:
https://zips.z.cash/zip-0317#deployment

So they have separate tickets:

• Select getblocktemplate transactions according to ZIP-317 #5473
• Relay and accept mempool transactions based on the ZIP-317 fee rules #5336

[codecov]

Codecov Report

Merging #5703 (a111fd6) into main (d5597f8) will increase coverage by 0.05%.
The diff coverage is 93.05%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5703      +/-   ##
==========================================
+ Coverage   78.73%   78.78%   +0.05%     
==========================================
  Files         305      306       +1     
  Lines       38503    38552      +49     
==========================================
+ Hits        30314    30372      +58     
+ Misses       8189     8180       -9     

[arya2]

Code looks clean and correct, just needs minor updates to a couple doc comments.

I like that zcash_serialized_size was updated to return a usize.

[arya2]

Thank you!