fix(inflation): daily epoch mint provision

[fedekunze]

Description

Closes: ENG-1207

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[linear]

ENG-1207 Inflation is only recalculate once a year, even when parameters change

Context

We want to create a proposal to change inflation, but there is a bug that prevents is. Check out the Specification.

Current

The inflation module mints new tokens every day (through hooks). To calculate how many tokens are minted per day (epoch mint provision), we calculate the total amount per year (period provision) and divide it by the epochs per year. The calculation, however, is only performed once a year to adjust inflation on a yearly basis, so that the same amount of tokens are minted every day for one year (illustration from token model)

The inflation parameters that calculate the inflation can be changed through governance. Since the epoch mint provision is only recalculated once a year, a proposal to change these parameters wouldn’t change the inflation until next April (one year after genesis).

evmosd q inflation params --node https://tendermint.bd.evmos.org:26657

enable_inflation: true
exponential_calculation:
  a: "300000000.000000000000000000"
  bonding_target: "0.660000000000000000"
  c: "9375000.000000000000000000"
  max_variance: "0.000000000000000000"
  r: "0.500000000000000000"
inflation_distribution:
  community_pool: "0.133333333000000000"
  staking_rewards: "0.533333334000000000"
  usage_incentives: "0.333333333000000000"
mint_denom: aevmos

Expected - Change calculation to run daily but keep yearly intervals

Inflation should be recalculated every day, so that it reacts to fluctuations in calculation parameters (e.g. bonding ratio or bonding target) without delay.

• move CalculateEpochMintProvision out of check if period has passed. Place before MintAndAllocateInflation so that inflation is calcualted every day instead of just when a period passes
• We don’t need to store EpochMintProvision in the store anymore, remove it from the store and its getter and setters. Queries need to be adjusted to calculate EpochMintProvision on the fly and push the response to the client
• Update tests and documentation

Test cases

• Epoch mint provision is recalculate every day using the current parameters
• If inflation parameters change (e.g. through governance), they are used for the next calculation => effective during calculation on next day.

[codecov]

Codecov Report

Merging #1193 (a1d86b1) into main (0e0a566) will increase coverage by 0.07%.
The diff coverage is 91.66%.

❗ Current head a1d86b1 differs from pull request most recent head e5b70a8. Consider uploading reports for the commit e5b70a8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1193      +/-   ##
==========================================
+ Coverage   76.34%   76.41%   +0.07%     
==========================================
  Files         149      148       -1     
  Lines        8099     8073      -26     
==========================================
- Hits         6183     6169      -14     
+ Misses       1716     1708       -8     
+ Partials      200      196       -4     

Impacted Files	Coverage Δ
app/app.go	81.70% <0.00%> (-0.12%)	⬇️
x/inflation/keeper/inflation.go	82.17% <90.90%> (+2.39%)	⬆️
x/inflation/keeper/grpc_query.go	100.00% <100.00%> (+8.57%)	⬆️
x/inflation/keeper/hooks.go	65.90% <100.00%> (-3.03%)	⬇️

[fedekunze]

LGTM, note that we also need to do a migration o remove the EpochMintProvision from the store in a separate PR.

cc @Vvaradinov

[Vvaradinov]

tACK. @fedekunze will follow up with the migration and tests in a separate PR.