R4R: Inflation bug fixes

[cwgoes]

Closes #2967

Changes:

• Tokens are minted for the previous block before distributing rewards
• Inflation rate is now recalculated each block instead of each hour
• For zero-height export, delegation withdrawal infos / validator withdrawal infos are not deleted but instead reset to height 0

Standard checklist:

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added entries in PENDING.md with issue #
• rereviewed Files changed in the github PR explorer

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #2982 into develop will increase coverage by 0.06%.
The diff coverage is 40%.

@@             Coverage Diff             @@
##           develop    #2982      +/-   ##
===========================================
+ Coverage     55.5%   55.57%   +0.06%     
===========================================
  Files          120      120              
  Lines         8494     8488       -6     
===========================================
+ Hits          4715     4717       +2     
+ Misses        3457     3449       -8     
  Partials       322      322

[cwgoes]

Interesting, one seed failing:

go test ./cmd/gaia/app -run TestGaiaSimulationAfterImport -SimulationEnabled=true -SimulationNumBlocks=50 -SimulationVerbose=true -SimulationCommit=true -SimulationSeed=981928 -v -timeout 24h

with

panic: invariant broken: loose token invariance:
	pool.LooseTokens: 78540599.9265062257
	sum of account tokens: 78540599.9265062218 [recovered]
	panic: invariant broken: loose token invariance:
	pool.LooseTokens: 78540599.9265062257
	sum of account tokens: 78540599.9265062218

[cwgoes]

Pinned to these lines in cmd/gaia/app/export.go:

  app.assertRuntimeInvariantsWith(ctx)
  fmt.Printf("pre-deletion")

  // delete all distribution infos
  // these will be recreated in InitGenesis
  app.distrKeeper.RemoveValidatorDistInfos(ctx)
  app.distrKeeper.RemoveDelegationDistInfos(ctx)

  // assert again
  app.assertRuntimeInvariantsWith(ctx)
  fmt.Printf("post-deletion")

Fails between "pre-deletion" and "post-deletion".

Somehow, it's not safe to delete distribution infos after (theoretically) withdrawing everything from them.

[cwgoes]

One ValidatorDistributionInfo has leftover delegator-pool stake:

--- FAIL: TestGaiaSimulationAfterImport (6.25s)
panic: valInfo: {OperatorAddr:35D5DAA716545B6F6A574E9F75061912DD08D0DF FeePoolWithdrawalHeight:50 DelAccum:{UpdateHeight:50 Accum:0.0000000000} DelPool:[{Denom:STAKE Amount:0.0000000039}] ValCommission:[]} [recovered]
	panic: valInfo: {OperatorAddr:35D5DAA716545B6F6A574E9F75061912DD08D0DF FeePoolWithdrawalHeight:50 DelAccum:{UpdateHeight:50 Accum:0.0000000000} DelPool:[{Denom:STAKE Amount:0.0000000039}] ValCommission:[]}

[cwgoes]

OK, confirmed that all the delegation infos have zero rewards, looks like occasional DelPool can be leftover by rounding. Instead now resetting delegation info height to 0 (and not deleting).

[rigelrozanski]

Let's benchmark the actual cost per block of this!

[cwgoes]

@rigelrozanski Added benchmarks in 60cf1f4 - total for both looks like ~2200 ns.

[alexanderbez]

Does this warrant any changes to the distribution spec (e.g. Affects on Staking)?

[alexanderbez]

Changes look good. Not sure if any further documentation needs to be updated? Also, not sure if this has any other implications on the state machine.

[jackzampolin]

Sounds like we should get this in for GoS as #2984 depends on it?

[cwgoes]

Sounds like we should get this in for GoS as #2984 depends on it?

Yup, and more importantly because it fixes #2967.

[rigelrozanski]

few minor comments which should be addressed still, but approving

[hleb-albau]

@cwgoes @jackzampolin is it correct, that NextAnnualProvisions() calculated using only bonded part of the tokens, but not really total supply?

// BeginBlocker
	totalSupply := k.sk.TotalPower(ctx)
	bondedRatio := k.sk.BondedRatio(ctx)
	minter.Inflation = minter.NextInflationRate(params, bondedRatio)
	minter.AnnualProvisions = minter.NextAnnualProvisions(params, totalSupply)