fix: Pool Coin Decimal Truncation During Deposit

[dongsam]

Description

closes: #445

For now, it is a draft version that reproduces the above issue and solves it at the PoC level, requiring more testing and edge case simulation.

Tasks

• Fix ExecuteDeposit function
• Add test code to verify that the issue has been resolved well
• Update MintingPoolCoinsInvariant and DepositInvariant
• Simulation to find edge cases
• Remove print codes temporary written for debugging

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[codecov]

Codecov Report

Merging #446 (f7df497) into develop (e118e21) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #446      +/-   ##
===========================================
- Coverage    85.10%   85.09%   -0.01%     
===========================================
  Files           23       23              
  Lines         2618     2604      -14     
===========================================
- Hits          2228     2216      -12     
+ Misses         226      225       -1     
+ Partials       164      163       -1     

Impacted Files	Coverage Δ
x/liquidity/keeper/invariants.go	66.31% <100.00%> (+1.79%)	⬆️
x/liquidity/keeper/liquidity_pool.go	83.16% <100.00%> (-0.50%)	⬇️

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[dongsam]

Suggested change

	sdk.NewCoin(depositCoins[0].Denom, lastReserveCoinA.Amount.ToDec().Mul(mintRate).TruncateInt()),
	sdk.NewCoin(depositCoins[1].Denom, lastReserveCoinB.Amount.ToDec().Mul(mintRate).TruncateInt()),
	sdk.NewCoin(depositCoins[0].Denom, lastReserveCoinA.Amount.ToDec().MulTruncate(mintRate).TruncateInt()),
	sdk.NewCoin(depositCoins[1].Denom, lastReserveCoinB.Amount.ToDec().MulTruncate(mintRate).TruncateInt()),

What do you think about using MulTruncate instead of Mul here? @hallazzang @typark391 @kogisin
I think both will have the same result but that is more intuitive, or is there a case that is not?

[hallazzang]

Using MulTruncate here can cause(very rarely) a depositor to take advantage against the pool.
Since the result of MulTruncate is always less or equal than the result of Mul, I suggest to use Mul here.
How do others think?

[dongsam]

That's right. Let's keep it Mul first, reproduce the corresponding edge case, analyze it, and specify it on spec through other PRs.

[dongsam]

Logic ExecuteDeposit was simplified, so the hit line was reduced, even codecov failed, and the actual functional coverage was not lowered.