feat(mint): Implement custom inflation function. (#10441)

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## Description

We want to use this feature in [Akash Network](https://akash.network/).

The [Akash whitepaper](https://whitepaper.io/document/632/akash-network-whitepaper) defines a time-based function for the inflation: `I(t)`. The way `cosmos-sdk` currently calculates inflation is different as compared to what the Akash whitepaper prescribes. So, we need the `cosmos-sdk` to accept a custom function to calculate inflation. When the custom function isn't provided, it should use the default inflation calculation logic as it currently does.

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---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [ ] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
- [ ] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [ ] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [ ] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [ ] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

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your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

CHANGELOG.md

@@ -52,6 +52,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ### API Breaking Changes
 
+* (x/mint) [\#10441](https://github.com/cosmos/cosmos-sdk/pull/10441) The `NewAppModule` function now accepts an inflation calculation function as an argument.
 * [\#10295](https://github.com/cosmos/cosmos-sdk/pull/10295) Remove store type aliases from /types
 * [\#9695](https://github.com/cosmos/cosmos-sdk/pull/9695) Migrate keys from `Info` -> `Record`
   * Add new `codec.Codec` argument in:

simapp/app.go

@@ -332,7 +332,7 @@ func NewSimApp(
 		crisis.NewAppModule(&app.CrisisKeeper, skipGenesisInvariants),
 		feegrantmodule.NewAppModule(appCodec, app.AccountKeeper, app.BankKeeper, app.FeeGrantKeeper, app.interfaceRegistry),
 		gov.NewAppModule(appCodec, app.GovKeeper, app.AccountKeeper, app.BankKeeper),
-		mint.NewAppModule(appCodec, app.MintKeeper, app.AccountKeeper),
+		mint.NewAppModule(appCodec, app.MintKeeper, app.AccountKeeper, nil),
 		slashing.NewAppModule(appCodec, app.SlashingKeeper, app.AccountKeeper, app.BankKeeper, app.StakingKeeper),
 		distr.NewAppModule(appCodec, app.DistrKeeper, app.AccountKeeper, app.BankKeeper, app.StakingKeeper),
 		staking.NewAppModule(appCodec, app.StakingKeeper, app.AccountKeeper, app.BankKeeper),
@@ -384,7 +384,7 @@ func NewSimApp(
 		capability.NewAppModule(appCodec, *app.CapabilityKeeper),
 		feegrantmodule.NewAppModule(appCodec, app.AccountKeeper, app.BankKeeper, app.FeeGrantKeeper, app.interfaceRegistry),
 		gov.NewAppModule(appCodec, app.GovKeeper, app.AccountKeeper, app.BankKeeper),
-		mint.NewAppModule(appCodec, app.MintKeeper, app.AccountKeeper),
+		mint.NewAppModule(appCodec, app.MintKeeper, app.AccountKeeper, nil),
 		staking.NewAppModule(appCodec, app.StakingKeeper, app.AccountKeeper, app.BankKeeper),
 		distr.NewAppModule(appCodec, app.DistrKeeper, app.AccountKeeper, app.BankKeeper, app.StakingKeeper),
 		slashing.NewAppModule(appCodec, app.SlashingKeeper, app.AccountKeeper, app.BankKeeper, app.StakingKeeper),

x/mint/abci.go

@@ -10,7 +10,7 @@ import (
 )
 
 // BeginBlocker mints new tokens for the previous block.
-func BeginBlocker(ctx sdk.Context, k keeper.Keeper) {
+func BeginBlocker(ctx sdk.Context, k keeper.Keeper, ic types.InflationCalculationFn) {
 	defer telemetry.ModuleMeasureSince(types.ModuleName, time.Now(), telemetry.MetricKeyBeginBlocker)
 
 	// fetch stored minter & params
@@ -20,7 +20,7 @@ func BeginBlocker(ctx sdk.Context, k keeper.Keeper) {
 	// recalculate inflation rate
 	totalStakingSupply := k.StakingTokenSupply(ctx)
 	bondedRatio := k.BondedRatio(ctx)
-	minter.Inflation = minter.NextInflationRate(params, bondedRatio)
+	minter.Inflation = ic(ctx, minter, params, bondedRatio)
 	minter.AnnualProvisions = minter.NextAnnualProvisions(params, totalStakingSupply)
 	k.SetMinter(ctx, minter)
 

x/mint/module.go

@@ -89,14 +89,23 @@ type AppModule struct {
 
 	keeper     keeper.Keeper
 	authKeeper types.AccountKeeper
+
+	// inflationCalculator is used to calculate the inflation rate during BeginBlock.
+	// If inflationCalculator is nil, the default inflation calculation logic is used.
+	inflationCalculator types.InflationCalculationFn
 }
 
-// NewAppModule creates a new AppModule object
-func NewAppModule(cdc codec.Codec, keeper keeper.Keeper, ak types.AccountKeeper) AppModule {
+// NewAppModule creates a new AppModule object. If the InflationCalculationFn
+// argument is nil, then the SDK's default inflation function will be used.
+func NewAppModule(cdc codec.Codec, keeper keeper.Keeper, ak types.AccountKeeper, ic types.InflationCalculationFn) AppModule {
+	if ic == nil {
+		ic = types.DefaultInflationCalculationFn
+	}
 	return AppModule{
-		AppModuleBasic: AppModuleBasic{cdc: cdc},
-		keeper:         keeper,
-		authKeeper:     ak,
+		AppModuleBasic:      AppModuleBasic{cdc: cdc},
+		keeper:              keeper,
+		authKeeper:          ak,
+		inflationCalculator: ic,
 	}
 }
 
@@ -149,7 +158,7 @@ func (AppModule) ConsensusVersion() uint64 { return 1 }
 
 // BeginBlock returns the begin blocker for the mint module.
 func (am AppModule) BeginBlock(ctx sdk.Context, _ abci.RequestBeginBlock) {
-	BeginBlocker(ctx, am.keeper)
+	BeginBlocker(ctx, am.keeper, am.inflationCalculator)
 }
 
 // EndBlock returns the end blocker for the mint module. It returns no validator

x/mint/types/genesis.go

@@ -1,5 +1,21 @@
 package types
 
+import (
+	sdk "github.com/cosmos/cosmos-sdk/types"
+)
+
+// InflationCalculationFn defines the function required to calculate inflation rate during
+// BeginBlock. It receives the minter and params stored in the keeper, along with the current
+// bondedRatio and returns the newly calculated inflation rate.
+// It can be used to specify a custom inflation calculation logic, instead of relying on the
+// default logic provided by the sdk.
+type InflationCalculationFn func(ctx sdk.Context, minter Minter, params Params, bondedRatio sdk.Dec) sdk.Dec
+
+// DefaultInflationCalculationFn is the default function used to calculate inflation.
+func DefaultInflationCalculationFn(_ sdk.Context, minter Minter, params Params, bondedRatio sdk.Dec) sdk.Dec {
+	return minter.NextInflationRate(params, bondedRatio)
+}
+
 // NewGenesisState creates a new GenesisState object
 func NewGenesisState(minter Minter, params Params) *GenesisState {
 	return &GenesisState{