feat(x/precisebank): Implement MintCoins (#1920)

Implement MintCoins method that matches x/bank MintCoins validation behavior

Makefile

@@ -327,6 +327,16 @@ test-cli: build
 test-migrate:
 	@$(GO_BIN) test -v -count=1 ./migrate/...
 
+# Use the old Apple linker to workaround broken xcode - https://github.com/golang/go/issues/65169
+ifeq ($(OS_FAMILY),Darwin)
+  FUZZLDFLAGS := -ldflags=-extldflags=-Wl,-ld_classic
+endif
+
+test-fuzz:
+	@$(GO_BIN) test $(FUZZLDFLAGS) -run NOTAREALTEST -v -fuzztime 10s -fuzz=FuzzMintCoins ./x/precisebank/keeper
+	@$(GO_BIN) test $(FUZZLDFLAGS) -run NOTAREALTEST -v -fuzztime 10s -fuzz=FuzzGenesisStateValidate_NonZeroRemainder ./x/precisebank/types
+	@$(GO_BIN) test $(FUZZLDFLAGS) -run NOTAREALTEST -v -fuzztime 10s -fuzz=FuzzGenesisStateValidate_ZeroRemainder ./x/precisebank/types
+
 # Kick start lots of sims on an AWS cluster.
 # This submits an AWS Batch job to run a lot of sims, each within a docker image. Results are uploaded to S3
 start-remote-sims:
@@ -347,4 +357,4 @@ update-kvtool:
 	git submodule update
 	cd tests/e2e/kvtool && make install
 
-.PHONY: all build-linux install clean build test test-cli test-all test-rest test-basic start-remote-sims
+.PHONY: all build-linux install clean build test test-cli test-all test-rest test-basic test-fuzz start-remote-sims

x/precisebank/keeper/fractional_balance.go

@@ -15,12 +15,12 @@ import (
 func (k *Keeper) GetFractionalBalance(
 	ctx sdk.Context,
 	address sdk.AccAddress,
-) (sdkmath.Int, bool) {
+) sdkmath.Int {
 	store := prefix.NewStore(ctx.KVStore(k.storeKey), types.FractionalBalancePrefix)
 
 	bz := store.Get(types.FractionalBalanceKey(address))
 	if bz == nil {
-		return sdkmath.ZeroInt(), false
+		return sdkmath.ZeroInt()
 	}
 
 	var bal sdkmath.Int
@@ -28,7 +28,7 @@ func (k *Keeper) GetFractionalBalance(
 		panic(fmt.Errorf("failed to unmarshal fractional balance: %w", err))
 	}
 
-	return bal, true
+	return bal
 }
 
 // SetFractionalBalance sets the fractional balance for an address.

x/precisebank/keeper/fractional_balance_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 
 	sdkmath "cosmossdk.io/math"
+	"github.com/cosmos/cosmos-sdk/store/prefix"
 	sdk "github.com/cosmos/cosmos-sdk/types"
 	"github.com/stretchr/testify/require"
 
@@ -11,9 +12,6 @@ import (
 )
 
 func TestSetGetFractionalBalance(t *testing.T) {
-	tk := NewMockedTestData(t)
-	ctx, k := tk.ctx, tk.keeper
-
 	addr := sdk.AccAddress([]byte("test-address"))
 
 	tests := []struct {
@@ -63,6 +61,9 @@ func TestSetGetFractionalBalance(t *testing.T) {
 	for _, tt := range tests {
 		tt := tt
 		t.Run(tt.name, func(t *testing.T) {
+			td := NewMockedTestData(t)
+			ctx, k := td.ctx, td.keeper
+
 			if tt.setPanicMsg != "" {
 				require.PanicsWithError(t, tt.setPanicMsg, func() {
 					k.SetFractionalBalance(ctx, tt.address, tt.amount)
@@ -75,23 +76,24 @@ func TestSetGetFractionalBalance(t *testing.T) {
 				k.SetFractionalBalance(ctx, tt.address, tt.amount)
 			})
 
-			// If its zero balance, check it was deleted
+			// If its zero balance, check it was deleted in store
 			if tt.amount.IsZero() {
-				_, exists := k.GetFractionalBalance(ctx, tt.address)
-				require.False(t, exists)
+				store := prefix.NewStore(ctx.KVStore(td.storeKey), types.FractionalBalancePrefix)
+				bz := store.Get(types.FractionalBalanceKey(tt.address))
+				require.Nil(t, bz)
 
 				return
 			}
 
-			gotAmount, exists := k.GetFractionalBalance(ctx, tt.address)
-			require.True(t, exists)
+			gotAmount := k.GetFractionalBalance(ctx, tt.address)
 			require.Equal(t, tt.amount, gotAmount)
 
 			// Delete balance
 			k.DeleteFractionalBalance(ctx, tt.address)
 
-			_, exists = k.GetFractionalBalance(ctx, tt.address)
-			require.False(t, exists)
+			store := prefix.NewStore(ctx.KVStore(td.storeKey), types.FractionalBalancePrefix)
+			bz := store.Get(types.FractionalBalanceKey(tt.address))
+			require.Nil(t, bz)
 		})
 	}
 }
@@ -111,23 +113,24 @@ func TestSetFractionalBalance_InvalidAddr(t *testing.T) {
 }
 
 func TestSetFractionalBalance_ZeroDeletes(t *testing.T) {
-	tk := NewMockedTestData(t)
-	ctx, k := tk.ctx, tk.keeper
+	td := NewMockedTestData(t)
+	ctx, k := td.ctx, td.keeper
 
 	addr := sdk.AccAddress([]byte("test-address"))
 
 	// Set balance
 	k.SetFractionalBalance(ctx, addr, sdkmath.NewInt(100))
 
-	bal, exists := k.GetFractionalBalance(ctx, addr)
-	require.True(t, exists)
+	bal := k.GetFractionalBalance(ctx, addr)
 	require.Equal(t, sdkmath.NewInt(100), bal)
 
 	// Set zero balance
 	k.SetFractionalBalance(ctx, addr, sdkmath.ZeroInt())
 
-	_, exists = k.GetFractionalBalance(ctx, addr)
-	require.False(t, exists)
+	// Check balance was deleted
+	store := prefix.NewStore(ctx.KVStore(td.storeKey), types.FractionalBalancePrefix)
+	bz := store.Get(types.FractionalBalanceKey(addr))
+	require.Nil(t, bz)
 
 	// Set zero balance again on non-existent balance
 	require.NotPanics(

x/precisebank/keeper/keeper.go

@@ -4,6 +4,7 @@ import (
 	"github.com/cosmos/cosmos-sdk/codec"
 	storetypes "github.com/cosmos/cosmos-sdk/store/types"
 	sdk "github.com/cosmos/cosmos-sdk/types"
+
 	evmtypes "github.com/evmos/ethermint/x/evm/types"
 
 	"github.com/kava-labs/kava/x/precisebank/types"
@@ -36,10 +37,6 @@ func NewKeeper(
 	}
 }
 
-func (k Keeper) MintCoins(ctx sdk.Context, moduleName string, amt sdk.Coins) error {
-	panic("unimplemented")
-}
-
 func (k Keeper) BurnCoins(ctx sdk.Context, moduleName string, amt sdk.Coins) error {
 	panic("unimplemented")
 }

x/precisebank/keeper/keeper_test.go

@@ -3,6 +3,7 @@ package keeper_test
 import (
 	"testing"
 
+	sdkmath "cosmossdk.io/math"
 	storetypes "github.com/cosmos/cosmos-sdk/store/types"
 	"github.com/cosmos/cosmos-sdk/testutil"
 	sdk "github.com/cosmos/cosmos-sdk/types"
@@ -46,3 +47,7 @@ func NewMockedTestData(t *testing.T) testData {
 		ak:       ak,
 	}
 }
+
+func c(denom string, amount int64) sdk.Coin        { return sdk.NewInt64Coin(denom, amount) }
+func ci(denom string, amount sdkmath.Int) sdk.Coin { return sdk.NewCoin(denom, amount) }
+func cs(coins ...sdk.Coin) sdk.Coins               { return sdk.NewCoins(coins...) }

x/precisebank/keeper/mint.go

@@ -0,0 +1,138 @@
+package keeper
+
+import (
+	"fmt"
+
+	errorsmod "cosmossdk.io/errors"
+	sdkmath "cosmossdk.io/math"
+	sdk "github.com/cosmos/cosmos-sdk/types"
+	sdkerrors "github.com/cosmos/cosmos-sdk/types/errors"
+	authtypes "github.com/cosmos/cosmos-sdk/x/auth/types"
+
+	"github.com/kava-labs/kava/x/precisebank/types"
+)
+
+// MintCoins creates new coins from thin air and adds it to the module account.
+// If ExtendedCoinDenom is provided, the corresponding fractional amount is
+// added to the module state.
+// It will panic if the module account does not exist or is unauthorized.
+func (k Keeper) MintCoins(ctx sdk.Context, moduleName string, amt sdk.Coins) error {
+	// Disallow minting to x/precisebank module
+	if moduleName == types.ModuleName {
+		panic(errorsmod.Wrapf(sdkerrors.ErrUnauthorized, "module account %s cannot be minted to", moduleName))
+	}
+
+	// Note: MintingRestrictionFn is not used in x/precisebank
+	// Panic errors are identical to x/bank for consistency.
+	acc := k.ak.GetModuleAccount(ctx, moduleName)
+	if acc == nil {
+		panic(errorsmod.Wrapf(sdkerrors.ErrUnknownAddress, "module account %s does not exist", moduleName))
+	}
+
+	if !acc.HasPermission(authtypes.Minter) {
+		panic(errorsmod.Wrapf(sdkerrors.ErrUnauthorized, "module account %s does not have permissions to mint tokens", moduleName))
+	}
+
+	// Ensure the coins are valid before minting
+	if !amt.IsValid() {
+		return errorsmod.Wrap(sdkerrors.ErrInvalidCoins, amt.String())
+	}
+
+	// Get non-ExtendedCoinDenom coins
+	passthroughCoins := amt
+
+	extendedAmount := amt.AmountOf(types.ExtendedCoinDenom)
+	if extendedAmount.IsPositive() {
+		// Remove ExtendedCoinDenom from the coins as it is managed by x/precisebank
+		removeCoin := sdk.NewCoin(types.ExtendedCoinDenom, extendedAmount)
+		passthroughCoins = amt.Sub(removeCoin)
+	}
+
+	// Coins unmanaged by x/precisebank are passed through to x/bank
+	if !passthroughCoins.Empty() {
+		if err := k.bk.MintCoins(ctx, moduleName, passthroughCoins); err != nil {
+			return err
+		}
+	}
+
+	// No more processing required if no ExtendedCoinDenom
+	if extendedAmount.IsZero() {
+		return nil
+	}
+
+	return k.mintExtendedCoin(ctx, moduleName, extendedAmount)
+}
+
+// mintExtendedCoin manages the minting of extended coins, and no other coins.
+func (k Keeper) mintExtendedCoin(
+	ctx sdk.Context,
+	moduleName string,
+	amt sdkmath.Int,
+) error {
+	moduleAddr := k.ak.GetModuleAddress(moduleName)
+
+	// Get current module account fractional balance - 0 if not found
+	fractionalAmount := k.GetFractionalBalance(ctx, moduleAddr)
+
+	// Get separated mint amounts
+	integerMintAmount := amt.Quo(types.ConversionFactor())
+	fractionalMintAmount := amt.Mod(types.ConversionFactor())
+
+	// Get new fractional balance after minting, this could be greater than
+	// the conversion factor and must be checked for carry over to integer mint
+	// amount as being set as-is may cause fractional balance exceeding max.
+	newFractionalBalance := fractionalAmount.Add(fractionalMintAmount)
+
+	// If it carries over, add 1 to integer mint amount. In this case, it will
+	// always be 1:
+	// fractional amounts x and y where both x and y < ConversionFactor
+	// x + y < (2 * ConversionFactor) - 2
+	// x + y < 1 integer amount + fractional amount
+	if newFractionalBalance.GTE(types.ConversionFactor()) {
+		// Carry over to integer mint amount
+		integerMintAmount = integerMintAmount.AddRaw(1)
+		// Subtract 1 integer equivalent amount of fractional balance. Same
+		// behavior as using .Mod() in this case.
+		newFractionalBalance = newFractionalBalance.Sub(types.ConversionFactor())
+	}
+
+	// Mint new integer amounts in x/bank - including carry over from fractional
+	// amount if any.
+	if integerMintAmount.IsPositive() {
+		integerMintCoin := sdk.NewCoin(types.IntegerCoinDenom, integerMintAmount)
+
+		if err := k.bk.MintCoins(
+			ctx,
+			moduleName,
+			sdk.NewCoins(integerMintCoin),
+		); err != nil {
+			return err
+		}
+	}
+
+	// Assign new fractional balance in x/precisebank
+	k.SetFractionalBalance(ctx, moduleAddr, newFractionalBalance)
+
+	// ----------------------------------------
+	// Update remainder & reserves to back minted fractional coins
+	prevRemainder := k.GetRemainderAmount(ctx)
+	// Deduct new remainder with minted fractional amount
+	newRemainder := prevRemainder.Sub(fractionalMintAmount)
+
+	if prevRemainder.LT(fractionalMintAmount) {
+		// Need additional 1 integer coin in reserve to back minted fractional
+		reserveMintCoins := sdk.NewCoins(sdk.NewCoin(types.IntegerCoinDenom, sdkmath.OneInt()))
+		if err := k.bk.MintCoins(ctx, types.ModuleName, reserveMintCoins); err != nil {
+			return fmt.Errorf("failed to mint %s for reserve: %w", reserveMintCoins, err)
+		}
+
+		// Update remainder with value of minted integer coin. newRemainder is
+		// currently negative at this point. This also means that it will always
+		// be < conversionFactor after this operation and not require a Mod().
+		newRemainder = newRemainder.Add(types.ConversionFactor())
+	}
+
+	k.SetRemainderAmount(ctx, newRemainder)
+
+	return nil
+}

x/precisebank/keeper/mint_integration_test.go

@@ -0,0 +1,359 @@
+package keeper_test
+
+import (
+	"fmt"
+	"testing"
+
+	sdkmath "cosmossdk.io/math"
+	sdk "github.com/cosmos/cosmos-sdk/types"
+	authtypes "github.com/cosmos/cosmos-sdk/x/auth/types"
+	bankkeeper "github.com/cosmos/cosmos-sdk/x/bank/keeper"
+	banktypes "github.com/cosmos/cosmos-sdk/x/bank/types"
+	minttypes "github.com/cosmos/cosmos-sdk/x/mint/types"
+
+	"github.com/kava-labs/kava/x/precisebank/keeper"
+	"github.com/kava-labs/kava/x/precisebank/testutil"
+	"github.com/kava-labs/kava/x/precisebank/types"
+	"github.com/stretchr/testify/suite"
+)
+
+type mintIntegrationTestSuite struct {
+	testutil.Suite
+}
+
+func (suite *mintIntegrationTestSuite) SetupTest() {
+	suite.Suite.SetupTest()
+}
+
+func TestMintIntegrationTest(t *testing.T) {
+	suite.Run(t, new(mintIntegrationTestSuite))
+}
+
+func (suite *mintIntegrationTestSuite) TestBlockedRecipient() {
+	// Tests that sending funds to x/precisebank is disallowed.
+	// x/precisebank balance is used as the reserve funds and should not be
+	// directly interacted with by external modules or users.
+	msgServer := bankkeeper.NewMsgServerImpl(suite.BankKeeper)
+
+	fromAddr := sdk.AccAddress{1}
+
+	// To x/precisebank
+	toAddr := suite.AccountKeeper.GetModuleAddress(types.ModuleName)
+	amount := cs(c("ukava", 1000))
+
+	msg := banktypes.NewMsgSend(fromAddr, toAddr, amount)
+
+	_, err := msgServer.Send(sdk.WrapSDKContext(suite.Ctx), msg)
+	suite.Require().Error(err)
+
+	suite.Require().EqualError(
+		err,
+		fmt.Sprintf("%s is not allowed to receive funds: unauthorized", toAddr.String()),
+	)
+}
+
+func (suite *mintIntegrationTestSuite) TestMintCoins_MatchingErrors() {
+	// x/precisebank MintCoins should be identical to x/bank MintCoins to
+	// consumers. This test ensures that the panics & errors returned by
+	// x/precisebank are identical to x/bank.
+
+	tests := []struct {
+		name            string
+		recipientModule string
+		mintAmount      sdk.Coins
+		wantErr         string
+		wantPanic       string
+	}{
+		{
+			"invalid module",
+			"notamodule",
+			cs(c("ukava", 1000)),
+			"",
+			"module account notamodule does not exist: unknown address",
+		},
+		{
+			"no mint permissions",
+			// Check app.go to ensure this module has no mint permissions
+			authtypes.FeeCollectorName,
+			cs(c("ukava", 1000)),
+			"",
+			"module account fee_collector does not have permissions to mint tokens: unauthorized",
+		},
+		{
+			"invalid amount",
+			minttypes.ModuleName,
+			sdk.Coins{sdk.Coin{Denom: "ukava", Amount: sdkmath.NewInt(-100)}},
+			"-100ukava: invalid coins",
+			"",
+		},
+	}
+
+	for _, tt := range tests {
+		suite.Run(tt.name, func() {
+			// Reset
+			suite.SetupTest()
+
+			if tt.wantErr == "" && tt.wantPanic == "" {
+				suite.Fail("test must specify either wantErr or wantPanic")
+			}
+
+			if tt.wantErr != "" {
+				// Check x/bank MintCoins for identical error
+				bankErr := suite.BankKeeper.MintCoins(suite.Ctx, tt.recipientModule, tt.mintAmount)
+				suite.Require().Error(bankErr)
+				suite.Require().EqualError(bankErr, tt.wantErr, "expected error should match x/bank MintCoins error")
+
+				pbankErr := suite.Keeper.MintCoins(suite.Ctx, tt.recipientModule, tt.mintAmount)
+				suite.Require().Error(pbankErr)
+				// Compare strings instead of errors, as error stack is still different
+				suite.Require().Equal(
+					bankErr.Error(),
+					pbankErr.Error(),
+					"x/precisebank error should match x/bank MintCoins error",
+				)
+			}
+
+			if tt.wantPanic != "" {
+				// First check the wantPanic string is correct.
+				// Actually specify the panic string in the test since it makes
+				// it more clear we are testing specific and different cases.
+				suite.Require().PanicsWithError(tt.wantPanic, func() {
+					_ = suite.BankKeeper.MintCoins(suite.Ctx, tt.recipientModule, tt.mintAmount)
+				}, "expected panic error should match x/bank MintCoins")
+
+				suite.Require().PanicsWithError(tt.wantPanic, func() {
+					_ = suite.Keeper.MintCoins(suite.Ctx, tt.recipientModule, tt.mintAmount)
+				}, "x/precisebank panic should match x/bank MintCoins")
+			}
+		})
+	}
+}
+
+func (suite *mintIntegrationTestSuite) TestMintCoins() {
+	type mintTest struct {
+		mintAmount sdk.Coins
+		// Expected **full** balances after MintCoins(mintAmount)
+		wantBalance sdk.Coins
+	}
+
+	tests := []struct {
+		name            string
+		recipientModule string
+		// Instead of having a start balance, we just have a list of mints to
+		// both test & get into desired non-default states.
+		mints []mintTest
+	}{
+		{
+			"passthrough - unrelated",
+			minttypes.ModuleName,
+			[]mintTest{
+				{
+					mintAmount:  cs(c("busd", 1000)),
+					wantBalance: cs(c("busd", 1000)),
+				},
+			},
+		},
+		{
+			"passthrough - integer denom",
+			minttypes.ModuleName,
+			[]mintTest{
+				{
+					mintAmount:  cs(c(types.IntegerCoinDenom, 1000)),
+					wantBalance: cs(c(types.ExtendedCoinDenom, 1000000000000000)),
+				},
+			},
+		},
+		{
+			"fractional only",
+			minttypes.ModuleName,
+			[]mintTest{
+				{
+					mintAmount:  cs(c(types.ExtendedCoinDenom, 1000)),
+					wantBalance: cs(c(types.ExtendedCoinDenom, 1000)),
+				},
+				{
+					mintAmount:  cs(c(types.ExtendedCoinDenom, 1000)),
+					wantBalance: cs(c(types.ExtendedCoinDenom, 2000)),
+				},
+			},
+		},
+		{
+			"exact carry",
+			minttypes.ModuleName,
+			[]mintTest{
+				{
+					mintAmount:  cs(ci(types.ExtendedCoinDenom, types.ConversionFactor())),
+					wantBalance: cs(ci(types.ExtendedCoinDenom, types.ConversionFactor())),
+				},
+				// Carry again - exact amount
+				{
+					mintAmount:  cs(ci(types.ExtendedCoinDenom, types.ConversionFactor())),
+					wantBalance: cs(ci(types.ExtendedCoinDenom, types.ConversionFactor().MulRaw(2))),
+				},
+			},
+		},
+		{
+			"carry with extra",
+			minttypes.ModuleName,
+			[]mintTest{
+				// MintCoins(C + 100)
+				{
+					mintAmount:  cs(ci(types.ExtendedCoinDenom, types.ConversionFactor().AddRaw(100))),
+					wantBalance: cs(ci(types.ExtendedCoinDenom, types.ConversionFactor().AddRaw(100))),
+				},
+				// MintCoins(C + 5), total = 2C + 105
+				{
+					mintAmount:  cs(ci(types.ExtendedCoinDenom, types.ConversionFactor().AddRaw(5))),
+					wantBalance: cs(ci(types.ExtendedCoinDenom, types.ConversionFactor().MulRaw(2).AddRaw(105))),
+				},
+			},
+		},
+		{
+			"integer with fractional",
+			minttypes.ModuleName,
+			[]mintTest{
+				{
+					mintAmount:  cs(ci(types.ExtendedCoinDenom, types.ConversionFactor().MulRaw(5).AddRaw(100))),
+					wantBalance: cs(ci(types.ExtendedCoinDenom, types.ConversionFactor().MulRaw(5).AddRaw(100))),
+				},
+				{
+					mintAmount:  cs(ci(types.ExtendedCoinDenom, types.ConversionFactor().MulRaw(2).AddRaw(5))),
+					wantBalance: cs(ci(types.ExtendedCoinDenom, types.ConversionFactor().MulRaw(7).AddRaw(105))),
+				},
+			},
+		},
+		{
+			"with passthrough",
+			minttypes.ModuleName,
+			[]mintTest{
+				{
+					mintAmount: cs(
+						ci(types.ExtendedCoinDenom, types.ConversionFactor().MulRaw(5).AddRaw(100)),
+						c("busd", 1000),
+					),
+					wantBalance: cs(
+						ci(types.ExtendedCoinDenom, types.ConversionFactor().MulRaw(5).AddRaw(100)),
+						c("busd", 1000),
+					),
+				},
+				{
+					mintAmount: cs(
+						ci(types.ExtendedCoinDenom, types.ConversionFactor().MulRaw(2).AddRaw(5)),
+						c("meow", 40),
+					),
+					wantBalance: cs(
+						ci(types.ExtendedCoinDenom, types.ConversionFactor().MulRaw(7).AddRaw(105)),
+						c("busd", 1000),
+						c("meow", 40),
+					),
+				},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		suite.Run(tt.name, func() {
+			// Reset
+			suite.SetupTest()
+
+			recipientAddr := suite.AccountKeeper.GetModuleAddress(tt.recipientModule)
+
+			for _, mt := range tt.mints {
+				err := suite.Keeper.MintCoins(suite.Ctx, tt.recipientModule, mt.mintAmount)
+				suite.Require().NoError(err)
+
+				// -------------------------------------------------------------
+				// Check FULL balances
+				// x/bank balances + x/precisebank balance
+				// Exclude "ukava" as x/precisebank balance will include it
+				bankCoins := suite.BankKeeper.GetAllBalances(suite.Ctx, recipientAddr)
+
+				// Only use x/bank balances for non-ukava denoms
+				var denoms []string
+				for _, coin := range bankCoins {
+					// Ignore integer coins, query the extended denom instead
+					if coin.Denom == types.IntegerCoinDenom {
+						continue
+					}
+
+					denoms = append(denoms, coin.Denom)
+				}
+
+				// Add the extended denom to the list of denoms to balance check
+				// Will be included in balance check even if x/bank doesn't have
+				// ukava.
+				denoms = append(denoms, types.ExtendedCoinDenom)
+
+				// All balance queries through x/precisebank
+				afterBalance := sdk.NewCoins()
+				for _, denom := range denoms {
+					coin := suite.Keeper.GetBalance(suite.Ctx, recipientAddr, denom)
+					afterBalance = afterBalance.Add(coin)
+				}
+
+				suite.Require().Equal(
+					mt.wantBalance.String(),
+					afterBalance.String(),
+					"unexpected balance after minting %s to %s",
+				)
+
+				// Ensure reserve is backing all minted fractions
+				allInvariantsFn := keeper.AllInvariants(suite.Keeper)
+				res, stop := allInvariantsFn(suite.Ctx)
+				suite.Require().False(stop, "invariant should not be broken")
+				suite.Require().Empty(res, "unexpected invariant message: %s", res)
+			}
+		})
+	}
+}
+
+func FuzzMintCoins(f *testing.F) {
+	f.Add(int64(0))
+	f.Add(int64(100))
+	f.Add(types.ConversionFactor().Int64())
+	f.Add(types.ConversionFactor().MulRaw(5).Int64())
+	f.Add(types.ConversionFactor().MulRaw(2).AddRaw(123948723).Int64())
+
+	f.Fuzz(func(t *testing.T, amount int64) {
+		// No negative amounts
+		if amount < 0 {
+			amount = -amount
+		}
+
+		// Manually setup test suite since no direct Fuzz support in test suites
+		suite := new(mintIntegrationTestSuite)
+		suite.SetT(t)
+		suite.SetS(suite)
+		suite.SetupTest()
+
+		mintCount := int64(10)
+
+		// Mint 10 times to include mints from non-zero balances
+		for i := int64(0); i < mintCount; i++ {
+			err := suite.Keeper.MintCoins(
+				suite.Ctx,
+				minttypes.ModuleName,
+				cs(c(types.ExtendedCoinDenom, amount)),
+			)
+			suite.Require().NoError(err)
+		}
+
+		// Check FULL balances
+		recipientAddr := suite.AccountKeeper.GetModuleAddress(minttypes.ModuleName)
+		bal := suite.Keeper.GetBalance(suite.Ctx, recipientAddr, types.ExtendedCoinDenom)
+
+		suite.Require().Equalf(
+			amount*mintCount,
+			bal.Amount.Int64(),
+			"unexpected balance after minting %d 5 times",
+			amount,
+		)
+
+		// Run Invariants to ensure remainder is backing all minted fractions
+		// and in a valid state
+		allInvariantsFn := keeper.AllInvariants(suite.Keeper)
+		res, stop := allInvariantsFn(suite.Ctx)
+		suite.Require().False(stop, "invariant should not be broken")
+		suite.Require().Empty(res, "unexpected invariant message: %s", res)
+	})
+}

x/precisebank/keeper/mint_test.go

@@ -0,0 +1,347 @@
+package keeper_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	sdkmath "cosmossdk.io/math"
+	sdk "github.com/cosmos/cosmos-sdk/types"
+	authtypes "github.com/cosmos/cosmos-sdk/x/auth/types"
+	minttypes "github.com/cosmos/cosmos-sdk/x/mint/types"
+
+	"github.com/kava-labs/kava/x/precisebank/types"
+)
+
+func TestMintCoins_PanicValidations(t *testing.T) {
+	// panic tests for invalid inputs
+
+	tests := []struct {
+		name            string
+		recipientModule string
+		setupFn         func(td testData)
+		mintAmount      sdk.Coins
+		wantPanic       string
+	}{
+		{
+			"invalid module",
+			"notamodule",
+			func(td testData) {
+				// Make module not found
+				td.ak.EXPECT().
+					GetModuleAccount(td.ctx, "notamodule").
+					Return(nil).
+					Once()
+			},
+			cs(c("ukava", 1000)),
+			"module account notamodule does not exist: unknown address",
+		},
+		{
+			"no permission",
+			minttypes.ModuleName,
+			func(td testData) {
+				td.ak.EXPECT().
+					GetModuleAccount(td.ctx, minttypes.ModuleName).
+					Return(authtypes.NewModuleAccount(
+						authtypes.NewBaseAccountWithAddress(sdk.AccAddress{1}),
+						minttypes.ModuleName,
+						// no mint permission
+					)).
+					Once()
+			},
+			cs(c("ukava", 1000)),
+			"module account mint does not have permissions to mint tokens: unauthorized",
+		},
+		{
+			"has mint permission",
+			minttypes.ModuleName,
+			func(td testData) {
+				td.ak.EXPECT().
+					GetModuleAccount(td.ctx, minttypes.ModuleName).
+					Return(authtypes.NewModuleAccount(
+						authtypes.NewBaseAccountWithAddress(sdk.AccAddress{1}),
+						minttypes.ModuleName,
+						// includes minter permission
+						authtypes.Minter,
+					)).
+					Once()
+
+				// Will call x/bank MintCoins coins
+				td.bk.EXPECT().
+					MintCoins(td.ctx, minttypes.ModuleName, cs(c("ukava", 1000))).
+					Return(nil).
+					Once()
+			},
+			cs(c("ukava", 1000)),
+			"",
+		},
+		{
+			"disallow minting to x/precisebank",
+			types.ModuleName,
+			func(td testData) {
+				// No mock setup needed since this is checked before module
+				// account checks
+			},
+			cs(c("ukava", 1000)),
+			"module account precisebank cannot be minted to: unauthorized",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			td := NewMockedTestData(t)
+			tt.setupFn(td)
+
+			if tt.wantPanic != "" {
+				require.PanicsWithError(t, tt.wantPanic, func() {
+					_ = td.keeper.MintCoins(td.ctx, tt.recipientModule, tt.mintAmount)
+				})
+				return
+			}
+
+			require.NotPanics(t, func() {
+				// Not testing errors, only panics for this test
+				_ = td.keeper.MintCoins(td.ctx, tt.recipientModule, tt.mintAmount)
+			})
+		})
+	}
+}
+
+func TestMintCoins_Errors(t *testing.T) {
+	// returned errors, not panics
+
+	tests := []struct {
+		name            string
+		recipientModule string
+		setupFn         func(td testData)
+		mintAmount      sdk.Coins
+		wantError       string
+	}{
+		{
+			"invalid coins",
+			minttypes.ModuleName,
+			func(td testData) {
+				// Valid module account minter
+				td.ak.EXPECT().
+					GetModuleAccount(td.ctx, minttypes.ModuleName).
+					Return(authtypes.NewModuleAccount(
+						authtypes.NewBaseAccountWithAddress(sdk.AccAddress{1}),
+						minttypes.ModuleName,
+						// includes minter permission
+						authtypes.Minter,
+					)).
+					Once()
+			},
+			sdk.Coins{sdk.Coin{
+				Denom:  "ukava",
+				Amount: sdk.NewInt(-1000),
+			}},
+			"-1000ukava: invalid coins",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			td := NewMockedTestData(t)
+			tt.setupFn(td)
+
+			require.NotPanics(t, func() {
+				err := td.keeper.MintCoins(td.ctx, tt.recipientModule, tt.mintAmount)
+
+				if tt.wantError != "" {
+					require.Error(t, err)
+					require.EqualError(t, err, tt.wantError)
+					return
+				}
+
+				require.NoError(t, err)
+			})
+		})
+	}
+}
+
+func TestMintCoins_ExpectedCalls(t *testing.T) {
+	// Tests the expected calls to the bank keeper when minting coins
+
+	tests := []struct {
+		name string
+		// Only care about starting fractional balance.
+		// MintCoins() doesn't care about the previous integer balance.
+		startFractionalBalance sdkmath.Int
+		mintAmount             sdk.Coins
+		// account x/precisebank balance (fractional amount)
+		wantPreciseBalance sdkmath.Int
+	}{
+		{
+			"passthrough mint - integer denom",
+			sdkmath.ZeroInt(),
+			cs(c("ukava", 1000)),
+			sdkmath.ZeroInt(),
+		},
+
+		{
+			"passthrough mint - unrelated denom",
+			sdkmath.ZeroInt(),
+			cs(c("meow", 1000)),
+			sdkmath.ZeroInt(),
+		},
+		{
+			"no carry - 0 starting fractional",
+			sdkmath.ZeroInt(),
+			cs(c(types.ExtendedCoinDenom, 1000)),
+			sdkmath.NewInt(1000),
+		},
+		{
+			"no carry - non-zero fractional",
+			sdkmath.NewInt(1_000_000),
+			cs(c(types.ExtendedCoinDenom, 1000)),
+			sdkmath.NewInt(1_001_000),
+		},
+		{
+			"fractional carry",
+			// max fractional amount
+			types.ConversionFactor().SubRaw(1),
+			cs(c(types.ExtendedCoinDenom, 1)), // +1 to carry
+			sdkmath.ZeroInt(),
+		},
+		{
+			"fractional carry max",
+			// max fractional amount + max fractional amount
+			types.ConversionFactor().SubRaw(1),
+			cs(ci(types.ExtendedCoinDenom, types.ConversionFactor().SubRaw(1))),
+			types.ConversionFactor().SubRaw(2),
+		},
+		{
+			"integer with fractional no carry",
+			sdkmath.NewInt(1234),
+			// mint 100 fractional
+			cs(c(types.ExtendedCoinDenom, 100)),
+			sdkmath.NewInt(1234 + 100),
+		},
+		{
+			"integer with fractional carry",
+			types.ConversionFactor().SubRaw(100),
+			// mint 105 fractional to carry
+			cs(c(types.ExtendedCoinDenom, 105)),
+			sdkmath.NewInt(5),
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			td := NewMockedTestData(t)
+
+			// Set initial fractional balance
+			// Initial integer balance doesn't matter for this test
+			moduleAddr := sdk.AccAddress{1}
+			td.keeper.SetFractionalBalance(
+				td.ctx,
+				moduleAddr,
+				tt.startFractionalBalance,
+			)
+			fBal := td.keeper.GetFractionalBalance(td.ctx, moduleAddr)
+			require.Equal(t, tt.startFractionalBalance, fBal)
+
+			// Always calls GetModuleAccount() to check if module exists &
+			// has permission
+			td.ak.EXPECT().
+				GetModuleAccount(td.ctx, minttypes.ModuleName).
+				Return(authtypes.NewModuleAccount(
+					authtypes.NewBaseAccountWithAddress(
+						moduleAddr,
+					),
+					minttypes.ModuleName,
+					// Include minter permissions - not testing permission in
+					// this test
+					authtypes.Minter,
+				)).
+				Once()
+
+			// ----------------------------------------
+			// Separate passthrough and extended coins
+			// Determine how much is passed through to x/bank
+			passthroughCoins := tt.mintAmount
+
+			found, extCoins := tt.mintAmount.Find(types.ExtendedCoinDenom)
+			if found {
+				// Remove extended coin from passthrough coins
+				passthroughCoins = passthroughCoins.Sub(extCoins)
+			} else {
+				extCoins = sdk.NewCoin(types.ExtendedCoinDenom, sdkmath.ZeroInt())
+			}
+
+			require.Equalf(
+				t,
+				sdkmath.ZeroInt(),
+				passthroughCoins.AmountOf(types.ExtendedCoinDenom),
+				"expected pass through coins should not include %v",
+				types.ExtendedCoinDenom,
+			)
+
+			// ----------------------------------------
+			// Set expectations for minting passthrough coins
+			// Only expect MintCoins to be called with passthrough coins with non-zero amount
+			if !passthroughCoins.Empty() {
+				t.Logf("Expecting MintCoins(%v)", passthroughCoins)
+
+				td.bk.EXPECT().
+					MintCoins(td.ctx, minttypes.ModuleName, passthroughCoins).
+					Return(nil).
+					Once()
+			}
+
+			// ----------------------------------------
+			// Set expectations for minting fractional coins
+			if !extCoins.IsNil() && extCoins.IsPositive() {
+				td.ak.EXPECT().
+					GetModuleAddress(minttypes.ModuleName).
+					Return(moduleAddr).
+					Once()
+
+				// Initial integer balance is always 0 for this test
+				totalNewBalance := tt.startFractionalBalance.Add(extCoins.Amount)
+				mintIntegerAmount := totalNewBalance.Quo(types.ConversionFactor())
+
+				mintCoins := cs(ci(types.IntegerCoinDenom, mintIntegerAmount))
+
+				// Only expect MintCoins to be called with mint coins with
+				// non-zero amount.
+				// Will fail if x/bank MintCoins is called with empty coins
+				if !mintCoins.Empty() {
+					t.Logf("Expecting MintCoins(%v)", mintCoins)
+
+					td.bk.EXPECT().
+						MintCoins(td.ctx, minttypes.ModuleName, mintCoins).
+						Return(nil).
+						Once()
+				}
+			}
+
+			// ----------------------------------------
+			// Set expectations for reserve minting when fractional amounts
+			// are minted & remainder is insufficient
+			mintFractionalAmount := extCoins.Amount.Mod(types.ConversionFactor())
+			currentRemainder := td.keeper.GetRemainderAmount(td.ctx)
+
+			remainderEnough := currentRemainder.GTE(mintFractionalAmount)
+			if !remainderEnough {
+				reserveMintCoins := cs(ci(types.IntegerCoinDenom, sdkmath.OneInt()))
+				td.bk.EXPECT().
+					// Mints to x/precisebank
+					MintCoins(td.ctx, types.ModuleName, reserveMintCoins).
+					Return(nil).
+					Once()
+			}
+
+			// ----------------------------------------
+			// Actual call after all setup and expectations
+			require.NotPanics(t, func() {
+				err := td.keeper.MintCoins(td.ctx, minttypes.ModuleName, tt.mintAmount)
+				require.NoError(t, err)
+			})
+
+			// Check final fractional balance
+			fBal = td.keeper.GetFractionalBalance(td.ctx, moduleAddr)
+			require.Equal(t, tt.wantPreciseBalance, fBal)
+		})
+	}
+}

x/precisebank/keeper/view.go

@@ -23,10 +23,7 @@ func (k Keeper) GetBalance(
 	integerAmount := spendableCoins.AmountOf(types.IntegerCoinDenom)
 
 	// x/precisebank for fractional balance
-	fractionalAmount, found := k.GetFractionalBalance(ctx, addr)
-	if !found {
-		fractionalAmount = sdk.ZeroInt()
-	}
+	fractionalAmount := k.GetFractionalBalance(ctx, addr)
 
 	// (Integer * ConversionFactor) + Fractional
 	fullAmount := integerAmount.

x/precisebank/testutil/fractional_balances.go

@@ -135,7 +135,7 @@ func GenerateEqualFractionalBalancesWithRemainder(
 ) (types.FractionalBalances, sdkmath.Int) {
 	t.Helper()
 
-	require.GreaterOrEqual(t, count, 3, "count must be at least 3 to generate both balances and remainder")
+	require.GreaterOrEqual(t, count, 2, "count must be at least 2 to generate both balances and remainder")
 
 	countWithRemainder := count + 1