feat: Burn Tax via utilization of existing Stability Tax code (#784)

* Burn Tax via utilization of existing Stability Tax code

* Added blockheights to the cosmos dependency

Author: edk208

custom/auth/ante/ante.go

@@ -8,13 +8,12 @@ import (
 	sdkerrors "github.com/cosmos/cosmos-sdk/types/errors"
 	cosmosante "github.com/cosmos/cosmos-sdk/x/auth/ante"
 	"github.com/cosmos/cosmos-sdk/x/auth/signing"
-	"github.com/cosmos/cosmos-sdk/x/auth/types"
 )
 
 // HandlerOptions are the options required for constructing a default SDK AnteHandler.
 type HandlerOptions struct {
 	AccountKeeper    cosmosante.AccountKeeper
-	BankKeeper       types.BankKeeper
+	BankKeeper       BankKeeper
 	FeegrantKeeper   cosmosante.FeegrantKeeper
 	OracleKeeper     OracleKeeper
 	TreasuryKeeper   TreasuryKeeper
@@ -62,7 +61,8 @@ func NewAnteHandler(options HandlerOptions) (sdk.AnteHandler, error) {
 		cosmosante.NewValidateMemoDecorator(options.AccountKeeper),
 		cosmosante.NewConsumeGasForTxSizeDecorator(options.AccountKeeper),
 		cosmosante.NewDeductFeeDecorator(options.AccountKeeper, options.BankKeeper, options.FeegrantKeeper),
-		cosmosante.NewSetPubKeyDecorator(options.AccountKeeper), // SetPubKeyDecorator must be called before all signature verification decorators
+		NewBurnTaxFeeDecorator(options.TreasuryKeeper, options.BankKeeper), // burn tax proceeds
+		cosmosante.NewSetPubKeyDecorator(options.AccountKeeper),            // SetPubKeyDecorator must be called before all signature verification decorators
 		cosmosante.NewValidateSigCountDecorator(options.AccountKeeper),
 		cosmosante.NewSigGasConsumeDecorator(options.AccountKeeper, sigGasConsumer),
 		NewSigVerificationDecorator(options.AccountKeeper, options.SignModeHandler),

custom/auth/ante/burntax.go

@@ -0,0 +1,61 @@
+package ante
+
+import (
+	sdk "github.com/cosmos/cosmos-sdk/types"
+	sdkerrors "github.com/cosmos/cosmos-sdk/types/errors"
+	"github.com/cosmos/cosmos-sdk/x/auth/types"
+	treasury "github.com/terra-money/core/x/treasury/types"
+)
+
+// TaxPowerUpgradeHeight is when taxes are allowed to go into effect
+// This will still need a parameter change proposal, but can be activated
+// anytime after this height
+const TaxPowerUpgradeHeight = 9346889
+
+// BurnTaxFeeDecorator will immediately burn the collected Tax
+type BurnTaxFeeDecorator struct {
+	treasuryKeeper TreasuryKeeper
+	bankKeeper     BankKeeper
+}
+
+// NewBurnTaxFeeDecorator returns new tax fee decorator instance
+func NewBurnTaxFeeDecorator(treasuryKeeper TreasuryKeeper, bankKeeper BankKeeper) BurnTaxFeeDecorator {
+	return BurnTaxFeeDecorator{
+		treasuryKeeper: treasuryKeeper,
+		bankKeeper:     bankKeeper,
+	}
+}
+
+// AnteHandle handles msg tax fee checking
+func (btfd BurnTaxFeeDecorator) AnteHandle(ctx sdk.Context, tx sdk.Tx, simulate bool, next sdk.AnteHandler) (newCtx sdk.Context, err error) {
+	// Do not proceed if you are below this block height
+	currHeight := ctx.BlockHeight()
+	if currHeight < TaxPowerUpgradeHeight {
+		return next(ctx, tx, simulate)
+	}
+
+	feeTx, ok := tx.(sdk.FeeTx)
+	if !ok {
+		return ctx, sdkerrors.Wrap(sdkerrors.ErrTxDecode, "Tx must be a FeeTx")
+	}
+
+	msgs := feeTx.GetMsgs()
+
+	// At this point we have already run the DeductFees AnteHandler and taken the fees from the sending account
+	// Now we remove the taxes from the gas reward and immediately burn it
+
+	if !simulate {
+		// Compute taxes again.
+		taxes := FilterMsgAndComputeTax(ctx, btfd.treasuryKeeper, msgs...)
+
+		// Record tax proceeds
+		if !taxes.IsZero() {
+			err = btfd.bankKeeper.SendCoinsFromModuleToModule(ctx, types.FeeCollectorName, treasury.BurnModuleName, taxes)
+			if err != nil {
+				return ctx, sdkerrors.Wrapf(sdkerrors.ErrInsufficientFunds, err.Error())
+			}
+		}
+	}
+
+	return next(ctx, tx, simulate)
+}

custom/auth/ante/burntax_test.go

@@ -0,0 +1,91 @@
+package ante_test
+
+import (
+	cryptotypes "github.com/cosmos/cosmos-sdk/crypto/types"
+	"github.com/cosmos/cosmos-sdk/testutil/testdata"
+	sdk "github.com/cosmos/cosmos-sdk/types"
+	banktypes "github.com/cosmos/cosmos-sdk/x/bank/types"
+
+	"github.com/terra-money/core/custom/auth/ante"
+	core "github.com/terra-money/core/types"
+	"github.com/cosmos/cosmos-sdk/types/query"
+	"github.com/cosmos/cosmos-sdk/x/auth/types"
+	minttypes "github.com/cosmos/cosmos-sdk/x/mint/types"
+)
+
+func (suite *AnteTestSuite) TestEnsureBurnTaxModule() {
+	suite.SetupTest(true) // setup
+	suite.txBuilder = suite.clientCtx.TxConfig.NewTxBuilder()
+
+	mfd := ante.NewBurnTaxFeeDecorator(suite.app.TreasuryKeeper, suite.app.BankKeeper)
+	antehandler := sdk.ChainAnteDecorators(mfd)
+
+	// keys and addresses
+	priv1, _, addr1 := testdata.KeyTestPubAddr()
+
+	// msg and signatures
+	sendAmount := int64(1000000)
+	sendCoins := sdk.NewCoins(sdk.NewInt64Coin(core.MicroSDRDenom, sendAmount))
+	msg := banktypes.NewMsgSend(addr1, addr1, sendCoins)
+
+	feeAmount := testdata.NewTestFeeAmount()
+	gasLimit := testdata.NewTestGasLimit()
+	suite.Require().NoError(suite.txBuilder.SetMsgs(msg))
+	suite.txBuilder.SetFeeAmount(feeAmount)
+	suite.txBuilder.SetGasLimit(gasLimit)
+
+	privs, accNums, accSeqs := []cryptotypes.PrivKey{priv1}, []uint64{0}, []uint64{0}
+	tx, err := suite.CreateTestTx(privs, accNums, accSeqs, suite.ctx.ChainID())
+	suite.Require().NoError(err)
+
+	// set zero gas prices
+	suite.ctx = suite.ctx.WithMinGasPrices(sdk.NewDecCoins())
+
+	// Set IsCheckTx to true
+	suite.ctx = suite.ctx.WithIsCheckTx(true)
+
+	// Luna must pass without burn before the specified tax block height
+	_, err = antehandler(suite.ctx, tx, false)
+	suite.Require().NoError(err, "Decorator should not have errored when block height is 1")
+
+	// Set the blockheight past the tax height block
+	suite.ctx = suite.ctx.WithBlockHeight(10000000)
+	// antehandler errors with insufficient fees due to tax
+	_, err = antehandler(suite.ctx, tx, false)
+	suite.Require().Error(err, "Decorator should errored on low fee for local gasPrice + tax")
+
+	tk := suite.app.TreasuryKeeper
+	expectedTax := tk.GetTaxRate(suite.ctx).MulInt64(sendAmount).TruncateInt()
+	if taxCap := tk.GetTaxCap(suite.ctx, core.MicroSDRDenom); expectedTax.GT(taxCap) {
+		expectedTax = taxCap
+	}
+
+	taxes := sdk.NewCoins(sdk.NewInt64Coin(core.MicroSDRDenom, expectedTax.Int64()))
+
+	bk := suite.app.BankKeeper
+	bk.MintCoins(suite.ctx, minttypes.ModuleName, sendCoins)
+
+	// Populate the FeeCollector module with taxes
+	bk.SendCoinsFromModuleToModule(suite.ctx, minttypes.ModuleName, types.FeeCollectorName, taxes)
+	feeCollector := suite.app.AccountKeeper.GetModuleAccount(suite.ctx, types.FeeCollectorName)
+
+	amountFee := bk.GetAllBalances(suite.ctx, feeCollector.GetAddress())
+	suite.Require().Equal(amountFee, taxes)
+	totalSupply, _, err := bk.GetPaginatedTotalSupply(suite.ctx, &query.PageRequest{})
+
+	// must pass with tax and burn
+	_, err = antehandler(suite.ctx, tx, false)
+	suite.Require().NoError(err, "Decorator should not have errored on fee higher than local gasPrice")
+
+	// Burn the taxes
+	tk.BurnCoinsFromBurnAccount(suite.ctx)
+	suite.Require().NoError(err)
+
+	supplyAfterBurn, _, err := bk.GetPaginatedTotalSupply(suite.ctx, &query.PageRequest{})
+
+	// Total supply should have decreased by the tax amount
+	suite.Require().Equal(taxes, totalSupply.Sub(supplyAfterBurn))
+
+
+}
+

custom/auth/ante/expected_keeper.go

@@ -15,3 +15,9 @@ type TreasuryKeeper interface {
 type OracleKeeper interface {
 	ValidateFeeder(ctx sdk.Context, feederAddr sdk.AccAddress, validatorAddr sdk.ValAddress) error
 }
+
+// BankKeeper defines the contract needed for supply related APIs (noalias)
+type BankKeeper interface {
+	SendCoinsFromAccountToModule(ctx sdk.Context, senderAddr sdk.AccAddress, recipientModule string, amt sdk.Coins) error
+	SendCoinsFromModuleToModule(ctx sdk.Context, senderModule string, recipientModule string, amt sdk.Coins) error
+}

custom/auth/ante/tax.go

@@ -144,14 +144,19 @@ func FilterMsgAndComputeTax(ctx sdk.Context, tk TreasuryKeeper, msgs ...sdk.Msg)
 
 // computes the stability tax according to tax-rate and tax-cap
 func computeTax(ctx sdk.Context, tk TreasuryKeeper, principal sdk.Coins) sdk.Coins {
+	currHeight := ctx.BlockHeight()
 	taxRate := tk.GetTaxRate(ctx)
 	if taxRate.Equal(sdk.ZeroDec()) {
 		return sdk.Coins{}
 	}
 
 	taxes := sdk.Coins{}
 	for _, coin := range principal {
-		if coin.Denom == core.MicroLunaDenom || coin.Denom == sdk.DefaultBondDenom {
+		// Originally only a stability tax on UST.  Changed to tax Luna as well after TaxPowerUpgradeHeight
+		if (coin.Denom == core.MicroLunaDenom || coin.Denom == sdk.DefaultBondDenom) && currHeight < TaxPowerUpgradeHeight {
+			continue
+		}
+		if coin.Denom == sdk.DefaultBondDenom {
 			continue
 		}