feat: add RefundGas function to GasMeter (cosmos#9403)

* feat: add RefundGas function to GasMeter

* changelog

* add comment about use case

* Apply suggestions from code review

Co-authored-by: Alessio Treglia <alessio@tendermint.com>
Co-authored-by: Amaury <1293565+amaurym@users.noreply.github.com>

CHANGELOG.md

@@ -128,8 +128,9 @@ if input key is empty, or input data contains empty key.
 
 ### Improvements
 
+* (store) [\#9403](https://github.com/cosmos/cosmos-sdk/pull/9403) Add `RefundGas` function to `GasMeter` interface
+* (baseapp, types) [\#9390](https://github.com/cosmos/cosmos-sdk/pull/9390) Add current block header hash to `Context`
 * (x/staking) [\#9423](https://github.com/cosmos/cosmos-sdk/pull/9423) Staking delegations now returns empty list instead of rpc error when no records found.
-* (baseapp, types) [#\9390](https://github.com/cosmos/cosmos-sdk/pull/9390) Add current block header hash to `Context`
 * (x/bank) [\#8614](https://github.com/cosmos/cosmos-sdk/issues/8614) Add `Name` and `Symbol` fields to denom metadata
 * (x/auth) [\#8522](https://github.com/cosmos/cosmos-sdk/pull/8522) Allow to query all stored accounts
 * (crypto/types) [\#8600](https://github.com/cosmos/cosmos-sdk/pull/8600) `CompactBitArray`: optimize the `NumTrueBitsBefore` method and add an `Equal` method.

store/types/gas.go

@@ -20,6 +20,12 @@ const (
 // Gas measured by the SDK
 type Gas = uint64
 
+// ErrorNegativeGasConsumed defines an error thrown when the amount of gas refunded results in a
+// negative gas consumed amount.
+type ErrorNegativeGasConsumed struct {
+	Descriptor string
+}
+
 // ErrorOutOfGas defines an error thrown when an action results in out of gas.
 type ErrorOutOfGas struct {
 	Descriptor string
@@ -37,6 +43,7 @@ type GasMeter interface {
 	GasConsumedToLimit() Gas
 	Limit() Gas
 	ConsumeGas(amount Gas, descriptor string)
+	RefundGas(amount Gas, descriptor string)
 	IsPastLimit() bool
 	IsOutOfGas() bool
 	String() string
@@ -91,7 +98,20 @@ func (g *basicGasMeter) ConsumeGas(amount Gas, descriptor string) {
 	if g.consumed > g.limit {
 		panic(ErrorOutOfGas{descriptor})
 	}
+}
+
+// RefundGas will deduct the given amount from the gas consumed. If the amount is greater than the
+// gas consumed, the function will panic.
+//
+// Use case: This functionality enables refunding gas to the transaction or block gas pools so that
+// EVM-compatible chains can fully support the go-ethereum StateDb interface.
+// See https://github.com/cosmos/cosmos-sdk/pull/9403 for reference.
+func (g *basicGasMeter) RefundGas(amount Gas, descriptor string) {
+	if g.consumed < amount {
+		panic(ErrorNegativeGasConsumed{Descriptor: descriptor})
+	}
 
+	g.consumed -= amount
 }
 
 func (g *basicGasMeter) IsPastLimit() bool {
@@ -138,6 +158,20 @@ func (g *infiniteGasMeter) ConsumeGas(amount Gas, descriptor string) {
 	}
 }
 
+// RefundGas will deduct the given amount from the gas consumed. If the amount is greater than the
+// gas consumed, the function will panic.
+//
+// Use case: This functionality enables refunding gas to the trasaction or block gas pools so that
+// EVM-compatible chains can fully support the go-ethereum StateDb interface.
+// See https://github.com/cosmos/cosmos-sdk/pull/9403 for reference.
+func (g *infiniteGasMeter) RefundGas(amount Gas, descriptor string) {
+	if g.consumed < amount {
+		panic(ErrorNegativeGasConsumed{Descriptor: descriptor})
+	}
+
+	g.consumed -= amount
+}
+
 func (g *infiniteGasMeter) IsPastLimit() bool {
 	return false
 }

store/types/gas_test.go

@@ -16,10 +16,13 @@ func TestInfiniteGasMeter(t *testing.T) {
 	meter.ConsumeGas(10, "consume 10")
 	require.Equal(t, uint64(10), meter.GasConsumed())
 	require.Equal(t, uint64(10), meter.GasConsumedToLimit())
+	meter.RefundGas(1, "refund 1")
+	require.Equal(t, uint64(9), meter.GasConsumed())
 	require.False(t, meter.IsPastLimit())
 	require.False(t, meter.IsOutOfGas())
 	meter.ConsumeGas(Gas(math.MaxUint64/2), "consume half max uint64")
 	require.Panics(t, func() { meter.ConsumeGas(Gas(math.MaxUint64/2)+2, "panic") })
+	require.Panics(t, func() { meter.RefundGas(meter.GasConsumed()+1, "refund greater than consumed") })
 }
 
 func TestGasMeter(t *testing.T) {
@@ -57,6 +60,11 @@ func TestGasMeter(t *testing.T) {
 		require.Panics(t, func() { meter.ConsumeGas(1, "") }, "Exceeded but not panicked. tc #%d", tcnum)
 		require.Equal(t, meter.GasConsumedToLimit(), meter.Limit(), "Gas consumption (to limit) not match limit")
 		require.Equal(t, meter.GasConsumed(), meter.Limit()+1, "Gas consumption not match limit+1")
+
+		require.NotPanics(t, func() { meter.RefundGas(1, "refund 1") })
+		require.Equal(t, meter.GasConsumed(), meter.Limit(), "Gas consumption not match limit+1")
+		require.Panics(t, func() { meter.RefundGas(meter.GasConsumed()+1, "refund greater than consumed") })
+
 		meter2 := NewGasMeter(math.MaxUint64)
 		meter2.ConsumeGas(Gas(math.MaxUint64/2), "consume half max uint64")
 		require.Panics(t, func() { meter2.ConsumeGas(Gas(math.MaxUint64/2)+2, "panic") })