fix(libevm/legacy): PrecompiledStatefulContract gas and remaining gas handling (#114)

- remaining gas higher than the input gas is not allowed (otherwise it would overflow as well)
- remaining gas can be equal to the input gas
- add new test and `vm.PrecompileEnvironment` stub

libevm/legacy/legacy.go

@@ -1,4 +1,4 @@
-// Copyright 2024 the libevm authors.
+// Copyright 2024-2025 the libevm authors.
 //
 // The libevm additions to go-ethereum are free software: you can redistribute
 // them and/or modify them under the terms of the GNU Lesser General Public License
@@ -18,7 +18,16 @@
 // equivalents.
 package legacy
 
-import "github.com/ava-labs/libevm/core/vm"
+import (
+	"errors"
+	"fmt"
+
+	"github.com/ava-labs/libevm/core/vm"
+)
+
+var (
+	errRemainingGasExceedsSuppliedGas = errors.New("remaining gas exceeds supplied gas")
+)
 
 // PrecompiledStatefulContract is the legacy signature of
 // [vm.PrecompiledStatefulContract], which explicitly accepts and returns gas
@@ -31,8 +40,11 @@ func (c PrecompiledStatefulContract) Upgrade() vm.PrecompiledStatefulContract {
 	return func(env vm.PrecompileEnvironment, input []byte) ([]byte, error) {
 		gas := env.Gas()
 		ret, remainingGas, err := c(env, input, gas)
-		if used := gas - remainingGas; used < gas {
+		if remainingGas > gas {
-			env.UseGas(used)
+			return ret, fmt.Errorf("%w: %d > %d", errRemainingGasExceedsSuppliedGas, remainingGas, gas)
+		}
+		if !env.UseGas(gas - remainingGas) {
+			return ret, vm.ErrOutOfGas
 		}
 		return ret, err
 	}

libevm/legacy/legacy_test.go

@@ -0,0 +1,111 @@
+// Copyright 2025 the libevm authors.
+//
+// The libevm additions to go-ethereum are free software: you can redistribute
+// them and/or modify them under the terms of the GNU Lesser General Public License
+// as published by the Free Software Foundation, either version 3 of the License,
+// or (at your option) any later version.
+//
+// The libevm additions are distributed in the hope that they will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser
+// General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see
+// <http://www.gnu.org/licenses/>.
+
+package legacy
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/ava-labs/libevm/core/vm"
+)
+
+// stubPrecompileEnvironment implements [vm.PrecompileEnvironment] for testing.
+type stubPrecompileEnvironment struct {
+	vm.PrecompileEnvironment
+	gas uint64
+}
+
+func (s *stubPrecompileEnvironment) Gas() uint64 {
+	return s.gas
+}
+
+func (s *stubPrecompileEnvironment) UseGas(gas uint64) (hasEnoughGas bool) {
+	if s.gas < gas {
+		return false
+	}
+	s.gas -= gas
+	return true
+}
+
+func TestPrecompiledStatefulContract_Upgrade(t *testing.T) {
+	t.Parallel()
+
+	errTest := errors.New("test error")
+
+	tests := map[string]struct {
+		suppliedGas   uint64
+		precompileRet []byte
+		remainingGas  uint64
+		precompileErr error
+		wantErr       error
+		wantGas       uint64
+	}{
+		"call_error": {
+			suppliedGas:   10,
+			precompileRet: []byte{2},
+			remainingGas:  6,
+			precompileErr: errTest,
+			wantErr:       errTest,
+			wantGas:       6,
+		},
+		"remaining_gas_exceeds_supplied_gas": {
+			suppliedGas:   10,
+			precompileRet: []byte{2},
+			remainingGas:  11,
+			wantErr:       errRemainingGasExceedsSuppliedGas,
+			wantGas:       10,
+		},
+		"zero_remaining_gas": {
+			suppliedGas:   10,
+			precompileRet: []byte{2},
+			remainingGas:  0,
+			wantGas:       0,
+		},
+		"used_one_gas": {
+			suppliedGas:   10,
+			precompileRet: []byte{2},
+			remainingGas:  9,
+			wantGas:       9,
+		},
+	}
+
+	for name, test := range tests {
+		testCase := test
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			c := PrecompiledStatefulContract(func(env vm.PrecompileEnvironment, input []byte, suppliedGas uint64) (ret []byte, remainingGas uint64, err error) {
+				return testCase.precompileRet, testCase.remainingGas, testCase.precompileErr
+			})
+
+			upgraded := c.Upgrade()
+
+			env := &stubPrecompileEnvironment{
+				gas: testCase.suppliedGas,
+			}
+			input := []byte("unused")
+
+			ret, err := upgraded(env, input)
+			require.ErrorIs(t, err, testCase.wantErr)
+			assert.Equal(t, testCase.precompileRet, ret, "bytes returned by upgraded contract")
+			assert.Equalf(t, testCase.wantGas, env.gas, "remaining gas in %T", env)
+		})
+	}
+}