fix: state.stateObject.empty() with extra payload (#62)

## Why this should be merged

Fixes invariant broken by introduction of `types.StateAccountExtra`.

## How this works

`state.stateObject.empty()` now also requires that the underlying
account's `Extra` field carries the zero value if a type is registered,
or is itself nil.

## How this was tested

New unit test.

---------

Co-authored-by: Darioush Jalali <darioush.jalali@avalabs.org>

core/state/state_object.go

@@ -93,7 +93,7 @@ type stateObject struct {
 
 // empty returns whether the account is considered empty.
 func (s *stateObject) empty() bool {
-	return s.data.Nonce == 0 && s.data.Balance.IsZero() && bytes.Equal(s.data.CodeHash, types.EmptyCodeHash.Bytes())
+	return s.data.Nonce == 0 && s.data.Balance.IsZero() && bytes.Equal(s.data.CodeHash, types.EmptyCodeHash.Bytes()) && s.data.Extra.IsZero()
 }
 
 // newObject creates a state object.

core/state/state_object.libevm_test.go

@@ -0,0 +1,79 @@
+// Copyright 2024 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 state
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/ava-labs/libevm/common"
+	"github.com/ava-labs/libevm/core/types"
+)
+
+func TestStateObjectEmpty(t *testing.T) {
+	tests := []struct {
+		name           string
+		registerAndSet func(*types.StateAccount)
+		wantEmpty      bool
+	}{
+		{
+			name:           "no registered types.StateAccount extra payload",
+			registerAndSet: func(*types.StateAccount) {},
+			wantEmpty:      true,
+		},
+		{
+			name: "erroneously non-nil types.StateAccountExtra when no registered payload",
+			registerAndSet: func(acc *types.StateAccount) {
+				acc.Extra = &types.StateAccountExtra{}
+			},
+			wantEmpty: true,
+		},
+		{
+			name: "explicit false bool",
+			registerAndSet: func(acc *types.StateAccount) {
+				types.RegisterExtras[bool]().SetOnStateAccount(acc, false)
+			},
+			wantEmpty: true,
+		},
+		{
+			name: "implicit false bool",
+			registerAndSet: func(*types.StateAccount) {
+				types.RegisterExtras[bool]()
+			},
+			wantEmpty: true,
+		},
+		{
+			name: "true bool",
+			registerAndSet: func(acc *types.StateAccount) {
+				types.RegisterExtras[bool]().SetOnStateAccount(acc, true)
+			},
+			wantEmpty: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			types.TestOnlyClearRegisteredExtras()
+			t.Cleanup(types.TestOnlyClearRegisteredExtras)
+
+			obj := newObject(nil, common.Address{}, nil)
+			tt.registerAndSet(&obj.data)
+			require.Equalf(t, tt.wantEmpty, obj.empty(), "%T.empty()", obj)
+		})
+	}
+}

core/types/rlp_payload.libevm.go

@@ -160,6 +160,13 @@ func (e *StateAccountExtra) Equal(f *StateAccountExtra) bool {
 	return e.t.Equal(f.t)
 }
 
+// IsZero reports whether e carries the the zero value for its type, as
+// registered via [RegisterExtras]. It returns true if no type was registered or
+// if `e == nil`.
+func (e *StateAccountExtra) IsZero() bool {
+	return e == nil || e.t == nil || e.t.IsZero()
+}
+
 var _ interface {
 	rlp.Encoder
 	rlp.Decoder