fix: hookstest.Stub JSON marshalling when persisting ChainConfig (#251)

## Why this should be merged

Allow calls to `core.SetupGenesisBlock()` when `hookstest.Stub` is
registered as a `ChainConfig` extra.

## How this works

Implements `json.Marshaler` and `json.Unmarshaler` interfaces.

## How this was tested

Unit test of the root cause and the desired usage.

libevm/hookstest/stub.go

@@ -19,6 +19,7 @@
 package hookstest
 
 import (
+	"encoding/json"
 	"math/big"
 	"testing"
 
@@ -62,6 +63,14 @@ func (s *Stub) Register(tb testing.TB) params.ExtraPayloads[*Stub, *Stub] {
 	})
 }
 
+// MarshalJSON implements [json.Marshaler] and always returns the JSON null
+// object. This avoids errors due to incompatible field types.
+func (Stub) MarshalJSON() ([]byte, error) { return []byte(`null`), nil }
+
+// UnmarshalJSON implements [json.Unmarshaler] and always returns nil, ignoring
+// its input.
+func (Stub) UnmarshalJSON([]byte) error { return nil }
+
 // PrecompileOverride uses the s.PrecompileOverrides map, if non-empty, as the
 // canonical source of all overrides. If the map is empty then no precompiles
 // are overridden.
@@ -135,4 +144,6 @@ func (s Stub) MinimumGasConsumption(limit uint64) uint64 {
 var _ interface {
 	params.ChainConfigHooks
 	params.RulesHooks
+	json.Marshaler
+	json.Unmarshaler
 } = Stub{}

libevm/hookstest/stub_test.go

@@ -0,0 +1,59 @@
+// 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
+// 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 hookstest
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"golang.org/x/exp/slog"
+
+	"github.com/ava-labs/libevm/common"
+	"github.com/ava-labs/libevm/core"
+	"github.com/ava-labs/libevm/core/rawdb"
+	"github.com/ava-labs/libevm/libevm/ethtest"
+	"github.com/ava-labs/libevm/log"
+	"github.com/ava-labs/libevm/params"
+)
+
+func TestSetupGenesisBlockWithStub(t *testing.T) {
+	// The original bug was due to [Stub] resulting in an error when being
+	// marshalled to JSON for [rawdb.WriteChainConfig], which resulted in
+	// [log.Crit] being called.
+	l := log.Root()
+	t.Cleanup(func() { log.SetDefault(l) })
+	log.SetDefault(log.NewLogger(slog.NewTextHandler(os.Stderr, nil)))
+
+	stub := &Stub{}
+	extras := stub.Register(t)
+
+	config := &params.ChainConfig{}
+	extras.ChainConfig.Set(config, stub)
+	gen := &core.Genesis{
+		Config: config,
+	}
+
+	db, cache, _ := ethtest.NewEmptyStateDB(t)
+
+	// An eventual call to this function was the root cause.
+	rawdb.WriteChainConfig(db, common.Hash{1, 2, 3, 4, 5, 6}, config)
+
+	// Also check calls to this function, which was the desired behaviour.
+	_, _, err := core.SetupGenesisBlock(db, cache.TrieDB(), gen)
+	require.NoError(t, err, "core.SetupGenesisBlock([%T with %T as registered %T extra])", gen, stub, config)
+}