eth: return error instead of panicking on debug_executionWitness (#35544)

eth/api_debug.go

@@ -500,6 +500,12 @@ func (api *DebugAPI) ExecutionWitness(bn rpc.BlockNumberOrHash) (*stateless.ExtW
 	if err != nil {
 		return &stateless.ExtWitness{}, fmt.Errorf("block %v not found", bn)
 	}
+	// BlockByNumberOrHash returns a nil block without an error when the
+	// requested block does not exist (per the RPC spec). Guard against it
+	// to avoid a nil pointer dereference below.
+	if block == nil {
+		return &stateless.ExtWitness{}, fmt.Errorf("block %v not found", bn)
+	}
 	parent := bc.GetHeader(block.ParentHash(), block.NumberU64()-1)
 	if parent == nil {
 		return &stateless.ExtWitness{}, fmt.Errorf("block %v found, but parent missing", bn)

eth/api_debug_test.go

@@ -41,6 +41,7 @@ import (
 	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/ethereum/go-ethereum/crypto"
 	"github.com/ethereum/go-ethereum/params"
+	"github.com/ethereum/go-ethereum/rpc"
 	"github.com/ethereum/go-ethereum/triedb"
 	"github.com/holiman/uint256"
 	"github.com/stretchr/testify/assert"
@@ -340,6 +341,34 @@ func TestGetModifiedAccounts(t *testing.T) {
 	})
 }
 
+// TestExecutionWitnessMissingBlock ensures that debug_executionWitness returns
+// an error, rather than panicking, when the requested block does not exist.
+// BlockByNumberOrHash returns a nil block without an error for an unknown hash,
+// which previously caused a nil pointer dereference.
+func TestExecutionWitnessMissingBlock(t *testing.T) {
+	t.Parallel()
+
+	accounts := newAccounts(1)
+	genesis := &core.Genesis{
+		Config: params.TestChainConfig,
+		Alloc: types.GenesisAlloc{
+			accounts[0].addr: {Balance: big.NewInt(params.Ether)},
+		},
+	}
+	blockChain := newTestBlockChain(t, 1, genesis, func(_ int, _ *core.BlockGen) {})
+	defer blockChain.Stop()
+
+	eth := &Ethereum{blockchain: blockChain}
+	eth.APIBackend = &EthAPIBackend{eth: eth}
+	api := NewDebugAPI(eth)
+
+	// A hash that does not correspond to any known block. This makes
+	// BlockByNumberOrHash return (nil, nil).
+	missing := rpc.BlockNumberOrHashWithHash(common.HexToHash("0xdeadbeef"), false)
+	_, err := api.ExecutionWitness(missing)
+	assert.Error(t, err, "expected an error for a missing block, got nil")
+}
+
 func TestDebugAPI_ClearTxpool(t *testing.T) {
 	// Create test key and genesis
 	testKey, _ := crypto.HexToECDSA("b71c71a67e1177ad4e901695e1b4b9ee17ae16c6668d313eac2f96dbcda3f291")