eth/protocols/eth: fix blockAccessList empty marker (#35286)

This PR addresses an issue in the eth71 `BlockAccessListsMsg` handler,

specifically: 
- if the requested bal is not accessible in the server side, 0x80
(EmptyString) will be returned as the marker
- at the client side, old message definition
`rlp.RawList[RawBlockAccessList]` assumes all the elements are List
- the message with 0x80 (kind = string) won't be decoded correctly
- the peer will be disconnected

The message definition has been changed to `rlp.RawList[rlp.RawValue]`,
which is aligned with the one in SNAP/2 protocol.

eth/protocols/eth/handler_test.go

@@ -742,7 +742,7 @@ func testGetBlockAccessLists(t *testing.T, protocol uint) {
 
 	var (
 		hashes []common.Hash
-		expect rlp.RawList[RawBlockAccessList]
+		expect rlp.RawList[rlp.RawValue]
 	)
 	for i := uint64(0); i <= backend.chain.CurrentBlock().Number.Uint64(); i++ {
 		block := backend.chain.GetBlockByNumber(i)
@@ -771,6 +771,42 @@ func testGetBlockAccessLists(t *testing.T, protocol uint) {
 	}
 }
 
+// TestBlockAccessListsUnavailableDecode checks that a BlockAccessLists response
+// containing the EIP-8159 unavailability marker (RLP empty string).
+func TestBlockAccessListsUnavailableDecode(t *testing.T) {
+	t.Parallel()
+
+	balRaw := makeTestBAL(t, common.Address{0x11})
+
+	// Assemble a response the way the serving side does, with the middle
+	// entry signaled as unavailable.
+	var list rlp.RawList[rlp.RawValue]
+	list.AppendRaw(balRaw)
+	list.AppendRaw(rlp.EmptyString)
+	list.AppendRaw(balRaw)
+
+	enc, err := rlp.EncodeToBytes(&BlockAccessListPacket{RequestId: 42, List: list})
+	if err != nil {
+		t.Fatalf("failed to encode packet: %v", err)
+	}
+	var packet BlockAccessListPacket
+	if err := rlp.DecodeBytes(enc, &packet); err != nil {
+		t.Fatalf("failed to decode packet: %v", err)
+	}
+	bals, err := packet.List.Items()
+	if err != nil {
+		t.Fatalf("failed to decode BAL entries: %v", err)
+	}
+	if len(bals) != 3 {
+		t.Fatalf("wrong entry count: got %d, want 3", len(bals))
+	}
+	for i, want := range [][]byte{balRaw, rlp.EmptyString, balRaw} {
+		if !bytes.Equal(bals[i], want) {
+			t.Errorf("entry %d mismatch: got %x, want %x", i, bals[i], want)
+		}
+	}
+}
+
 type decoder struct {
 	msg []byte
 }

eth/protocols/eth/handlers.go

@@ -679,10 +679,10 @@ func handleGetBlockAccessLists(backend Backend, msg Decoder, peer *Peer) error {
 
 // serviceGetBlockAccessListsQuery assembles the response to a BAL query.
 // Unavailable BALs are returned as empty list entries.
-func serviceGetBlockAccessListsQuery(chain *core.BlockChain, query GetBlockAccessListsRequest) rlp.RawList[RawBlockAccessList] {
+func serviceGetBlockAccessListsQuery(chain *core.BlockChain, query GetBlockAccessListsRequest) rlp.RawList[rlp.RawValue] {
 	var (
 		bytes int
-		bals  rlp.RawList[RawBlockAccessList]
+		bals  rlp.RawList[rlp.RawValue]
 	)
 	for _, hash := range query {
 		if bytes >= softResponseLimit || bals.Len() >= maxBALsServe {
@@ -720,7 +720,12 @@ func handleBlockAccessLists(backend Backend, msg Decoder, peer *Peer) error {
 	metadata := func() interface{} {
 		hashes := make([]common.Hash, len(bals))
 		for i := range bals {
-			hashes[i] = crypto.Keccak256Hash(bals[i].Bytes())
+			// Unavailable BALs (signaled by the empty string) are marked
+			// with the zero hash
+			if bytes.Equal(bals[i], rlp.EmptyString) {
+				continue
+			}
+			hashes[i] = crypto.Keccak256Hash(bals[i])
 		}
 		return hashes
 	}

eth/protocols/eth/peer.go

@@ -258,7 +258,7 @@ func (p *Peer) ReplyReceiptsRLP70(id uint64, receipts rlp.RawList[*ReceiptList],
 }
 
 // ReplyBlockAccessLists is the response to GetBlockAccessLists (EIP-8159).
-func (p *Peer) ReplyBlockAccessLists(id uint64, list rlp.RawList[RawBlockAccessList]) error {
+func (p *Peer) ReplyBlockAccessLists(id uint64, list rlp.RawList[rlp.RawValue]) error {
 	return p2p.Send(p.rw, BlockAccessListsMsg, &BlockAccessListPacket{
 		RequestId: id,
 		List:      list,

eth/protocols/eth/protocol.go

@@ -24,7 +24,6 @@ import (
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core/forkid"
 	"github.com/ethereum/go-ethereum/core/types"
-	"github.com/ethereum/go-ethereum/core/types/bal"
 	"github.com/ethereum/go-ethereum/rlp"
 )
 
@@ -299,15 +298,14 @@ type GetBlockAccessListsPacket struct {
 	GetBlockAccessListsRequest
 }
 
-type RawBlockAccessList struct {
-	rlp.RawList[bal.AccountAccess]
-}
-
-type BlockAccessListResponse []RawBlockAccessList
+// BlockAccessListResponse holds one raw entry per requested hash. Entries are
+// kept as raw values because, per EIP-8159, the RLP empty string signals an
+// unavailable BAL (an empty list is itself a valid BAL).
+type BlockAccessListResponse []rlp.RawValue
 
 type BlockAccessListPacket struct {
 	RequestId uint64
-	List      rlp.RawList[RawBlockAccessList]
+	List      rlp.RawList[rlp.RawValue]
 }
 
 func (*StatusPacket) Name() string { return "Status" }