rlp, eth: batch-allocate headers and reuse stream in decode path

eth/protocols/eth/handlers.go

@@ -21,6 +21,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"math"
+	"math/big"
 
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core"
@@ -374,15 +375,31 @@ func handleBlockHeaders(backend Backend, msg Decoder, peer *Peer) error {
 	}, metadata)
 }
 
+type headerAlloc struct {
+	header     types.Header
+	difficulty big.Int
+	number     big.Int
+}
+
 func decodeBlockHeaders(list *rlp.RawList[*types.Header]) ([]*types.Header, error) {
 	headers := make([]*types.Header, 0, list.Len())
+	stream := rlp.NewBytesStream(nil)
+	defer rlp.PutStream(stream)
+
 	it := list.ContentIterator()
 	for it.Next() {
-		header := new(types.Header)
-		if err := types.DecodeHeader(it.Value(), header); err != nil {
+		a := new(headerAlloc)
+		a.header.Difficulty = &a.difficulty
+		a.header.Number = &a.number
+
+		stream.ResetBytes(it.Value())
+		if err := a.header.DecodeRLP(stream); err != nil {
 			return headers, err
 		}
-		headers = append(headers, header)
+		if stream.Remaining() != 0 {
+			return headers, rlp.ErrMoreThanOneValue
+		}
+		headers = append(headers, &a.header)
 	}
 	return headers, nil
 }

rlp/decode.go

@@ -642,6 +642,14 @@ func PutStream(stream *Stream) {
 	streamPool.Put(stream)
 }
 
+// ResetBytes resets the stream to decode from b, reusing the inline slice
+// reader. This allows decoding a batch of values with one pooled stream
+// instead of doing a pool round-trip per value.
+func (s *Stream) ResetBytes(b []byte) {
+	s.sliceRdr = b
+	s.Reset(&s.sliceRdr, uint64(len(b)))
+}
+
 // NewListStream creates a new stream that pretends to be positioned
 // at an encoded list of the given length.
 func NewListStream(r io.Reader, len uint64) *Stream {