[#1262] metabase: Remove intermediate allocations in decodeList

```
name              old time/op    new time/op    delta
Put/parallel-8      1.57ms ±11%    1.51ms ± 3%   -4.06%  (p=0.043 n=9+10)
Put/sequential-8    5.16ms ± 2%    5.16ms ± 3%     ~     (p=1.000 n=9+10)

name              old alloc/op   new alloc/op   delta
Put/parallel-8       126kB ± 4%     123kB ± 4%   -2.54%  (p=0.016 n=8+10)
Put/sequential-8     171kB ± 1%     170kB ± 1%     ~     (p=0.182 n=9+10)

name              old allocs/op  new allocs/op  delta
Put/parallel-8         565 ± 2%       473 ± 1%  -16.18%  (p=0.000 n=9+10)
Put/sequential-8       819 ± 1%       792 ± 0%   -3.34%  (p=0.000 n=9+10)
```

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

pkg/local_object_storage/metabase/index_test.go

@@ -0,0 +1,62 @@
+package meta
+
+import (
+	"math"
+	"math/rand"
+	"testing"
+
+	"github.com/nspcc-dev/neo-go/pkg/io"
+	"github.com/stretchr/testify/require"
+)
+
+func Test_getVarUint(t *testing.T) {
+	data := make([]byte, 10)
+	for _, val := range []uint64{0, 0xfc, 0xfd, 0xfffe, 0xffff, 0xfffffffe, 0xffffffff, math.MaxUint64} {
+		expSize := io.PutVarUint(data, val)
+		actual, actSize, err := getVarUint(data)
+		require.NoError(t, err)
+		require.Equal(t, val, actual)
+		require.Equal(t, expSize, actSize, "value: %x", val)
+
+		_, _, err = getVarUint(data[:expSize-1])
+		require.Error(t, err)
+	}
+}
+
+func Test_decodeList(t *testing.T) {
+	t.Run("empty", func(t *testing.T) {
+		lst, err := decodeList(nil)
+		require.NoError(t, err)
+		require.True(t, len(lst) == 0)
+	})
+	t.Run("empty, 0 len", func(t *testing.T) {
+		lst, err := decodeList([]byte{0})
+		require.NoError(t, err)
+		require.True(t, len(lst) == 0)
+	})
+	t.Run("bad len", func(t *testing.T) {
+		_, err := decodeList([]byte{0xfe})
+		require.Error(t, err)
+	})
+	t.Run("random", func(t *testing.T) {
+		expected := make([][]byte, 20)
+		for i := range expected {
+			expected[i] = make([]byte, rand.Uint32()%10)
+			rand.Read(expected[i])
+		}
+
+		data, err := encodeList(expected)
+		require.NoError(t, err)
+
+		actual, err := decodeList(data)
+		require.NoError(t, err)
+		require.Equal(t, expected, actual)
+
+		t.Run("unexpected EOF", func(t *testing.T) {
+			for i := 1; i < len(data)-1; i++ {
+				_, err := decodeList(data[:i])
+				require.Error(t, err)
+			}
+		})
+	})
+}

pkg/local_object_storage/metabase/put.go

@@ -1,8 +1,10 @@
 package meta
 
 import (
+	"encoding/binary"
 	"errors"
 	"fmt"
+	gio "io"
 
 	"github.com/nspcc-dev/neo-go/pkg/io"
 	"github.com/nspcc-dev/neofs-node/pkg/core/object"
@@ -377,18 +379,58 @@ func decodeList(data []byte) (lst [][]byte, err error) {
 	if len(data) == 0 {
 		return nil, nil
 	}
-	r := io.NewBinReaderFromBuf(data)
-	l := r.ReadVarUint()
-	lst = make([][]byte, l, l+1)
-	for i := range lst {
-		lst[i] = r.ReadVarBytes()
+
+	var offset uint64
+	size, n, err := getVarUint(data)
+	if err != nil {
+		return nil, err
 	}
-	if r.Err != nil {
-		return nil, r.Err
+
+	offset += uint64(n)
+	lst = make([][]byte, size, size+1)
+	for i := range lst {
+		sz, n, err := getVarUint(data[offset:])
+		if err != nil {
+			return nil, err
+		}
+		offset += uint64(n)
+
+		next := offset + sz
+		if uint64(len(data)) < next {
+			return nil, gio.ErrUnexpectedEOF
+		}
+		lst[i] = data[offset:next]
+		offset = next
 	}
 	return lst, nil
 }
 
+func getVarUint(data []byte) (uint64, int, error) {
+	if len(data) == 0 {
+		return 0, 0, gio.ErrUnexpectedEOF
+	}
+
+	switch b := data[0]; b {
+	case 0xfd:
+		if len(data) < 3 {
+			return 0, 1, gio.ErrUnexpectedEOF
+		}
+		return uint64(binary.LittleEndian.Uint16(data[1:])), 3, nil
+	case 0xfe:
+		if len(data) < 5 {
+			return 0, 1, gio.ErrUnexpectedEOF
+		}
+		return uint64(binary.LittleEndian.Uint32(data[1:])), 5, nil
+	case 0xff:
+		if len(data) < 9 {
+			return 0, 1, gio.ErrUnexpectedEOF
+		}
+		return binary.LittleEndian.Uint64(data[1:]), 9, nil
+	default:
+		return uint64(b), 1, nil
+	}
+}
+
 // updateBlobovniczaID for existing objects if they were moved from from
 // one blobovnicza to another.
 func updateBlobovniczaID(tx *bbolt.Tx, addr *addressSDK.Address, id *blobovnicza.ID) error {