Make binary serialization of value keys appendable

The previous binary serialisation format for value-keys, which is a 2D
slice of bytes, started the number rows. This meant that given a set of
bytes the function could determine how many rows to expect. But the
tradeoff is that in order to append a new row to an existing serialized
value-keys one had to change the uvarint representing the number of
rows at the beginning of the value. Because the size is a uvarint, this
could mean shifting bytes, etc.

Instead, do not bother with storing the number of rows at all. Instead,
simply keep reading a uvarint-sized bytes and expect that reading the
last row should result in no leftover bytes.

This allows further optimisations inside the store implementations,
where rather than reading the entire value of value-keys from the store
adding a value and re-serializing it, one could simply append a new
value to the end of the already serialized value-keys.

value.go

@@ -5,7 +5,6 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"math"
 
 	"github.com/libp2p/go-libp2p-core/peer"
 	"github.com/multiformats/go-varint"
@@ -165,7 +164,6 @@ func (BinaryValueSerde) UnmarshalValue(b []byte) (Value, error) {
 
 func (BinaryValueSerde) MarshalValueKeys(vk [][]byte) ([]byte, error) {
 	var buf bytes.Buffer
-	buf.Write(varint.ToUvarint(uint64(len(vk))))
 	for _, v := range vk {
 		vl := len(v)
 		uvl := uint64(vl)
@@ -184,24 +182,8 @@ func (BinaryValueSerde) MarshalValueKeys(vk [][]byte) ([]byte, error) {
 func (BinaryValueSerde) UnmarshalValueKeys(b []byte) ([][]byte, error) {
 	var vk [][]byte
 	buf := bytes.NewBuffer(b)
-
-	// Decode vk length.
-	ulen, err := varint.ReadUvarint(buf)
-	if err != nil {
-		return vk, err
-	}
-	if ulen > math.MaxUint32 {
-		return vk, ErrSerdeOverflow
-	}
-	l := int(ulen)
-	// There should at least be l number of bytes since length of each inner byte slice length
-	// should have been written, even if it was zero, and minimum size of a uvarint is a byte.
-	if l < 0 || l > buf.Len() {
-		return vk, ErrSerdeOverflow
-	}
-
 	// Decode each value key.
-	for i := 0; i < l; i++ {
+	for buf.Len() != 0 {
 		usize, err := varint.ReadUvarint(buf)
 		if err != nil {
 			return vk, err
@@ -212,8 +194,5 @@ func (BinaryValueSerde) UnmarshalValueKeys(b []byte) ([][]byte, error) {
 		}
 		vk = append(vk, buf.Next(size))
 	}
-	if buf.Len() != 0 {
-		return vk, fmt.Errorf("too many bytes; %d remain unread", buf.Len())
-	}
 	return vk, nil
 }

value_serde_test.go

@@ -145,7 +145,7 @@ func TestBinaryValueSerde_MarshalUnmarshalEmptyValues(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if !reflect.DeepEqual(svk, []byte{0}) {
+	if len(svk) != 0 {
 		t.Fatal()
 	}
 
@@ -154,7 +154,7 @@ func TestBinaryValueSerde_MarshalUnmarshalEmptyValues(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if !reflect.DeepEqual(svk, []byte{1, 0}) {
+	if !reflect.DeepEqual(svk, []byte{0}) {
 		t.Fatal()
 	}
 }