capability: allow to decode unknown capabilities

It's more symmetric than the C# counterpart, but the essence is the same: we
can accept capabilities we can't interpret. Refs. neo-project/neo#3639

Signed-off-by: Roman Khimov <roman@nspcc.ru>

pkg/network/capability/capability.go

@@ -6,8 +6,13 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/io"
 )
 
-// MaxCapabilities is the maximum number of capabilities per payload.
-const MaxCapabilities = 32
+const (
+	// MaxCapabilities is the maximum number of capabilities per payload.
+	MaxCapabilities = 32
+
+	// MaxDataSize is the maximum size of capability payload.
+	MaxDataSize = 1024
+)
 
 // Capabilities is a list of Capability.
 type Capabilities []Capability
@@ -65,8 +70,7 @@ func (c *Capability) DecodeBinary(br *io.BinReader) {
 	case TCPServer, WSServer:
 		c.Data = &Server{}
 	default:
-		br.Err = errors.New("unknown node capability type")
-		return
+		c.Data = &Unknown{}
 	}
 	c.Data.DecodeBinary(br)
 }
@@ -111,3 +115,19 @@ func (s *Server) DecodeBinary(br *io.BinReader) {
 func (s *Server) EncodeBinary(bw *io.BinWriter) {
 	bw.WriteU16LE(s.Port)
 }
+
+// Unknown represents an unknown capability with some data. Other nodes can
+// decode it even if they can't interpret it. This is not expected to be used
+// for sending data directly (proper new types should be used), but it allows
+// for easier protocol extensibility (old nodes won't reject new capabilities).
+type Unknown []byte
+
+// DecodeBinary implements io.Serializable.
+func (u *Unknown) DecodeBinary(br *io.BinReader) {
+	*u = br.ReadVarBytes()
+}
+
+// EncodeBinary implements io.Serializable.
+func (u *Unknown) EncodeBinary(bw *io.BinWriter) {
+	bw.WriteVarBytes(*u)
+}

pkg/network/capability/capability_test.go

@@ -0,0 +1,15 @@
+package capability
+
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neo-go/internal/testserdes"
+)
+
+func TestUnknownEncodeDecode(t *testing.T) {
+	var (
+		u  = Unknown{0x55, 0xaa}
+		ud Unknown
+	)
+	testserdes.EncodeDecodeBinary(t, &u, &ud)
+}

pkg/network/payload/version_test.go

@@ -29,12 +29,20 @@ func TestVersionEncodeDecode(t *testing.T) {
 				Port: wsPort,
 			},
 		},
+		{
+			Type: 0xff,
+			Data: &capability.Unknown{},
+		},
 		{
 			Type: capability.FullNode,
 			Data: &capability.Node{
 				StartHeight: height,
 			},
 		},
+		{
+			Type: 0xf0,
+			Data: &capability.Unknown{0x55, 0xaa},
+		},
 	}
 
 	version := NewVersion(magic, id, useragent, capabilities)