more better CNPeer.Send()

this fixes a race condition where if 2 goroutines try to send a packet at the same time, they could end up being
malformed due to the 2 separate calls to peer.conn.Write().

instead of writing the packet size to peer.conn.Write() directly, we make space in buf for the packet size,
and patch it in place. this lets us get away with only having 1 call to peer.conn.Write() which will ensure that
the full packet is written properly and be goroutine safe :3

core/protocol/cnpeer.go

@@ -51,6 +51,9 @@ func (peer *CNPeer) Send(typeID uint32, data ...interface{}) error {
 	buf := pool.Get()
 	defer pool.Put(buf)
 
+	// allocate space for packet size
+	buf.Write(make([]byte, 4))
+
 	// body start
 	pkt := NewPacket(buf)
 
@@ -66,25 +69,22 @@ func (peer *CNPeer) Send(typeID uint32, data ...interface{}) error {
 		}
 	}
 
+	// prepend the packet size
+	binary.LittleEndian.PutUint32(buf.Bytes()[:4], uint32(buf.Len()-4))
+
 	// encrypt body
 	switch peer.whichKey {
 	case USE_E:
-		EncryptData(buf.Bytes(), peer.E_key)
+		EncryptData(buf.Bytes()[4:], peer.E_key)
 	case USE_FE:
-		EncryptData(buf.Bytes(), peer.FE_key)
+		EncryptData(buf.Bytes()[4:], peer.FE_key)
 	}
 
-	// write packet size
-	if err := binary.Write(peer.conn, binary.LittleEndian, uint32(buf.Len())); err != nil {
-		return err
-	}
-
-	// write packet body
-	log.Printf("Sending %#v, sizeof: %d", data, buf.Len())
+	// send full packet
+	log.Printf("Sending %#v, sizeof: %d, buffer: %v", data, buf.Len(), buf.Bytes())
 	if _, err := peer.conn.Write(buf.Bytes()); err != nil {
-		return fmt.Errorf("[FATAL] failed to write packet body! %v", err)
+		return fmt.Errorf("failed to write packet body! %v", err)
 	}
-
 	return nil
 }