added num.go for NBO stuff

limit.go

@@ -6,11 +6,7 @@ import (
 
 // LimitedReader wraps an io.Reader with a msgio framed reader. The LimitedReader
 // will return a reader which will io.EOF when the msg length is done.
-func LimitedReader(r io.Reader) io.Reader {
-	l := int64(0)
-	lbuf := make([]byte, lengthSize)
-	if _, err := io.ReadFull(r, lbuf); err == nil {
-		l = int64(NBO.Uint32(lbuf))
-	}
-	return io.LimitReader(r, l)
+func LimitedReader(r io.Reader) (io.Reader, error) {
+	l, err := ReadLen(r, nil)
+	return io.LimitReader(r, int64(l)), err
 }

msgio.go

@@ -1,17 +1,13 @@
 package msgio
 
 import (
-	"encoding/binary"
 	"errors"
 	"io"
 	"sync"
 
 	mpool "github.com/jbenet/go-msgio/mpool"
 )
 
-// NBO is NetworkByteOrder
-var NBO = binary.BigEndian
-
 //  ErrMsgTooLarge is returned when the message length is exessive
 var ErrMsgTooLarge = errors.New("message too large")
 
@@ -98,9 +94,7 @@ func (s *writer) Write(msg []byte) (int, error) {
 func (s *writer) WriteMsg(msg []byte) (err error) {
 	s.lock.Lock()
 	defer s.lock.Unlock()
-
-	length := uint32(len(msg))
-	if err := binary.Write(s.W, NBO, &length); err != nil {
+	if err := WriteLen(s.W, len(msg)); err != nil {
 		return err
 	}
 	_, err = s.W.Write(msg)
@@ -159,14 +153,11 @@ func (s *reader) NextMsgLen() (int, error) {
 	return s.nextMsgLen()
 }
 
-func (s *reader) nextMsgLen() (int, error) {
+func (s *reader) nextMsgLen() (n int, err error) {
 	if s.next == -1 {
-		if _, err := io.ReadFull(s.R, s.lbuf); err != nil {
-			return 0, err
-		}
-		s.next = int(NBO.Uint32(s.lbuf))
+		s.next, err = ReadLen(s.R, s.lbuf)
 	}
-	return s.next, nil
+	return s.next, err
 }
 
 func (s *reader) Read(msg []byte) (int, error) {

num.go

@@ -0,0 +1,33 @@
+package msgio
+
+import (
+	"encoding/binary"
+	"io"
+)
+
+// NBO is NetworkByteOrder
+var NBO = binary.BigEndian
+
+// WriteLen writes a length to the given writer.
+func WriteLen(w io.Writer, l int) error {
+	ul := uint32(l)
+	return binary.Write(w, NBO, &ul)
+}
+
+// ReadLen reads a length from the given reader.
+// if buf is non-nil, it reuses the buffer. Ex:
+//    l, err := ReadLen(r, nil)
+//    _, err := ReadLen(r, buf)
+func ReadLen(r io.Reader, buf []byte) (int, error) {
+	if len(buf) < 4 {
+		buf = make([]byte, 4)
+	}
+	buf = buf[:4]
+
+	if _, err := io.ReadFull(r, buf); err != nil {
+		return 0, err
+	}
+
+	n := int(NBO.Uint32(buf))
+	return n, nil
+}