secio: buffer remainders in calls to Read()

used to return io.ErrShortBuffer, but this makes client
code much more complicated. we're already allocating
buffers when it's too large, so might as well just
keep it for later.

crypto/secio/rw.go

@@ -76,6 +76,9 @@ type etmReader struct {
 	msgio.Reader
 	io.Closer
 
+	// buffer
+	buf []byte
+
 	// params
 	msg msgio.ReadCloser // msgio for knowing where boundaries lie
 	str cipher.Stream    // the stream cipher to encrypt with
@@ -91,20 +94,35 @@ func (r *etmReader) NextMsgLen() (int, error) {
 	return r.msg.NextMsgLen()
 }
 
+func (r *etmReader) drainBuf(buf []byte) int {
+	if r.buf == nil {
+		return 0
+	}
+
+	n := copy(buf, r.buf)
+	r.buf = r.buf[n:]
+	return n
+}
+
 func (r *etmReader) Read(buf []byte) (int, error) {
-	// first, check the buffer has enough space.
+	// first, check if we have anything in the buffer
+	copied := r.drainBuf(buf)
+	buf = buf[copied:]
+	if copied > 0 {
+		return copied, nil
+		// return here to avoid complicating the rest...
+		// user can call io.ReadFull.
+	}
+
+	// check the buffer has enough space for the next msg
 	fullLen, err := r.msg.NextMsgLen()
 	if err != nil {
 		return 0, err
 	}
 
-	dataLen := fullLen - r.mac.size
-	if cap(buf) < dataLen {
-		return 0, io.ErrShortBuffer
-	}
-
 	buf2 := buf
 	changed := false
+	// if not enough space, allocate a new buffer.
 	if cap(buf) < fullLen {
 		buf2 = make([]byte, fullLen)
 		changed = true
@@ -121,10 +139,15 @@ func (r *etmReader) Read(buf []byte) (int, error) {
 		return 0, err
 	}
 	buf2 = buf2[:m]
-	if changed {
-		return copy(buf, buf2), nil
+	if !changed {
+		return m, nil
 	}
-	return m, nil
+
+	n = copy(buf, buf2)
+	if len(buf2) > len(buf) {
+		r.buf = buf2[len(buf):] // had some left over? save it.
+	}
+	return n, nil
 }
 
 func (r *etmReader) ReadMsg() ([]byte, error) {