code review 3069041: crypto/cipher: add package

src/pkg/crypto/cipher/io.go

 // Copyright 2010 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 package cipher
 
 import (
         "os"
         "io"
 )
@@ skipping 11 matching lines @@
 func (r StreamReader) Read(dst []byte) (n int, err os.Error) {
         n, err = r.R.Read(dst)
         r.S.XORKeyStream(dst[:n], dst[:n])
         return
 }
 
 // StreamWriter wraps a Stream into an io.Writer. It simply calls XORKeyStream
 // to process each slice of data which passes through. If any Write call
 // returns short then the StreamWriter is out of sync and must be discarded.
 type StreamWriter struct {
-        S        Stream
-        W        io.Writer
-        Desynced os.Error
+        S   Stream
+        W   io.Writer
+        Err os.Error
 }
 
 func (w StreamWriter) Write(src []byte) (n int, err os.Error) {
-        if w.Desynced != nil {
-                return 0, w.Desynced
+        if w.Err != nil {
+                return 0, w.Err
         }
         c := make([]byte, len(src))
         w.S.XORKeyStream(c, src)
         n, err = w.W.Write(c)
         if n != len(src) {
-                w.Desynced = os.ErrorString("cipher.StreamWriter: stream out of sync")
+                if err == nil { // should never happen
+                        err = io.ErrShortWrite
+                }
+                w.Err = err
         }
         return
 }
 
 func (w StreamWriter) Close() os.Error {
         // This saves us from either requiring a WriteCloser or having a
         // StreamWriterCloser.
         return w.W.(io.Closer).Close()
 }