go/token: document deserialization property

FileSet deserialization (Read) uses its own instance of a gob decoder.
If the FileSet data may be followed by other data on the reader, Read
may consume too much data that is lost unless the reader implements
ReadByte.

Also: Minor internal refactoring for symmetry.

R=r
CC=golang-dev
https://golang.org/cl/5233041

src/pkg/go/token/serialize.go

@@ -24,10 +24,19 @@ type serializedFileSet struct {
 	Files []serializedFile
 }
 
+func (s *serializedFileSet) Read(r io.Reader) os.Error {
+	return gob.NewDecoder(r).Decode(s)
+}
+
+func (s *serializedFileSet) Write(w io.Writer) os.Error {
+	return gob.NewEncoder(w).Encode(s)
+}
+
 // Read reads the fileset from r into s; s must not be nil.
+// If r does not also implement io.ByteReader, it will be wrapped in a bufio.Reader.
 func (s *FileSet) Read(r io.Reader) os.Error {
 	var ss serializedFileSet
-	if err := gob.NewDecoder(r).Decode(&ss); err != nil {
+	if err := ss.Read(r); err != nil {
 		return err
 	}
 
@@ -58,5 +67,5 @@ func (s *FileSet) Write(w io.Writer) os.Error {
 	ss.Files = files
 	s.mutex.Unlock()
 
-	return gob.NewEncoder(w).Encode(ss)
+	return ss.Write(w)
 }