go/bindings: update code to pass structs versions tests

mojo/public/go/bindings/decoder.go

 // Copyright 2015 The Chromium 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 bindings
 
 import (
         "encoding/binary"
         "fmt"
         "math"
@@ skipping 107 matching lines @@
         }
         if header != mapHeader {
                 return fmt.Errorf("invalid map header: %v", header)
         }
         if err := d.pushState(header, pointerBitSize); err != nil {
                 return err
         }
         return nil
 }
 
-// StartStruct starts decoding a struct and reads its data header,
-// returning struct version declared in data header.
+// StartStruct starts decoding a struct and reads its data header.
+// Returns the read data header. The caller should check if it is valid.
 // Note: it doesn't read a pointer to the encoded struct.
 // Call |Finish()| after reading all fields.
-func (d *Decoder) StartStruct() (uint32, error) {
+func (d *Decoder) StartStruct() (DataHeader, error) {
         header, err := d.readDataHeader()
         if err != nil {
-                return 0, err
+                return DataHeader{}, err
         }
         if header.Size < dataHeaderSize {
-                return 0, fmt.Errorf("data header size is too small: is %d, but should be at least %d", header.Size, dataHeaderSize)
+                return DataHeader{}, fmt.Errorf("data header size(%d) should be at least %d", header.Size, dataHeaderSize)
         }
         if err := d.pushState(header, 0); err != nil {
-                return 0, err
+                return DataHeader{}, err
         }
-        return header.ElementsOrVersion, nil
+        return header, nil
 }
 
 func (d *Decoder) readDataHeader() (DataHeader, error) {
         if err := d.claimData(dataHeaderSize); err != nil {
                 return DataHeader{}, err
         }
         oldEnd := d.end - dataHeaderSize
         header := DataHeader{
                 Size:              binary.LittleEndian.Uint32(d.buf[oldEnd:]),
                 ElementsOrVersion: binary.LittleEndian.Uint32(d.buf[oldEnd+4:]),
@@ skipping 204 matching lines @@
 }
 
 // ReadSharedBufferHandle reads a shared buffer handle.
 func (d *Decoder) ReadSharedBufferHandle() (system.SharedBufferHandle, error) {
         if handle, err := d.ReadUntypedHandle(); err != nil {
                 return nil, err
         } else {
                 return handle.ToSharedBufferHandle(), nil
         }
 }