go/bindings: update code to pass structs versions tests

mojo/public/tools/bindings/generators/go_templates/struct.tmpl

 // 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.
 
 {% macro define(struct, exported=True) %}
 type {{struct|name(exported)}} struct {
 {% for field in struct.fields %}
         {{field|name(exported)}} {{field.kind|go_type}}
 {% endfor %}
 }
 
 func (s *{{struct|name(exported)}}) Encode(encoder *bindings.Encoder) error {
 {% set HEADER_SIZE = 8 %}
         encoder.StartStruct({{struct.versions[-1].num_bytes - HEADER_SIZE}}, {{struct.versions[-1].version}})
 {% for byte in struct.bytes %}
 {%   for packed_field in byte.packed_fields %}
         {{encode('s.'~packed_field.field|name(exported), packed_field.field.kind)|tab_indent()}}
 {%   endfor %}
 {% endfor %}
         if err := encoder.Finish(); err != nil {
                 return err
         }
         return nil
 }
 
+var {{struct|name(False)}}_Versions []bindings.DataHeader = []bindings.DataHeader{
+{% for versionInfo in struct.versions %}
+        bindings.DataHeader{{'{'}}{{versionInfo.num_bytes}}, {{versionInfo.version}}{{'}'}},
+{% endfor %}
+}
+
 func (s *{{struct|name(exported)}}) Decode(decoder *bindings.Decoder) error {
-{% if struct.bytes %}
-        version, err := decoder.StartStruct()
-{% else %}
-        _, err := decoder.StartStruct()
-{% endif %}
+        header, err := decoder.StartStruct()
         if err != nil {
                 return err
         }
+        index := sort.Search(len({{struct|name(False)}}_Versions), func(i int) bool {
+                return {{struct|name(False)}}_Versions[i].ElementsOrVersion >= header.ElementsOrVersion
+        })
+        if index < len({{struct|name(False)}}_Versions) {
+                if {{struct|name(False)}}_Versions[index].ElementsOrVersion > header.ElementsOrVersion {
+                        index--
+                }
+                expectedSize := {{struct|name(False)}}_Versions[index].Size
+                if expectedSize != header.Size {
+                        return fmt.Errorf("invalid struct header size: should be %d, but was %d", expectedSize, header.Size)
+                }
+        }
 {% for byte in struct.bytes %}
 {%   for packed_field in byte.packed_fields %}
-        if version >= {{packed_field.min_version}} {
+        if header.ElementsOrVersion >= {{packed_field.min_version}} {
                 {{decode('s.'~packed_field.field|name(exported), packed_field.field.kind)|tab_indent(2)}}
         }
 {%   endfor %}
 {% endfor %}
         if err := decoder.Finish(); err != nil {
                 return err
         }
         return nil
 }
 
@@ skipping 172 matching lines @@
 }
 {{value}} = map{{level}}
 {% else %}
 value{{level}}, err := decoder.Read{{kind|decode_suffix}}()
 if err != nil {
         return err
 }
 {{value}} = value{{level}}
 {% endif %}
 {% endmacro %}