Fix error message from KeyForObj when passing an invalid struct.

service/datastore/pls.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 datastore
 
 import (
         "reflect"
 )
 
@@ skipping 140 matching lines @@
 //     // split on "|"
 //     // assign to n.First, n.Last
 //   }
 //
 //   type Person struct {
 //     Name `gae:"$id"`
 //   }
 func GetPLS(obj interface{}) interface {
         PropertyLoadSaver
         MetaGetterSetter
+        Problem() error

[dnj, 2015/12/11 06:36:17]

Since Problem() is only used by tests, why export it here? WDYT about either:

(a) making it a private interface (Problem -> problem)?
(b) removing it here and just type-asserting the test struct to "*structPLS"?

 } {
         v := reflect.ValueOf(obj)
         if v.Kind() != reflect.Ptr || v.Elem().Kind() != reflect.Struct {
                 return &structPLS{c: &structCodec{problem: ErrInvalidEntityType}}
         }
         if v.IsNil() {
                 return &structPLS{c: &structCodec{problem: ErrInvalidEntityType}}
         }
         v = v.Elem()
         c := getCodec(v.Type())
@@ skipping 12 matching lines @@
         c, ok := structCodecs[structType]
         structCodecsMutex.RUnlock()
         if ok {
                 return c
         }
 
         structCodecsMutex.Lock()
         defer structCodecsMutex.Unlock()
         return getStructCodecLocked(structType)
 }