Change RawDatastore to do less reflection.

go/src/infra/gae/libs/gae/upstream_errors.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.
 
 // This file contains types and values which are mirrors/duplicates of the
 // upstream SDK errors. This exists so that users can depend solely on this
 // wrapper library without also needing to import the SDK.
 //
 // Format of the errors is Err<sub>Name, where <sub> is one of the 2/3-letter
 // All Caps subpackage codes (e.g. DS, TQ, MC, etc.). Name is the same as the
@@ skipping 28 matching lines @@
         ErrMCCASConflict = memcache.ErrCASConflict
         ErrMCNoStats     = memcache.ErrNoStats
         ErrMCNotStored   = memcache.ErrNotStored
         ErrMCServerError = memcache.ErrServerError
 
         ErrTQTaskAlreadyAdded = taskqueue.ErrTaskAlreadyAdded
 )
 
 /////////////////////////////// Supporting code ////////////////////////////////
 
+// ErrDSFieldMismatch is returned when a field is to be loaded into a different
+// type than the one it was stored from, or when a field is missing or
+// unexported in the destination struct.
+// StructType is the type of the struct pointed to by the destination argument
+// passed to Get or to Iterator.Next.
+type ErrDSFieldMismatch struct {
+        StructType reflect.Type
+        FieldName  string
+        Reason     string
+}
+
+func (e *ErrDSFieldMismatch) Error() string {
+        return fmt.Sprintf("gae: cannot load field %q into a %q: %s",
+                e.FieldName, e.StructType, e.Reason)
+}
+
 // MultiError is returned by batch operations when there are errors with
 // particular elements. Errors will be in a one-to-one correspondence with
 // the input elements; successful elements will have a nil entry.
 type MultiError []error
 
 func (m MultiError) Error() string {
         s, n := "", 0
         for _, e := range m {
                 if e != nil {
                         if n == 0 {
@@ skipping 41 matching lines @@
                 // we know that err already conforms to the error interface (or the caller's
                 // method wouldn't compile), so check to see if the error's underlying type
                 // looks like one of the special error types we implement.
                 v := reflect.ValueOf(err)
                 if v.Type().ConvertibleTo(multiErrorType) {
                         err = v.Convert(multiErrorType).Interface().(error)
                 }
         }
         return err
 }