Milo: Use custom config caching layer

luci_config/server/cfgclient/backend/erroring/erroring.go

 // Copyright 2015 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 // Package erroring implements config.Backend that simply returns an error.
 //
 // May be handy as a placeholder in case some more useful implementation is not
 // available.
 package erroring
 
 import (
+        "errors"
         "net/url"
 
+        "github.com/luci/luci-go/common/config"
+        "github.com/luci/luci-go/common/config/impl/memory"
         "github.com/luci/luci-go/luci_config/server/cfgclient/backend"
 
         "golang.org/x/net/context"
 )
 
 // New produces backend.B instance that returns the given error for all calls.
 //
 // Panics if given err is nil.
 func New(err error) backend.B {
         if err == nil {
@@ skipping 18 matching lines @@
 
 func (e erroringImpl) GetAll(c context.Context, t backend.GetAllTarget, path string, p backend.Params) (
         []*backend.Item, error) {
 
         return nil, e.err
 }
 
 func (e erroringImpl) ConfigSetURL(c context.Context, configSet string, p backend.Params) (url.URL, error) {
         return url.URL{}, e.err
 }
+
+func (e erroringImpl) GetConfigInterface(c context.Context, a backend.Authority) config.Interface {
+        emptySet := map[string]memory.ConfigSet{}
+        i := memory.New(emptySet)
+        memory.SetError(i, errors.New("error"))
+        return i
+}