server/config: Add an erroring config wrapper.

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 (
+        "net/url"
+
+        "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 {
+                panic("the error must not be nil")
+        }
+        return erroringImpl{err}
+}
+
+type erroringImpl struct {
+        err error
+}
+
+func (e erroringImpl) ServiceURL(context.Context) url.URL {
+        return url.URL{
+                Scheme: "error",
+        }
+}
+
+func (e erroringImpl) Get(c context.Context, configSet, path string, p backend.Params) (*backend.Item, error) {
+        return nil, e.err
+}
+
+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
+}