Better attempt at an appengine wrapper.

go/src/infra/gae/libs/wrapper/featurebreaker.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 wrapper
+
+import (
+        "runtime"
+        "strings"
+        "sync"
+)
+
+// FeatureBreaker allows a fake implementation to set and unset broken features.
+// A feature is the Name of some methog on the fake. So if you had:

[Vadim Sh., 2015/05/24 19:01:57]

typo: methog

[iannucci, 2015/05/24 20:04:31]

Done

+//   var fake interface{ FeatureBreaker, MCSingleReadWriter } = ...
+//
+// you could do:
+//   fake.SetBrokenFeatures("Add", "Set")
+//
+// and then
+//   fake.Add(...) and fake.Set(...)
+//
+// would return an error (likely memcache.ErrServerError in this case), as if
+// the service were disconnected or broken.
+type FeatureBreaker interface {
+        SetBrokenFeatures(feature ...string)
+        UnsetBrokenFeatures(feature ...string)
+}
+
+// BrokenFeatures implements the FeatureBreaker interface, and is suitable for
+// embedding within a fake service.
+type BrokenFeatures struct {
+        lock sync.Mutex
+
+        broken map[string]struct{}
+        err    error
+}
+
+// NewBrokenFeatures creates an embeddable *BrokenFeatures which is set to
+// return err for a given broken feature.
+func NewBrokenFeatures(err error) *BrokenFeatures {
+        return &BrokenFeatures{broken: map[string]struct{}{}, err: err}
+}
+
+// SetBrokenFeatures allows you to specify an MCSingleReadWriter function name
+// to cause it to return memcache.ErrServerError. e.g.
+//
+//   m.SetBrokenFeatures("Add")
+//
+// would return memcache.ErrServerError. You can reverse this by calling
+// UnsetBrokenFeatures("Add").
+func (b *BrokenFeatures) SetBrokenFeatures(feature ...string) {
+        b.lock.Lock()
+        defer b.lock.Unlock()
+        for _, f := range feature {
+                b.broken[f] = struct{}{}
+        }
+}
+
+// UnsetBrokenFeatures is the inverse of SetBrokenFeatures.
+func (b *BrokenFeatures) UnsetBrokenFeatures(feature ...string) {
+        b.lock.Lock()
+        defer b.lock.Unlock()
+        for _, f := range feature {
+                delete(b.broken, f)
+        }
+}
+
+// IsBroken is to be called internally by the fake service on every
+// publically-facing method. If it returns an error, the fake should return

[Vadim Sh., 2015/05/24 19:01:57]

explicitly from the body of this method?

[iannucci, 2015/05/24 20:04:31]

Added example

+// the error.
+func (b *BrokenFeatures) IsBroken() error {
+        b.lock.Lock()
+        defer b.lock.Unlock()
+        if len(b.broken) == 0 {
+                return nil
+        }
+        off := 1
+
+        for {
+                fn, _, _, _ := runtime.Caller(off)
+                name := runtime.FuncForPC(fn).Name()
+                toks := strings.Split(name, ".")
+                name = toks[len(toks)-1]
+                if name == "IsBroken" {
+                        // Allow users to override IsBroken
+                        // ITS MAGIC!!!!!
+                        off++
+                        continue
+                }
+                if _, ok := b.broken[name]; ok {
+                        return b.err
+                }
+                break
+        }
+
+        return nil
+}