Better attempt at an appengine wrapper.

go/src/infra/gae/libs/wrapper/brokenfeatures.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 (
+        "errors"
+        "runtime"
+        "strings"
+        "sync"
+        "unicode"
+        "unicode/utf8"
+)
+
+// FeatureBreaker allows a fake implementation to set and unset broken features.
+// A feature is the Name of some method on the fake. So if you had:
+//   var fake interface{ FeatureBreaker, MCSingleReadWriter } = ...
+//
+// you could do:
+//   fake.BreakFeatures(memcache.ErrServerError, "Add", "Set")
+//
+// and then
+//   fake.Add(...) and fake.Set(...)
+//
+// would return the error.
+//
+// You may also pass nil as the error for BreakFeatures, and the fake will
+// provide some suitable (but generic) error for those features (like a
+// BAD_REQUEST or something like that).
+type FeatureBreaker interface {
+        BreakFeatures(err error, feature ...string)
+        UnbreakFeatures(feature ...string)
+}
+
+// ErrBrokenFeaturesBroken is returned from IsBroken when BrokenFeatures itself
+// isn't working correctly.
+var ErrBrokenFeaturesBroken = errors.New("brokenFeatures: Unable to retrieve caller information")
+
+// BrokenFeatures implements the FeatureBreaker interface, and is suitable for
+// embedding within a fake service.
+type BrokenFeatures struct {
+        lock sync.Mutex
+
+        broken  map[string]error
+        dfltErr error
+}
+
+// NewBrokenFeatures creates an embeddable *BrokenFeatures which is set to
+// return err for a given broken feature.
+func NewBrokenFeatures(err error) *BrokenFeatures {

[M-A Ruel, 2015/05/27 00:27:11]

I still think this function is unneeded.

[iannucci, 2015/05/27 02:40:56]

Removed, made dfltErr exported

+        return &BrokenFeatures{broken: map[string]error{}, dfltErr: err}
+}
+
+// initLocked makes sure the map is allocated, in case the user decided to
+// do &BrokenFeatures{} themselves.
+func (b *BrokenFeatures) initLocked() {
+        if b.broken == nil {
+                b.broken = map[string]error{}
+        }
+}
+
+// BreakFeatures 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
+// UnbreakFeatures("Add").
+func (b *BrokenFeatures) BreakFeatures(err error, feature ...string) {
+        b.lock.Lock()
+        defer b.lock.Unlock()
+        b.initLocked()

[M-A Ruel, 2015/05/27 00:27:11]

Inline initLocked here since it's now the only call site.

[iannucci, 2015/05/27 02:40:56]

done

+
+        for _, f := range feature {
+                b.broken[f] = err
+        }
+}
+
+// UnbreakFeatures is the inverse of BreakFeatures, and will return the named
+// features back to their original functionality.
+func (b *BrokenFeatures) UnbreakFeatures(feature ...string) {
+        b.lock.Lock()
+        defer b.lock.Unlock()
+        b.initLocked()

[M-A Ruel, 2015/05/27 00:27:11]

It's not needed, delete(nil, ...) works, e.g.
http://play.golang.org/p/WI0_F4m_Rz

[iannucci, 2015/05/27 02:40:57]

cool, done

+
+        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
+// the error.
+//
+// Example:
+//   type MyService struct { *BrokenFeatures }
+//   func (ms *MyService) Thingy() error {
+//     if err := ms.IsBroken(); err != nil {
+//       return err
+//     }
+//               ...
+//   }
+//
+//  You can now do ms.SetBrokenFeatures("Thingy"), and Thingy will return an
+//  error.
+//
+//  Note that IsBroken will keep walking the stack until it finds the first
+//  publically-exported method, which will allow you to put the IsBroken call
+//  in an internal helper method of your service implementation.
+//
+//  Additionaly, IsBroken allows a very primitive form of overriding; it walks
+//  the stack until it finds the first method which is not called "IsBroken".
+//  This allows the embedding struct to call into BrokenFeatures.IsBroken from
+//  another IsBroken function, and still have it behave correctly.
+func (b *BrokenFeatures) IsBroken() error {
+        b.lock.Lock()
+        defer b.lock.Unlock()
+        b.initLocked()

[M-A Ruel, 2015/05/27 00:27:11]

it's not needed here.

[iannucci, 2015/05/27 02:40:56]

done

+
+        if len(b.broken) == 0 {
+                return nil
+        }
+        off := 1 // offset of 1 skips ourselves by default
+
+        for {

[M-A Ruel, 2015/05/27 00:27:11]

for off := 1; ; off++ {

[iannucci, 2015/05/27 02:40:57]

done

+                // TODO(riannucci): Profile this to see if it's having an adverse
+                // performance impact ont tests.
+                fn, _, _, ok := runtime.Caller(off)
+                if !ok {
+                        return ErrBrokenFeaturesBroken
+                }
+                toks := strings.Split(runtime.FuncForPC(fn).Name(), ".")
+                name := toks[len(toks)-1]
+                firstRune, _ := utf8.DecodeRuneInString(name)
+                if !unicode.IsUpper(firstRune) {
+                        // unexported method, keep walking till we find the first exported
+                        // method. Do !IsUpper, since exported is defined by IsUpper and not
+                        // !IsLower, and afaik, in unicode-land they're not direct opposites.
+                        off++

[M-A Ruel, 2015/05/27 00:27:11]

remove

[iannucci, 2015/05/27 02:40:56]

done

+                        continue
+                }
+                if name == "IsBroken" {
+                        // Allow users to override IsBroken, keep walking until we see a function
+                        // which is named differently than IsBroken.
+                        off++

[M-A Ruel, 2015/05/27 00:27:11]

remove

[iannucci, 2015/05/27 02:40:57]

done

+                        continue
+                }
+                if err, ok := b.broken[name]; ok {
+                        if err != nil {
+                                return err
+                        }
+                        return b.dfltErr
+                }
+                break
+        }
+
+        return nil
+}