Better attempt at an appengine wrapper.

go/src/infra/gae/libs/wrapper/brokenfeatures.go

Index: go/src/infra/gae/libs/wrapper/brokenfeatures.go
diff --git a/go/src/infra/gae/libs/wrapper/brokenfeatures.go b/go/src/infra/gae/libs/wrapper/brokenfeatures.go
new file mode 100644
index 0000000000000000000000000000000000000000..4e4524b203bf57efc8e3af5150761512ce4b1282
--- /dev/null
+++ b/go/src/infra/gae/libs/wrapper/brokenfeatures.go
@@ -0,0 +1,157 @@
+// 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"
+	"fmt"
+	"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
+
+	// DefaultError is the default error to return when you call
+	// BreakFeatures(nil, ...). If this is unset and the user calls BreakFeatures
+	// with nil, BrokenFeatures will return a generic error.
+	DefaultError 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()
+	if b.broken == nil {
+		b.broken = map[string]error{}
+	}
+
+	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()
+
+	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 {
+	if b.noBrokenFeatures() {
+		return nil
+	}
+
+	var name string
+	for off := 1; ; off++ { // offset of 1 skips ourselves by default
+		// 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.
+			continue
+		}
+		if name == "IsBroken" {
+			// Allow users to override IsBroken, keep walking until we see a function
+			// which is named differently than IsBroken.
+			continue
+		}
+		break
+	}
+
+	b.lock.Lock()
+	defer b.lock.Unlock()
+	if err, ok := b.broken[name]; ok {
+		if err != nil {
+			return err
+		}
+		if b.DefaultError != nil {
+			return b.DefaultError
+		}
+		return fmt.Errorf("feature %q is broken", name)
+	}
+
+	return nil
+}
+
+func (b *BrokenFeatures) noBrokenFeatures() bool {
+	b.lock.Lock()
+	defer b.lock.Unlock()
+	return len(b.broken) == 0
+}