Better attempt at an appengine wrapper.

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

Index: go/src/infra/gae/libs/wrapper/globalinfo.go
diff --git a/go/src/infra/gae/libs/wrapper/globalinfo.go b/go/src/infra/gae/libs/wrapper/globalinfo.go
new file mode 100644
index 0000000000000000000000000000000000000000..1833f1dd59e1c938eb3d6a9fd2715990dce65be9
--- /dev/null
+++ b/go/src/infra/gae/libs/wrapper/globalinfo.go
@@ -0,0 +1,80 @@
+// 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 (
+	"time"
+
+	"appengine"
+
+	"golang.org/x/net/context"
+)
+
+// GlobalInfo is the interface for all of the package methods which normally
+// would be in the 'appengine' package.
+type GlobalInfo interface {
+	// methods usually requiring a Context
+
+	AccessToken(scopes ...string) (token string, expiry time.Time, err error)
+	AppID() string
+	DefaultVersionHostname() string
+	ModuleHostname(module, version, instance string) (string, error)
+	ModuleName() string
+	PublicCertificates() ([]appengine.Certificate, error)
+	RequestID() string
+	ServiceAccount() (string, error)
+	SignBytes(bytes []byte) (keyName string, signature []byte, err error)
+	VersionID() string
+
+	// our tweaked interface
+
+	// Namespace takes the new namespace as a string, and returns a context

[M-A Ruel, 2015/05/25 17:14:52]

I think it's better to leave it out.

If we don't know what's the use case of a function and do not expect to use it,
we should probably leave it out. Adding it back once it's actually needed is
likely less painful.

I don't mind that much, having near 100% coverage is also useful in some ways.

[iannucci, 2015/05/26 18:25:06]

I actually do plan on using it, and I know some other devs are thinking about
using it as well. It's a pretty good way to implement multi-tenancy.

+	// set to use that namespace, or an error. I'm not sure what the error
+	// could be, since it's not documented on the appengine SDK.
+	Namespace(namespace string) (context.Context, error)
+
+	// Really global functions... these don't normally even require context, but
+	// for the purposes of testing+consistency, they're included here.
+
+	Datacenter() string
+	InstanceID() string
+	IsDevAppserver() bool
+	ServerSoftware() string
+
+	IsCapabilityDisabled(err error) bool
+	IsOverQuota(err error) bool
+	IsTimeoutError(err error) bool
+
+	// Backends are deprecated in favor of modules, so simplify this a bit by
+	// omitting them from the interface.
+	// BackendHostname(name string, index int) string
+	// BackendInstance() (name string, index int)
+}
+
+// GIFactory is the function signature for factory methods compatible with
+// SetGIFactory.
+type GIFactory func(context.Context) GlobalInfo
+
+// GetGI gets gets the GlobalInfo implementation from context.
+func GetGI(c context.Context) GlobalInfo {
+	obj := c.Value(globalInfoKey)
+	if obj == nil || obj.(GIFactory) == nil {

[M-A Ruel, 2015/05/25 17:14:52]

http://golang.org/ref/spec#Type_assertions

[iannucci, 2015/05/26 18:25:06]

http://play.golang.org/p/g1bN9UNSkS

Typed v. untyped nil w.r.t functions. :)

+		return nil
+	}
+	return obj.(GIFactory)(c)
+}
+
+// SetGIFactory sets the function to produce GlobalInfo instances, as returned by
+// the GetGI method.
+func SetGIFactory(c context.Context, gif GIFactory) context.Context {
+	return context.WithValue(c, globalInfoKey, gif)
+}
+
+// SetGI sets the current GlobalInfo object in the context. Useful for testing
+// with a quick mock. This is just a shorthand SetGIFactory invocation to set
+// a factory which always returns the same object.
+func SetGI(c context.Context, gi GlobalInfo) context.Context {
+	return SetGIFactory(c, func(context.Context) GlobalInfo { return gi })
+}