How can I create cloud context.Context from appengine.Context

I can't figure out how to call cloud.WithContext and google.DefaultClient if I have appengine.Context and not context.Context.

There are (old) "appengine" and (new) "google.golang.org/appengine" packages. The first one brings custom appengine.Context when second one comes with context.Context from "golang.org/x/net/context"

The whole google.golang.org/cloud expects context.Context only.

I would be happy to move to using new "google.golang.org/appengine", but I've stuck with runtime.RunInBackground that have not been ported yet. From https://github.com/golang/appengine:

> appengine/aetest, appengine/cloudsql and appengine/runtime have not been ported yet.

What I could write if appengine/runtime have been ported already:

import (
    "golang.org/x/net/context"

    "google.golang.org/appengine"
    "google.golang.org/appengine/runtime"
    "google.golang.org/cloud"
    "google.golang.org/cloud/storage"
)

func handler(w http.ResponseWriter, r *http.Request) {
    c := appengine.NewContext(r)
    runtime.RunInBackground(c, func(ctx context.Context) {
        hc, _ := google.DefaultClient(ctx, storage.ScopeFullControl)
        cc := cloud.WithContext(ctx, appengine.AppID(ctx), hc)
        …
   })
}

but there is no "google.golang.org/appengine/runtime" yet. So I have

runtime.RunInBackground(c, func(ctx appengine.Context) {

At the moment there is no way to get cloud context.Context from (old) appengine.Context.

Solution for Managed VM

A month ago BackgroundContext() method was added to google.golang.org/appengine (available for Managed VM only). This allows start goroutine and get cloud context.Context without passing it into.

Solution for classic Appengine

There is no solution at the moment.

Do this:

func getCloudContext(appengineContext context.Context) context.Context {
	hc := &http.Client{
		Transport: &oauth2.Transport{
			Source: google.AppEngineTokenSource(appengineContext, storage.ScopeFullControl),
			Base:   &urlfetch.Transport{Context: appengineContext},
		},
	}

	return cloud.NewContext(appengine.AppID(appengineContext), hc)
}

Or if passing the credentials through the dev server isn't working, you can also use explicit credentials:

func getCloudContext(aeCtx context.Context) (context.Context, error) {
	data, err := ioutil.ReadFile("/path/to/credentials.json")
	if err != nil {
		return nil, err
	}

	conf, err := google.JWTConfigFromJSON(
		data,
		storage.ScopeFullControl,
	)
	if err != nil {
		return nil, err
	}

	tokenSource := conf.TokenSource(aeCtx)

	hc := &http.Client{
		Transport: &oauth2.Transport{
			Source: tokenSource,
			Base:   &urlfetch.Transport{Context: aeCtx},
		},
	}

	return cloud.NewContext(appengine.AppID(aeCtx), hc), nil
}

Here is how you can do what you want:

import (
    "code.google.com/p/goauth2/appengine/serviceaccount"
	"golang.org/x/net/context"
	"appengine"
)

// oauth2 module requires a context.Context so use goauth2 for now
func CloudContext(c appengine.Context, scopes ...string) context.Context {
	client, _ := serviceaccount.NewClient(c, scopes...)
	return cloud.WithContext(context.Background(), appengine.AppID(c), client)
}

You can get the goauth2 library from https://code.google.com/p/goauth2/source/browse/