oauth2: redesign the API

Tests and examples aren't updated yet. The tree will be broken after this,
but nobody should be using this yet anyway.

Change-Id: I0004c738f40919ab46d107c71c011c510fbc748f
Reviewed-on: https://go-review.googlesource.com/1246
Reviewed-by: Burcu Dogan <jbd@google.com>

client_appengine.go

@@ -0,0 +1,39 @@
+// Copyright 2014 The oauth2 Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build appengine,!appenginevm
+
+// App Engine hooks.
+
+package oauth2
+
+import (
+	"log"
+	"net/http"
+	"sync"
+
+	"appengine"
+	"appengine/urlfetch"
+)
+
+var warnOnce sync.Once
+
+func init() {
+	registerContextClientFunc(contextClientAppEngine)
+}
+
+func contextClientAppEngine(ctx Context) (*http.Client, error) {
+	if actx, ok := ctx.(appengine.Context); ok {
+		return urlfetch.NewClient(actx), nil
+	}
+	// The user did it wrong. We'll log once (and hope they see it
+	// in dev_appserver), but stil return (nil, nil) in case some
+	// other contextClientFunc hook finds a way to proceed.
+	warnOnce.Do(gaeDoingItWrongHelp)
+	return nil, nil
+}
+
+func gaeDoingItWrongHelp() {
+	log.Printf("WARNING: you attempted to use the oauth2 package without passing a valid appengine.Context or *http.Request as the oauth2.Context. App Engine requires that all service RPCs (including urlfetch) be associated with an *http.Request/appengine.Context.")
+}