Make UseRemote work for dev_appserver too.

impl/prod/context.go

Index: impl/prod/context.go
diff --git a/impl/prod/context.go b/impl/prod/context.go
index 73fec355fd739518a432bc310b43ae6acb7344b6..bccbf7934cd648ad1cb748ec7fad3b0d3f46b38e 100644
--- a/impl/prod/context.go
+++ b/impl/prod/context.go
@@ -5,9 +5,14 @@
 package prod
 
 import (
+	"fmt"
 	"net/http"
+	"net/http/cookiejar"
+	"net/url"
+	"strings"
 
 	"github.com/luci/gae/service/info"
+	"github.com/luci/gae/service/urlfetch"
 	"golang.org/x/net/context"
 	gOAuth "golang.org/x/oauth2/google"
 	"google.golang.org/appengine"
@@ -29,6 +34,9 @@ var (
 // anyway).
 func AEContext(c context.Context) context.Context {
 	aeCtx, _ := c.Value(prodContextKey).(context.Context)
+	if aeCtx == nil {
+		return nil
+	}
 	if deadline, ok := c.Deadline(); ok {
 		aeCtx, _ = context.WithDeadline(aeCtx, deadline)
 	}
@@ -39,6 +47,9 @@ func AEContext(c context.Context) context.Context {
 // Context that's not part of a transaction.
 func AEContextNoTxn(c context.Context) context.Context {
 	aeCtx, _ := c.Value(prodContextNoTxnKey).(context.Context)
+	if aeCtx == nil {
+		return nil
+	}
 	aeCtx, err := appengine.Namespace(aeCtx, info.Get(c).GetNamespace())
 	if err != nil {
 		panic(err)
@@ -81,27 +92,68 @@ func Use(c context.Context, r *http.Request) context.Context {
 //
 // docs: https://cloud.google.com/appengine/docs/go/tools/remoteapi
 //
-// If client is nil, this will use a default Google OAuth2 client. Otherwise the
-// client must be configured to have the following OAuth2 scopes:
-//		"https://www.googleapis.com/auth/appengine.apis"
-//		"https://www.googleapis.com/auth/userinfo.email"
-//		"https://www.googleapis.com/auth/cloud.platform"
-func UseRemote(c context.Context, host string, client *http.Client) (context.Context, error) {
-	err := error(nil)
+// inOutCtx will be replaced with the new, derived context, if err is nil,
+// otherwise it's unchanged and continues to be safe-to-use.
+//
+// If client is nil, this will use create a new client, and will try to be
+// clever about it:
+//   * If you're creating a remote context FROM AppEngine, this will use
+//     urlfetch.Transport. This can be used to allow app-to-app remote_api
+//     control.
+//
+//   * If host starts with "localhost", this will create a regular http.Client
+//     with a cookiejar, and call the _ah/login API to log in as an admin with
+//     the user "admin@example.com".
+//
+//   * Otherwise, it will create a Google OAuth2 client with the following scopes:
+//       - "https://www.googleapis.com/auth/appengine.apis"
+//       - "https://www.googleapis.com/auth/userinfo.email"
+//       - "https://www.googleapis.com/auth/cloud.platform"
+func UseRemote(inOutCtx *context.Context, host string, client *http.Client) (err error) {
 	if client == nil {
-		client, err = gOAuth.DefaultClient(context.Background(),
-			"https://www.googleapis.com/auth/appengine.apis",
-			"https://www.googleapis.com/auth/userinfo.email",
-			"https://www.googleapis.com/auth/cloud.platform",
-		)
-		if err != nil {
-			return nil, err
+		if strings.HasPrefix(host, "localhost") {
+			transp := http.DefaultTransport
+			if aeCtx := AEContextNoTxn(*inOutCtx); aeCtx != nil {
+				transp = urlfetch.Get(aeCtx)
+			}
+
+			client = &http.Client{Transport: transp}
+			client.Jar, err = cookiejar.New(nil)
+			if err != nil {
+				return
+			}
+			u := fmt.Sprintf("http://%s/_ah/login?%s", host, url.Values{
+				"email":  {"admin@example.com"},
+				"admin":  {"True"},
+				"action": {"Login"},
+			}.Encode())
+
+			var rsp *http.Response
+			rsp, err = client.Get(u)
+			if err != nil {
+				return
+			}
+			defer rsp.Body.Close()
+		} else {
+			aeCtx := AEContextNoTxn(*inOutCtx)
+			if aeCtx == nil {
+				aeCtx = context.Background()
+			}
+			client, err = gOAuth.DefaultClient(aeCtx,
+				"https://www.googleapis.com/auth/appengine.apis",
+				"https://www.googleapis.com/auth/userinfo.email",
+				"https://www.googleapis.com/auth/cloud.platform",
+			)
+			if err != nil {
+				return
+			}
 		}
 	}
 
 	aeCtx, err := remote_api.NewRemoteContext(host, client)
 	if err != nil {
-		return nil, err
+		return
 	}
-	return setupAECtx(c, aeCtx), nil
+	*inOutCtx = setupAECtx(*inOutCtx, aeCtx)
+	return nil
 }