google: implement application default credentials

Google is moving toward a default, language-agnostic way of obtaining
authentication credentials on App Engine and Compute Engine.
This change implements this mechanism.

Fixes #90.

Change-Id: Iaae4d60bbc75b1a9f2ec6ad14ab76fb2e279d756
Reviewed-on: https://go-review.googlesource.com/6074
Reviewed-by: Burcu Dogan <jbd@google.com>

google/default.go

@@ -0,0 +1,154 @@
+// Copyright 2015 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.
+
+package google
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"os"
+	"path/filepath"
+	"runtime"
+
+	"golang.org/x/net/context"
+	"golang.org/x/oauth2"
+	"golang.org/x/oauth2/jwt"
+	"google.golang.org/cloud/compute/metadata"
+)
+
+// DefaultClient returns an HTTP Client that uses the
+// DefaultTokenSource to obtain authentication credentials.
+//
+// This client should be used when developing services
+// that run on Google App Engine or Google Compute Engine
+// and use "Application Default Credentials."
+//
+// For more details, see:
+// https://developers.google.com/accounts/application-default-credentials
+//
+func DefaultClient(ctx context.Context, scope ...string) (*http.Client, error) {
+	ts, err := DefaultTokenSource(ctx, scope...)
+	if err != nil {
+		return nil, err
+	}
+	return oauth2.NewClient(ctx, ts), nil
+}
+
+// DefaultTokenSource is a token source that uses
+// "Application Default Credentials".
+//
+// It looks for credentials in the following places,
+// preferring the first location found:
+//
+//   1. A JSON file whose path is specified by the
+//      GOOGLE_APPLICATION_CREDENTIALS environment variable.
+//   2. A JSON file in a location known to the gcloud command-line tool.
+//      On Windows, this is %APPDATA%/gcloud/application_default_credentials.json.
+//      On other systems, $HOME/.config/gcloud/application_default_credentials.json.
+//   3. On Google App Engine it uses the appengine.AccessToken function.
+//   4. On Google Compute Engine, it fetches credentials from the metadata server.
+//      (In this final case any provided scopes are ignored.)
+//
+// For more details, see:
+// https://developers.google.com/accounts/application-default-credentials
+//
+func DefaultTokenSource(ctx context.Context, scope ...string) (oauth2.TokenSource, error) {
+	// First, try the environment variable.
+	const envVar = "GOOGLE_APPLICATION_CREDENTIALS"
+	if filename := os.Getenv(envVar); filename != "" {
+		ts, err := tokenSourceFromFile(ctx, filename, scope)
+		if err != nil {
+			return nil, fmt.Errorf("google: error getting credentials using %v environment variable: %v", envVar, err)
+		}
+		return ts, nil
+	}
+
+	// Second, try a well-known file.
+	filename := wellKnownFile()
+	_, err := os.Stat(filename)
+	if err == nil {
+		ts, err2 := tokenSourceFromFile(ctx, filename, scope)
+		if err2 == nil {
+			return ts, nil
+		}
+		err = err2
+	} else if os.IsNotExist(err) {
+		err = nil // ignore this error
+	}
+	if err != nil {
+		return nil, fmt.Errorf("google: error getting credentials using well-known file (%v): %v", filename, err)
+	}
+
+	// Third, if we're on Google App Engine use those credentials.
+	if appengineTokenFunc != nil {
+		return AppEngineTokenSource(ctx, scope...), nil
+	}
+
+	// Fourth, if we're on Google Compute Engine use the metadata server.
+	if metadata.OnGCE() {
+		return ComputeTokenSource(""), nil
+	}
+
+	// None are found; return helpful error.
+	const url = "https://developers.google.com/accounts/application-default-credentials"
+	return nil, fmt.Errorf("google: could not find default credentials. See %v for more information.", url)
+}
+
+func wellKnownFile() string {
+	const f = "application_default_credentials.json"
+	if runtime.GOOS == "windows" {
+		return filepath.Join(os.Getenv("APPDATA"), "gcloud", f)
+	}
+	return filepath.Join(guessUnixHomeDir(), ".config", "gcloud", f)
+}
+
+func tokenSourceFromFile(ctx context.Context, filename string, scopes []string) (oauth2.TokenSource, error) {
+	b, err := ioutil.ReadFile(filename)
+	if err != nil {
+		return nil, err
+	}
+	var d struct {
+		// Common fields
+		Type     string
+		ClientID string `json:"client_id"`
+
+		// User Credential fields
+		ClientSecret string `json:"client_secret"`
+		RefreshToken string `json:"refresh_token"`
+
+		// Service Account fields
+		ClientEmail  string `json:"client_email"`
+		PrivateKeyID string `json:"private_key_id"`
+		PrivateKey   string `json:"private_key"`
+	}
+	if err := json.Unmarshal(b, &d); err != nil {
+		return nil, err
+	}
+	switch d.Type {
+	case "authorized_user":
+		cfg := &oauth2.Config{
+			ClientID:     d.ClientID,
+			ClientSecret: d.ClientSecret,
+			Scopes:       append([]string{}, scopes...), // copy
+			Endpoint:     Endpoint,
+		}
+		tok := &oauth2.Token{RefreshToken: d.RefreshToken}
+		return cfg.TokenSource(ctx, tok), nil
+	case "service_account":
+		cfg := &jwt.Config{
+			Email:      d.ClientEmail,
+			PrivateKey: []byte(d.PrivateKey),
+			Scopes:     append([]string{}, scopes...), // copy
+			TokenURL:   JWTTokenURL,
+		}
+		return cfg.TokenSource(ctx), nil
+	case "":
+		return nil, errors.New("missing 'type' field in credentials")
+	default:
+		return nil, fmt.Errorf("unknown credential type: %q", d.Type)
+	}
+}

google/example_test.go

@@ -19,6 +19,15 @@ import (
 	"google.golang.org/appengine/urlfetch"
 )
 
+func ExampleDefaultClient() {
+	client, err := google.DefaultClient(oauth2.NoContext,
+		"https://www.googleapis.com/auth/devstorage.full_control")
+	if err != nil {
+		log.Fatal(err)
+	}
+	client.Get("...")
+}
+
 func Example_webServer() {
 	// Your credentials should be obtained from the Google
 	// Developer Console (https://console.developers.google.com).

google/google.go

@@ -2,15 +2,16 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package google provides support for making
-// OAuth2 authorized and authenticated HTTP requests
-// to Google APIs. It supports Web server, client-side,
-// service accounts, Google Compute Engine service accounts,
-// and Google App Engine service accounts authorization
-// and authentications flows:
+// Package google provides support for making OAuth2 authorized and
+// authenticated HTTP requests to Google APIs.
+// It supports the Web server flow, client-side credentials, service accounts,
+// Google Compute Engine service accounts, and Google App Engine service
+// accounts.
 //
 // For more information, please read
-// https://developers.google.com/accounts/docs/OAuth2.
+// https://developers.google.com/accounts/docs/OAuth2
+// and
+// https://developers.google.com/accounts/application-default-credentials.
 package google // import "golang.org/x/oauth2/google"
 
 import (
@@ -34,7 +35,7 @@ var Endpoint = oauth2.Endpoint{
 // JWTTokenURL is Google's OAuth 2.0 token URL to use with the JWT flow.
 const JWTTokenURL = "https://accounts.google.com/o/oauth2/token"
 
-// JWTConfigFromJSON uses a Google Developers Console client_credentials.json
+// ConfigFromJSON uses a Google Developers Console client_credentials.json
 // file to construct a config.
 // client_credentials.json can be downloadable from https://console.developers.google.com,
 // under "APIs & Auth" > "Credentials". Download the Web application credentials in the