oauth2/google: add client_credentials.json Config constructor

Change-Id: I27969a381784f83db7cc8287f65f4ba6a1d972bb
Reviewed-on: https://go-review.googlesource.com/4968
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

google/google.go

@@ -34,6 +34,39 @@ 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
+// 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
+// JSON format and provide the contents of the file as jsonKey.
+func ConfigFromJSON(jsonKey []byte, scope ...string) (*oauth2.Config, error) {
+	var j struct {
+		Web struct {
+			ClientID     string   `json:"client_id"`
+			ClientSecret string   `json:"client_secret"`
+			RedirectURIs []string `json:"redirect_uris"`
+			AuthURI      string   `json:"auth_uri"`
+			TokenURI     string   `json:"token_uri"`
+		} `json:"web"`
+	}
+	if err := json.Unmarshal(jsonKey, &j); err != nil {
+		return nil, err
+	}
+	if len(j.Web.RedirectURIs) < 1 {
+		return nil, errors.New("oauth2/google: missing redirect URL in the client_credentials.json")
+	}
+	return &oauth2.Config{
+		ClientID:     j.Web.ClientID,
+		ClientSecret: j.Web.ClientSecret,
+		RedirectURL:  j.Web.RedirectURIs[0],
+		Scopes:       scope,
+		Endpoint: oauth2.Endpoint{
+			AuthURL:  j.Web.AuthURI,
+			TokenURL: j.Web.TokenURI,
+		},
+	}, nil
+}
+
 // JWTConfigFromJSON uses a Google Developers service account JSON key file to read
 // the credentials that authorize and authenticate the requests.
 // Create a service account on "Credentials" page under "APIs & Auth" for your

google/google_test.go

@@ -0,0 +1,50 @@
+// 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 (
+	"strings"
+	"testing"
+)
+
+var webJSONKey = []byte(`
+{
+    "web": {
+        "auth_uri": "https://google.com/o/oauth2/auth",
+        "client_secret": "3Oknc4jS_wA2r9i",
+        "token_uri": "https://google.com/o/oauth2/token",
+        "client_email": "222-nprqovg5k43uum874cs9osjt2koe97g8@developer.gserviceaccount.com",
+        "redirect_uris": ["https://www.example.com/oauth2callback"],
+        "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/222-nprqovg5k43uum874cs9osjt2koe97g8@developer.gserviceaccount.com",
+        "client_id": "222-nprqovg5k43uum874cs9osjt2koe97g8.apps.googleusercontent.com",
+        "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
+        "javascript_origins": ["https://www.example.com"]
+    }
+}`)
+
+func TestConfigFromJSON(t *testing.T) {
+	conf, err := ConfigFromJSON(webJSONKey, "scope1", "scope2")
+	if err != nil {
+		t.Error(err)
+	}
+	if got, want := conf.ClientID, "222-nprqovg5k43uum874cs9osjt2koe97g8.apps.googleusercontent.com"; got != want {
+		t.Errorf("ClientID = %q; want %q", got, want)
+	}
+	if got, want := conf.ClientSecret, "3Oknc4jS_wA2r9i"; got != want {
+		t.Errorf("ClientSecret = %q; want %q", got, want)
+	}
+	if got, want := conf.RedirectURL, "https://www.example.com/oauth2callback"; got != want {
+		t.Errorf("RedictURL = %q; want %q", got, want)
+	}
+	if got, want := strings.Join(conf.Scopes, ","), "scope1,scope2"; got != want {
+		t.Errorf("Scopes = %q; want %q", got, want)
+	}
+	if got, want := conf.Endpoint.AuthURL, "https://google.com/o/oauth2/auth"; got != want {
+		t.Errorf("AuthURL = %q; want %q", got, want)
+	}
+	if got, want := conf.Endpoint.TokenURL, "https://google.com/o/oauth2/token"; got != want {
+		t.Errorf("TokenURL = %q; want %q", got, want)
+	}
+}