oauth2: add support for client credential grant type

Creates a new package called clientcredentials and
adds transport and token information to the internal
package. Also modifies the oauth2 package to make
use of the newly added files in the internal package.

The clientcredentials package allows for token requests
using a "client credentials" grant type.

Fixes https://github.com/golang/oauth2/issues/7

Change-Id: Iec649d1029870c27a2d1023baa9d52db42ff45e8
Reviewed-on: https://go-review.googlesource.com/2983
Reviewed-by: Burcu Dogan <jbd@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

internal/oauth2.go

@@ -67,3 +67,10 @@ func ParseINI(ini io.Reader) (map[string]map[string]string, error) {
 	}
 	return result, nil
 }
+
+func CondVal(v string) []string {
+	if v == "" {
+		return nil
+	}
+	return []string{v}
+}

internal/token.go

@@ -0,0 +1,210 @@
+// 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.
+
+// Package internal contains support packages for oauth2 package.
+package internal
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"mime"
+	"net/http"
+	"net/url"
+	"strconv"
+	"strings"
+	"time"
+
+	"golang.org/x/net/context"
+)
+
+// Token represents the crendentials used to authorize
+// the requests to access protected resources on the OAuth 2.0
+// provider's backend.
+//
+// This type is a mirror of oauth2.Token and exists to break
+// an otherwise-circular dependency. Other internal packages
+// should convert this Token into an oauth2.Token before use.
+type Token struct {
+	// AccessToken is the token that authorizes and authenticates
+	// the requests.
+	AccessToken string
+
+	// TokenType is the type of token.
+	// The Type method returns either this or "Bearer", the default.
+	TokenType string
+
+	// RefreshToken is a token that's used by the application
+	// (as opposed to the user) to refresh the access token
+	// if it expires.
+	RefreshToken string
+
+	// Expiry is the optional expiration time of the access token.
+	//
+	// If zero, TokenSource implementations will reuse the same
+	// token forever and RefreshToken or equivalent
+	// mechanisms for that TokenSource will not be used.
+	Expiry time.Time
+
+	// Raw optionally contains extra metadata from the server
+	// when updating a token.
+	Raw interface{}
+}
+
+// tokenJSON is the struct representing the HTTP response from OAuth2
+// providers returning a token in JSON form.
+type tokenJSON struct {
+	AccessToken  string         `json:"access_token"`
+	TokenType    string         `json:"token_type"`
+	RefreshToken string         `json:"refresh_token"`
+	ExpiresIn    expirationTime `json:"expires_in"` // at least PayPal returns string, while most return number
+	Expires      expirationTime `json:"expires"`    // broken Facebook spelling of expires_in
+}
+
+func (e *tokenJSON) expiry() (t time.Time) {
+	if v := e.ExpiresIn; v != 0 {
+		return time.Now().Add(time.Duration(v) * time.Second)
+	}
+	if v := e.Expires; v != 0 {
+		return time.Now().Add(time.Duration(v) * time.Second)
+	}
+	return
+}
+
+type expirationTime int32
+
+func (e *expirationTime) UnmarshalJSON(b []byte) error {
+	var n json.Number
+	err := json.Unmarshal(b, &n)
+	if err != nil {
+		return err
+	}
+	i, err := n.Int64()
+	if err != nil {
+		return err
+	}
+	*e = expirationTime(i)
+	return nil
+}
+
+var brokenAuthHeaderProviders = []string{
+	"https://accounts.google.com/",
+	"https://www.googleapis.com/",
+	"https://github.com/",
+	"https://api.instagram.com/",
+	"https://www.douban.com/",
+	"https://api.dropbox.com/",
+	"https://api.soundcloud.com/",
+	"https://www.linkedin.com/",
+	"https://api.twitch.tv/",
+	"https://oauth.vk.com/",
+	"https://api.odnoklassniki.ru/",
+	"https://connect.stripe.com/",
+	"https://api.pushbullet.com/",
+	"https://oauth.sandbox.trainingpeaks.com/",
+	"https://oauth.trainingpeaks.com/",
+	"https://www.strava.com/oauth/",
+}
+
+// providerAuthHeaderWorks reports whether the OAuth2 server identified by the tokenURL
+// implements the OAuth2 spec correctly
+// See https://code.google.com/p/goauth2/issues/detail?id=31 for background.
+// In summary:
+// - Reddit only accepts client secret in the Authorization header
+// - Dropbox accepts either it in URL param or Auth header, but not both.
+// - Google only accepts URL param (not spec compliant?), not Auth header
+// - Stripe only accepts client secret in Auth header with Bearer method, not Basic
+func providerAuthHeaderWorks(tokenURL string) bool {
+	for _, s := range brokenAuthHeaderProviders {
+		if strings.HasPrefix(tokenURL, s) {
+			// Some sites fail to implement the OAuth2 spec fully.
+			return false
+		}
+	}
+
+	// Assume the provider implements the spec properly
+	// otherwise. We can add more exceptions as they're
+	// discovered. We will _not_ be adding configurable hooks
+	// to this package to let users select server bugs.
+	return true
+}
+
+func RetrieveToken(ctx context.Context, ClientID, ClientSecret, TokenURL string, v url.Values) (*Token, error) {
+	hc, err := ContextClient(ctx)
+	if err != nil {
+		return nil, err
+	}
+	v.Set("client_id", ClientID)
+	bustedAuth := !providerAuthHeaderWorks(TokenURL)
+	if bustedAuth && ClientSecret != "" {
+		v.Set("client_secret", ClientSecret)
+	}
+	req, err := http.NewRequest("POST", TokenURL, strings.NewReader(v.Encode()))
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	if !bustedAuth {
+		req.SetBasicAuth(ClientID, ClientSecret)
+	}
+	r, err := hc.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer r.Body.Close()
+	body, err := ioutil.ReadAll(io.LimitReader(r.Body, 1<<20))
+	if err != nil {
+		return nil, fmt.Errorf("oauth2: cannot fetch token: %v", err)
+	}
+	if code := r.StatusCode; code < 200 || code > 299 {
+		return nil, fmt.Errorf("oauth2: cannot fetch token: %v\nResponse: %s", r.Status, body)
+	}
+
+	var token *Token
+	content, _, _ := mime.ParseMediaType(r.Header.Get("Content-Type"))
+	switch content {
+	case "application/x-www-form-urlencoded", "text/plain":
+		vals, err := url.ParseQuery(string(body))
+		if err != nil {
+			return nil, err
+		}
+		token = &Token{
+			AccessToken:  vals.Get("access_token"),
+			TokenType:    vals.Get("token_type"),
+			RefreshToken: vals.Get("refresh_token"),
+			Raw:          vals,
+		}
+		e := vals.Get("expires_in")
+		if e == "" {
+			// TODO(jbd): Facebook's OAuth2 implementation is broken and
+			// returns expires_in field in expires. Remove the fallback to expires,
+			// when Facebook fixes their implementation.
+			e = vals.Get("expires")
+		}
+		expires, _ := strconv.Atoi(e)
+		if expires != 0 {
+			token.Expiry = time.Now().Add(time.Duration(expires) * time.Second)
+		}
+	default:
+		var tj tokenJSON
+		if err = json.Unmarshal(body, &tj); err != nil {
+			return nil, err
+		}
+		token = &Token{
+			AccessToken:  tj.AccessToken,
+			TokenType:    tj.TokenType,
+			RefreshToken: tj.RefreshToken,
+			Expiry:       tj.expiry(),
+			Raw:          make(map[string]interface{}),
+		}
+		json.Unmarshal(body, &token.Raw) // no error checks for optional fields
+	}
+	// Don't overwrite `RefreshToken` with an empty value
+	// if this was a token refreshing request.
+	if token.RefreshToken == "" {
+		token.RefreshToken = v.Get("refresh_token")
+	}
+	return token, nil
+}

internal/token_test.go

@@ -0,0 +1,28 @@
+// 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.
+
+// Package internal contains support packages for oauth2 package.
+package internal
+
+import (
+	"fmt"
+	"testing"
+)
+
+func Test_providerAuthHeaderWorks(t *testing.T) {
+	for _, p := range brokenAuthHeaderProviders {
+		if providerAuthHeaderWorks(p) {
+			t.Errorf("URL: %s not found in list", p)
+		}
+		p := fmt.Sprintf("%ssomesuffix", p)
+		if providerAuthHeaderWorks(p) {
+			t.Errorf("URL: %s not found in list", p)
+		}
+	}
+	p := "https://api.not-in-the-list-example.com/"
+	if !providerAuthHeaderWorks(p) {
+		t.Errorf("URL: %s found in list", p)
+	}
+
+}

internal/transport.go

@@ -0,0 +1,67 @@
+// 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.
+
+// Package internal contains support packages for oauth2 package.
+package internal
+
+import (
+	"net/http"
+
+	"golang.org/x/net/context"
+)
+
+// HTTPClient is the context key to use with golang.org/x/net/context's
+// WithValue function to associate an *http.Client value with a context.
+var HTTPClient ContextKey
+
+// ContextKey is just an empty struct. It exists so HTTPClient can be
+// an immutable public variable with a unique type. It's immutable
+// because nobody else can create a ContextKey, being unexported.
+type ContextKey struct{}
+
+// ContextClientFunc is a func which tries to return an *http.Client
+// given a Context value. If it returns an error, the search stops
+// with that error.  If it returns (nil, nil), the search continues
+// down the list of registered funcs.
+type ContextClientFunc func(context.Context) (*http.Client, error)
+
+var contextClientFuncs []ContextClientFunc
+
+func RegisterContextClientFunc(fn ContextClientFunc) {
+	contextClientFuncs = append(contextClientFuncs, fn)
+}
+
+func ContextClient(ctx context.Context) (*http.Client, error) {
+	for _, fn := range contextClientFuncs {
+		c, err := fn(ctx)
+		if err != nil {
+			return nil, err
+		}
+		if c != nil {
+			return c, nil
+		}
+	}
+	if hc, ok := ctx.Value(HTTPClient).(*http.Client); ok {
+		return hc, nil
+	}
+	return http.DefaultClient, nil
+}
+
+func ContextTransport(ctx context.Context) http.RoundTripper {
+	hc, err := ContextClient(ctx)
+	// This is a rare error case (somebody using nil on App Engine).
+	if err != nil {
+		return ErrorTransport{err}
+	}
+	return hc.Transport
+}
+
+// ErrorTransport returns the specified error on RoundTrip.
+// This RoundTripper should be used in rare error cases where
+// error handling can be postponed to response handling time.
+type ErrorTransport struct{ Err error }
+
+func (t ErrorTransport) RoundTrip(*http.Request) (*http.Response, error) {
+	return nil, t.Err
+}