oauth2: Add support for custom params in Exchange

Allows implementation of PKCE https://www.oauth.com/oauth2-servers/pkce/
for secure code exchange.

Fixes golang/oauth2#286

Change-Id: I07b6fefe6834a2406e8ca2fcbf118d38fc4875d1
GitHub-Last-Rev: 31c5ccbed3
GitHub-Pull-Request: golang/oauth2#285
Reviewed-on: https://go-review.googlesource.com/111515
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

oauth2.go

@@ -124,6 +124,8 @@ func SetAuthURLParam(key, value string) AuthCodeOption {
 //
 // Opts may include AccessTypeOnline or AccessTypeOffline, as well
 // as ApprovalForce.
+// It can also be used to pass the PKCE challange.
+// See https://www.oauth.com/oauth2-servers/pkce/ for more info.
 func (c *Config) AuthCodeURL(state string, opts ...AuthCodeOption) string {
 	var buf bytes.Buffer
 	buf.WriteString(c.Endpoint.AuthURL)
@@ -186,7 +188,10 @@ func (c *Config) PasswordCredentialsToken(ctx context.Context, username, passwor
 //
 // The code will be in the *http.Request.FormValue("code"). Before
 // calling Exchange, be sure to validate FormValue("state").
-func (c *Config) Exchange(ctx context.Context, code string) (*Token, error) {
+//
+// Opts may include the PKCE verifier code if previously used in AuthCodeURL.
+// See https://www.oauth.com/oauth2-servers/pkce/ for more info.
+func (c *Config) Exchange(ctx context.Context, code string, opts ...AuthCodeOption) (*Token, error) {
 	v := url.Values{
 		"grant_type": {"authorization_code"},
 		"code":       {code},
@@ -194,6 +199,9 @@ func (c *Config) Exchange(ctx context.Context, code string) (*Token, error) {
 	if c.RedirectURL != "" {
 		v.Set("redirect_uri", c.RedirectURL)
 	}
+	for _, opt := range opts {
+		opt.setValue(v)
+	}
 	return retrieveToken(ctx, c, v)
 }
 

oauth2_test.go

@@ -135,6 +135,52 @@ func TestExchangeRequest(t *testing.T) {
 	}
 }
 
+func TestExchangeRequest_CustomParam(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.String() != "/token" {
+			t.Errorf("Unexpected exchange request URL, %v is found.", r.URL)
+		}
+		headerAuth := r.Header.Get("Authorization")
+		if headerAuth != "Basic Q0xJRU5UX0lEOkNMSUVOVF9TRUNSRVQ=" {
+			t.Errorf("Unexpected authorization header, %v is found.", headerAuth)
+		}
+		headerContentType := r.Header.Get("Content-Type")
+		if headerContentType != "application/x-www-form-urlencoded" {
+			t.Errorf("Unexpected Content-Type header, %v is found.", headerContentType)
+		}
+		body, err := ioutil.ReadAll(r.Body)
+		if err != nil {
+			t.Errorf("Failed reading request body: %s.", err)
+		}
+		if string(body) != "code=exchange-code&foo=bar&grant_type=authorization_code&redirect_uri=REDIRECT_URL" {
+			t.Errorf("Unexpected exchange payload, %v is found.", string(body))
+		}
+		w.Header().Set("Content-Type", "application/x-www-form-urlencoded")
+		w.Write([]byte("access_token=90d64460d14870c08c81352a05dedd3465940a7c&scope=user&token_type=bearer"))
+	}))
+	defer ts.Close()
+	conf := newConf(ts.URL)
+
+	param := SetAuthURLParam("foo", "bar")
+	tok, err := conf.Exchange(context.Background(), "exchange-code", param)
+	if err != nil {
+		t.Error(err)
+	}
+	if !tok.Valid() {
+		t.Fatalf("Token invalid. Got: %#v", tok)
+	}
+	if tok.AccessToken != "90d64460d14870c08c81352a05dedd3465940a7c" {
+		t.Errorf("Unexpected access token, %#v.", tok.AccessToken)
+	}
+	if tok.TokenType != "bearer" {
+		t.Errorf("Unexpected token type, %#v.", tok.TokenType)
+	}
+	scope := tok.Extra("scope")
+	if scope != "user" {
+		t.Errorf("Unexpected value for scope: %v", scope)
+	}
+}
+
 func TestExchangeRequest_JSONResponse(t *testing.T) {
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if r.URL.String() != "/token" {