google: add ExchangeToken() to run STS exchanges.

Adds the ExchangeToken() function and support structs, but depends on  https://github.com/golang/oauth2/pull/439

Change-Id: Id738a27b0c2ac083409156af1f60283b9140b159
GitHub-Last-Rev: 1aa066dc21e11a278f2698f982b7e7c857114d0a
GitHub-Pull-Request: golang/oauth2#444
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/261918
Run-TryBot: Tyler Bui-Palsulich <tbp@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Tyler Bui-Palsulich <tbp@google.com>
Trust: Cody Oss <codyoss@google.com>
Reviewed-by: Tyler Bui-Palsulich <tbp@google.com>

google/internal/externalaccount/sts_exchange.go

@@ -0,0 +1,96 @@
+// Copyright 2020 The Go 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 externalaccount
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"golang.org/x/oauth2"
+	"io"
+	"net/http"
+	"net/url"
+	"strconv"
+	"strings"
+)
+
+// ExchangeToken performs an oauth2 token exchange with the provided endpoint.
+// The first 4 fields are all mandatory.  headers can be used to pass additional
+// headers beyond the bare minimum required by the token exchange.  options can
+// be used to pass additional JSON-structured options to the remote server.
+func ExchangeToken(ctx context.Context, endpoint string, request *STSTokenExchangeRequest, authentication ClientAuthentication, headers http.Header, options map[string]interface{}) (*STSTokenExchangeResponse, error) {
+
+	client := oauth2.NewClient(ctx, nil)
+
+	data := url.Values{}
+	data.Set("audience", request.Audience)
+	data.Set("grant_type", "urn:ietf:params:oauth:grant-type:token-exchange")
+	data.Set("requested_token_type", "urn:ietf:params:oauth:token-type:access_token")
+	data.Set("subject_token_type", request.SubjectTokenType)
+	data.Set("subject_token", request.SubjectToken)
+	data.Set("scope", strings.Join(request.Scope, " "))
+	opts, err := json.Marshal(options)
+	if err != nil {
+		return nil, fmt.Errorf("oauth2/google: failed to marshal additional options: %v", err)
+	}
+	data.Set("options", string(opts))
+
+	authentication.InjectAuthentication(data, headers)
+	encodedData := data.Encode()
+
+	req, err := http.NewRequestWithContext(ctx, "POST", endpoint, strings.NewReader(encodedData))
+	if err != nil {
+		return nil, fmt.Errorf("oauth2/google: failed to properly build http request: %v", err)
+
+	}
+	for key, list := range headers {
+		for _, val := range list {
+			req.Header.Add(key, val)
+		}
+	}
+	req.Header.Add("Content-Length", strconv.Itoa(len(encodedData)))
+
+	resp, err := client.Do(req)
+
+	if err != nil {
+		return nil, fmt.Errorf("oauth2/google: invalid response from Secure Token Server: %v", err)
+	}
+	defer resp.Body.Close()
+
+	bodyJson := json.NewDecoder(io.LimitReader(resp.Body, 1<<20))
+	var stsResp STSTokenExchangeResponse
+	err = bodyJson.Decode(&stsResp)
+	if err != nil {
+		return nil, fmt.Errorf("oauth2/google: failed to unmarshal response body from Secure Token Server: %v", err)
+
+	}
+
+	return &stsResp, nil
+}
+
+// STSTokenExchangeRequest contains fields necessary to make an oauth2 token exchange.
+type STSTokenExchangeRequest struct {
+	ActingParty struct {
+		ActorToken     string
+		ActorTokenType string
+	}
+	GrantType          string
+	Resource           string
+	Audience           string
+	Scope              []string
+	RequestedTokenType string
+	SubjectToken       string
+	SubjectTokenType   string
+}
+
+// STSTokenExchangeResponse is used to decode the remote server response during an oauth2 token exchange.
+type STSTokenExchangeResponse struct {
+	AccessToken     string `json:"access_token"`
+	IssuedTokenType string `json:"issued_token_type"`
+	TokenType       string `json:"token_type"`
+	ExpiresIn       int    `json:"expires_in"`
+	Scope           string `json:"scope"`
+	RefreshToken    string `json:"refresh_token"`
+}