Supporting service account impersonation.

Change-Id: I6481964659c2c852e50bf8b19a1306629e7cc4ae

google/internal/externalaccount/basecredentials.go

@@ -89,6 +89,14 @@ type tokenSource struct {
 func (ts tokenSource) Token() (*oauth2.Token, error) {
 	conf := ts.conf
 
+	if conf.ServiceAccountImpersonationURL != "" {
+		token, err := ts.impersonate()
+		if err != nil {
+			return nil, err
+		}
+		return token, err
+	}
+
 	credSource := conf.parse(ts.ctx)
 	if credSource == nil {
 		return nil, fmt.Errorf("oauth2/google: unable to parse credential source")
@@ -130,6 +138,5 @@ func (ts tokenSource) Token() (*oauth2.Token, error) {
 	if stsResp.RefreshToken != "" {
 		accessToken.RefreshToken = stsResp.RefreshToken
 	}
-
 	return accessToken, nil
 }

google/internal/externalaccount/basecredentials_test.go

@@ -22,7 +22,6 @@ var testConfig = Config{
 	Audience:         "32555940559.apps.googleusercontent.com",
 	SubjectTokenType: "urn:ietf:params:oauth:token-type:jwt",
 	TokenInfoURL:     "http://localhost:8080/v1/tokeninfo",
-	ServiceAccountImpersonationURL: "https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/service-gcs-admin@$PROJECT_ID.iam.gserviceaccount.com:generateAccessToken",
 	ClientSecret:     "notsosecret",
 	ClientID:         "rbrgnognrhongo3bi4gb9ghg9g",
 	CredentialSource: testBaseCredSource,
@@ -55,7 +54,7 @@ func TestToken(t *testing.T) {
 		}
 		body, err := ioutil.ReadAll(r.Body)
 		if err != nil {
-			t.Errorf("Failed reading request body: %s.", err)
+			t.Fatalf("Failed reading request body: %s.", err)
 		}
 		if got, want := string(body), baseCredsRequestBody; got != want {
 			t.Errorf("Unexpected exchange payload: got %v but want %v", got, want)

google/internal/externalaccount/impersonate.go

@@ -0,0 +1,79 @@
+// 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 (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"golang.org/x/oauth2"
+	"io"
+	"io/ioutil"
+	"net/http"
+	"time"
+)
+
+// generateAccesstokenReq is used for service account impersonation
+type generateAccessTokenReq struct {
+	Delegates []string `json:"delegates,omitempty"`
+	Lifetime  string   `json:"lifetime,omitempty"`
+	Scope     []string `json:"scope,omitempty"`
+}
+
+type impersonateTokenResponse struct {
+	AccessToken string `json:"accessToken"`
+	ExpireTime  string `json:"expireTime"`
+}
+
+func (ts tokenSource) impersonate() (*oauth2.Token, error) {
+	reqBody := generateAccessTokenReq{
+		Lifetime: "3600s",
+		Scope:    ts.conf.Scopes,
+	}
+	b, err := json.Marshal(reqBody)
+
+	serviceAccountImpersonationURL := ts.conf.ServiceAccountImpersonationURL
+	ts.conf.ServiceAccountImpersonationURL = ""
+	ts.conf.Scopes = []string{"https://www.googleapis.com/auth/cloud-platform"}
+
+	client := oauth2.NewClient(ts.ctx, ts)
+	if err != nil {
+		return &oauth2.Token{}, fmt.Errorf("google: unable to marshal request: %v", err)
+	}
+	req, err := http.NewRequest("POST", serviceAccountImpersonationURL, bytes.NewReader(b))
+	if err != nil {
+		return nil, fmt.Errorf("impersonate: unable to create request: %v", err)
+	}
+	req = req.WithContext(ts.ctx)
+	req.Header.Set("Content-Type", "application/json")
+
+	resp, err := client.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("impersonate: unable to generate access token: %v", err)
+	}
+	defer resp.Body.Close()
+	body, err := ioutil.ReadAll(io.LimitReader(resp.Body, 1<<20))
+	if err != nil {
+		return nil, fmt.Errorf("impersonate: unable to read body: %v", err)
+	}
+	if c := resp.StatusCode; c < 200 || c > 299 {
+		return nil, fmt.Errorf("impersonate: status code %d: %s", c, body)
+	}
+
+	var accessTokenResp impersonateTokenResponse
+	if err := json.Unmarshal(body, &accessTokenResp); err != nil {
+		return nil, fmt.Errorf("impersonate: unable to parse response: %v", err)
+	}
+	expiry, err := time.Parse(time.RFC3339, accessTokenResp.ExpireTime)
+	if err != nil {
+		return nil, fmt.Errorf("impersonate: unable to parse expiry: %v", err)
+	}
+	return &oauth2.Token{
+		AccessToken: accessTokenResp.AccessToken,
+		Expiry:      expiry,
+		TokenType:   "Bearer",
+	}, nil
+
+}

google/internal/externalaccount/impersonate_test.go

@@ -0,0 +1,100 @@
+// 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"
+	"io/ioutil"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+var testImpersonateConfig = Config{
+	Audience:         "32555940559.apps.googleusercontent.com",
+	SubjectTokenType: "urn:ietf:params:oauth:token-type:jwt",
+	TokenInfoURL:     "http://localhost:8080/v1/tokeninfo",
+	ClientSecret:     "notsosecret",
+	ClientID:         "rbrgnognrhongo3bi4gb9ghg9g",
+	CredentialSource: testBaseCredSource,
+	Scopes:           []string{"https://www.googleapis.com/auth/devstorage.full_control"},
+}
+
+var (
+	baseImpersonateCredsReqBody  = "audience=32555940559.apps.googleusercontent.com&grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Atoken-exchange&options=null&requested_token_type=urn%3Aietf%3Aparams%3Aoauth%3Atoken-type%3Aaccess_token&scope=https%3A%2F%2Fwww.googleapis.com%2Fauth%2Fcloud-platform&subject_token=street123&subject_token_type=urn%3Aietf%3Aparams%3Aoauth%3Atoken-type%3Ajwt"
+	baseImpersonateCredsRespBody = `{"accessToken":"Second.Access.Token","expireTime":"2020-12-28T15:01:23Z"}`
+)
+
+func TestImpersonation(t *testing.T) {
+	impersonateServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+
+		if got, want := r.URL.String(), "/"; got != want {
+			t.Errorf("URL.String(): got %v but want %v", got, want)
+		}
+		headerAuth := r.Header.Get("Authorization")
+		if got, want := headerAuth, "Bearer Sample.Access.Token"; got != want {
+			t.Errorf("got %v but want %v", got, want)
+		}
+		headerContentType := r.Header.Get("Content-Type")
+		if got, want := headerContentType, "application/json"; got != want {
+			t.Errorf("got %v but want %v", got, want)
+		}
+		body, err := ioutil.ReadAll(r.Body)
+		if err != nil {
+			t.Fatalf("Failed reading request body: %v.", err)
+		}
+		if got, want := string(body), "{\"lifetime\":\"3600s\",\"scope\":[\"https://www.googleapis.com/auth/devstorage.full_control\"]}"; got != want {
+			t.Errorf("Unexpected impersonation payload: got %v but want %v", got, want)
+		}
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(baseImpersonateCredsRespBody))
+	}))
+	testImpersonateConfig.ServiceAccountImpersonationURL = impersonateServer.URL
+	targetServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if got, want := r.URL.String(), "/"; got != want {
+			t.Errorf("URL.String(): got %v but want %v", got, want)
+		}
+		headerAuth := r.Header.Get("Authorization")
+		if got, want := headerAuth, "Basic cmJyZ25vZ25yaG9uZ28zYmk0Z2I5Z2hnOWc6bm90c29zZWNyZXQ="; got != want {
+			t.Errorf("got %v but want %v", got, want)
+		}
+		headerContentType := r.Header.Get("Content-Type")
+		if got, want := headerContentType, "application/x-www-form-urlencoded"; got != want {
+			t.Errorf("got %v but want %v", got, want)
+		}
+		body, err := ioutil.ReadAll(r.Body)
+		if err != nil {
+			t.Fatalf("Failed reading request body: %v.", err)
+		}
+		if got, want := string(body), baseImpersonateCredsReqBody; got != want {
+			t.Errorf("Unexpected exchange payload: got %v but want %v", got, want)
+		}
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(baseCredsResponseBody))
+	}))
+	defer targetServer.Close()
+
+	testImpersonateConfig.TokenURL = targetServer.URL
+	ourTS := tokenSource{
+		ctx:  context.Background(),
+		conf: &testImpersonateConfig,
+	}
+
+	oldNow := now
+	defer func() { now = oldNow }()
+	now = testNow
+
+	tok, err := ourTS.Token()
+	if err != nil {
+		t.Fatalf("Unexpected error: %e", err)
+	}
+	if got, want := tok.AccessToken, "Second.Access.Token"; got != want {
+		t.Errorf("Unexpected access token: got %v, but wanted %v", got, want)
+	}
+	if got, want := tok.TokenType, "Bearer"; got != want {
+		t.Errorf("Unexpected TokenType: got %v, but wanted %v", got, want)
+	}
+
+}