xds: use google default creds (#3673)

- use google default creds, so the client works not only on GCE (e.g. it also reads env variable for creds).
- Change google default creds to use jwt directly if scope is not set.

- Leak check is disabled temporarily due to googleapis/google-cloud-go#2417

credentials/oauth/oauth.go

@@ -74,6 +74,8 @@ func NewJWTAccessFromKey(jsonKey []byte) (credentials.PerRPCCredentials, error)
 }
 
 func (j jwtAccess) GetRequestMetadata(ctx context.Context, uri ...string) (map[string]string, error) {
+	// TODO: the returned TokenSource is reusable. Store it in a sync.Map, with
+	// uri as the key, to avoid recreating for every RPC.
 	ts, err := google.JWTAccessTokenSourceFromJSON(j.jsonKey, uri[0])
 	if err != nil {
 		return nil, err
@@ -177,9 +179,43 @@ func NewServiceAccountFromFile(keyFile string, scope ...string) (credentials.Per
 // NewApplicationDefault returns "Application Default Credentials". For more
 // detail, see https://developers.google.com/accounts/docs/application-default-credentials.
 func NewApplicationDefault(ctx context.Context, scope ...string) (credentials.PerRPCCredentials, error) {
-	t, err := google.DefaultTokenSource(ctx, scope...)
+	creds, err := google.FindDefaultCredentials(ctx, scope...)
 	if err != nil {
 		return nil, err
 	}
-	return TokenSource{t}, nil
+
+	// If JSON is nil, the authentication is provided by the environment and not
+	// with a credentials file, e.g. when code is running on Google Cloud
+	// Platform. Use the returned token source.
+	if creds.JSON == nil {
+		return TokenSource{creds.TokenSource}, nil
+	}
+
+	// If auth is provided by env variable or creds file, the behavior will be
+	// different based on whether scope is set. Because the returned
+	// creds.TokenSource does oauth with jwt by default, and it requires scope.
+	// We can only use it if scope is not empty, otherwise it will fail with
+	// missing scope error.
+	//
+	// If scope is set, use it, it should just work.
+	//
+	// If scope is not set, we try to use jwt directly without oauth (this only
+	// works if it's a service account).
+
+	if len(scope) != 0 {
+		return TokenSource{creds.TokenSource}, nil
+	}
+
+	// Try to convert JSON to a jwt config without setting the optional scope
+	// parameter to check if it's a service account (the function errors if it's
+	// not). This is necessary because the returned config doesn't show the type
+	// of the account.
+	if _, err := google.JWTConfigFromJSON(creds.JSON); err != nil {
+		// If this fails, it's not a service account, return the original
+		// TokenSource from above.
+		return TokenSource{creds.TokenSource}, nil
+	}
+
+	// If it's a service account, create a JWT only access with the key.
+	return NewJWTAccessFromKey(creds.JSON)
 }

xds/internal/client/bootstrap/bootstrap.go

@@ -139,7 +139,7 @@ func NewConfig() (*Config, error) {
 			config.BalancerName = xs.ServerURI
 			for _, cc := range xs.ChannelCreds {
 				if cc.Type == googleDefaultCreds {
-					config.Creds = grpc.WithCredentialsBundle(google.NewComputeEngineCredentials())
+					config.Creds = grpc.WithCredentialsBundle(google.NewDefaultCredentials())
 					// We stop at the first credential type that we support.
 					break
 				}

xds/internal/client/bootstrap/bootstrap_test.go

@@ -1,3 +1,5 @@
+// +build !appengine
+
 /*
  *
  * Copyright 2019 gRPC authors.
@@ -22,13 +24,11 @@ import (
 	"os"
 	"testing"
 
+	corepb "github.com/envoyproxy/go-control-plane/envoy/api/v2/core"
 	"github.com/golang/protobuf/proto"
+	structpb "github.com/golang/protobuf/ptypes/struct"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/credentials/google"
-	"google.golang.org/grpc/internal/grpctest"
-
-	corepb "github.com/envoyproxy/go-control-plane/envoy/api/v2/core"
-	structpb "github.com/golang/protobuf/ptypes/struct"
 )
 
 var (
@@ -58,19 +58,14 @@ var (
 	}
 )
 
-type s struct {
-	grpctest.Tester
-}
-
-func Test(t *testing.T) {
-	grpctest.RunSubTests(t, s{})
-}
+// TODO: enable leak check for this package when
+// https://github.com/googleapis/google-cloud-go/issues/2417 is fixed.
 
 // TestNewConfig exercises the functionality in NewConfig with different
 // bootstrap file contents. It overrides the fileReadFunc by returning
 // bootstrap file contents defined in this test, instead of reading from a
 // file.
-func (s) TestNewConfig(t *testing.T) {
+func TestNewConfig(t *testing.T) {
 	bootstrapFileMap := map[string]string{
 		"empty":          "",
 		"badJSON":        `["test": 123]`,
@@ -283,7 +278,7 @@ func (s) TestNewConfig(t *testing.T) {
 	}
 }
 
-func (s) TestNewConfigEnvNotSet(t *testing.T) {
+func TestNewConfigEnvNotSet(t *testing.T) {
 	os.Unsetenv(fileEnv)
 	config, err := NewConfig()
 	if err == nil {