credentials/google: stub out the oauth package in test

credentials/google/google.go

@@ -50,7 +50,7 @@ func NewDefaultCredentialsWithOptions(opts DefaultCredentialsOptions) credential
 		ctx, cancel := context.WithTimeout(context.Background(), tokenRequestTimeout)
 		defer cancel()
 		var err error
-		opts.PerRPCCreds, err = oauth.NewApplicationDefault(ctx)
+		opts.PerRPCCreds, err = newADC(ctx)
 		if err != nil {
 			logger.Warningf("NewDefaultCredentialsWithOptions: failed to create application oauth: %v", err)
 		}
@@ -112,6 +112,9 @@ var (
 	newALTS = func() credentials.TransportCredentials {
 		return alts.NewClientCreds(alts.DefaultClientOptions())
 	}
+	newADC = func(ctx context.Context) (credentials.PerRPCCredentials, error) {
+		return oauth.NewApplicationDefault(ctx)
+	}
 )
 
 // NewWithMode should make a copy of Bundle, and switch mode. Modifying the

credentials/google/google_test.go

@@ -65,17 +65,24 @@ var (
 )
 
 func overrideNewCredsFuncs() func() {
-	oldNewTLS := newTLS
+	origNewTLS := newTLS
 	newTLS = func() credentials.TransportCredentials {
 		return testTLS
 	}
-	oldNewALTS := newALTS
+	origNewALTS := newALTS
 	newALTS = func() credentials.TransportCredentials {
 		return testALTS
 	}
+	origNewADC := newADC
+	newADC = func(context.Context) (credentials.PerRPCCredentials, error) {
+		// We do not use perRPC creds in this test. It is safe to return nil here.
+		return nil, nil
+	}
+
 	return func() {
-		newTLS = oldNewTLS
-		newALTS = oldNewALTS
+		newTLS = origNewTLS
+		newALTS = origNewALTS
+		newADC = origNewADC
 	}
 }