enable c++ for ALTS kokoro tests

src/core/lib/security/transport/security_handshaker.cc

@@ -232,6 +232,12 @@ static grpc_error* on_handshake_next_done_locked(
     const unsigned char* bytes_to_send, size_t bytes_to_send_size,
     tsi_handshaker_result* handshaker_result) {
   grpc_error* error = GRPC_ERROR_NONE;
+
+  // Handshaker was shutdown.
+  if (h->shutdown) {
+    security_handshake_failed_locked(h, GRPC_ERROR_REF(error));
+    return GRPC_ERROR_CREATE_FROM_STATIC_STRING("Handshaker shutdown");
+  }
   // Read more if we need to.
   if (result == TSI_INCOMPLETE_DATA) {
     GPR_ASSERT(bytes_to_send_size == 0);

tools/run_tests/run_interop_tests.py

@@ -638,10 +638,10 @@ _LANGUAGES_WITH_HTTP2_CLIENTS_FOR_HTTP2_SERVER_TEST_CASES = [
 ]
 
 #TODO: Add c++ when c++ ALTS interop client is ready.
-_LANGUAGES_FOR_ALTS_TEST_CASES = ['java', 'go']
+_LANGUAGES_FOR_ALTS_TEST_CASES = ['java', 'go', 'c++']
 
 #TODO: Add c++ when c++ ALTS interop server is ready.
-_SERVERS_FOR_ALTS_TEST_CASES = ['java', 'go']
+_SERVERS_FOR_ALTS_TEST_CASES = ['java', 'go', 'c++']
 
 _TRANSPORT_SECURITY_OPTIONS = ['tls', 'alts', 'insecure']