set SSL_MODE_ASYNC if neverbleed enabled
@@ -1308,6 +1308,7 @@ static int listener_setup_ssl(h2o_configurator_command_t *cmd, h2o_configurator_
identity->ossl = SSL_CTX_new(SSLv23_server_method());
SSL_CTX_set_options(identity->ossl, ssl_options);
#if PTLS_OPENSSL_HAVE_ASYNC
+ if (use_neverbleed)
SSL_CTX_set_mode(identity->ossl, SSL_MODE_ASYNC);
#endif
