Allow trust anchor in gRPC ssl transport security

src/core/tsi/ssl_transport_security.cc

@@ -651,6 +651,8 @@ static tsi_result ssl_ctx_load_verification_certs(SSL_CTX* context,
                                                   STACK_OF(X509_NAME) *
                                                       *root_name) {
   X509_STORE* cert_store = SSL_CTX_get_cert_store(context);
+  X509_STORE_set_flags(cert_store,
+                       X509_V_FLAG_PARTIAL_CHAIN | X509_V_FLAG_TRUSTED_FIRST);
   return x509_store_load_certs(cert_store, pem_roots, pem_roots_size,
                                root_name);
 }