Fixing an inifinite loop in case of multi-statements and closed backend connection

If a backend connection was terminated in the middle of the execution of multiple
statements in a multi-statements command, the worker thread was entering an infinite loop.
Furthermore, this commit disables query retry in case of multi-statements if the first one
completed successfully

This fixes #3339

lib/MySQL_Session.cpp

@@ -4066,8 +4066,15 @@ handler_again:
 									if (myds->myconn->MyRS && myds->myconn->MyRS->transfer_started) {
 									// transfer to frontend has started, we cannot retry
 									} else {
-										retry_conn=true;
-										proxy_warning("Retrying query.\n");
+										if (myds->myconn->mysql->server_status & SERVER_MORE_RESULTS_EXIST) {
+											// transfer to frontend has started, because this is, at least,
+											// the second resultset coming from the server
+											// we cannot retry
+											proxy_warning("Disabling query retry because SERVER_MORE_RESULTS_EXIST is set\n");
+										} else {
+											retry_conn=true;
+											proxy_warning("Retrying query.\n");
+										}
 									}
 								}
 							}

lib/mysql_connection.cpp

@@ -1404,15 +1404,20 @@ handler_again:
 				} else {
 					compute_unknown_transaction_status();
 				}
+				if (_myerrno < 2000) {
+					// we can continue only if the error is coming from the backend.
+					// (or if zero)
+					// if the error comes from the client library, something terribly
+					// wrong happened and we cannot continue
+					if (mysql->server_status & SERVER_MORE_RESULTS_EXIST) {
+						async_state_machine=ASYNC_NEXT_RESULT_START;
+					}
+				}
 			}
 			if (mysql_result) {
 				mysql_free_result(mysql_result);
 				mysql_result=NULL;
 			}
-			//if (mysql_next_result(mysql)==0) {
-			if (mysql->server_status & SERVER_MORE_RESULTS_EXIST) {
-				async_state_machine=ASYNC_NEXT_RESULT_START;
-			}
 			break;
 		case ASYNC_SET_AUTOCOMMIT_START:
 			set_autocommit_start();