MXS-2521: Add test case

The test case doesn't reproduce the problem due to MDEV-19811 getting in
the way but it is likely to reproduce it once that bug has been avoided.

maxscale-system-test/CMakeLists.txt

@@ -966,6 +966,9 @@ add_test_executable(mxs2464_sescmd_reconnect.cpp mxs2464_sescmd_reconnect mxs246
 # MXS-2563: Failing debug assertion at rwsplitsession.cc:1129 : m_expected_responses == 0
 add_test_executable(mxs2563_concurrent_slave_failure.cpp mxs2563_concurrent_slave_failure mxs2563_concurrent_slave_failure LABELS REPL_BACKEND readwritesplit)
 
+# MXS-2521: COM_STMT_EXECUTE maybe return empty result
+add_test_executable(mxs2521_double_exec.cpp mxs2521_double_exec mxs2521_double_exec LABELS REPL_BACKEND readwritesplit)
+
 ############################################
 # BEGIN: binlogrouter and avrorouter tests #
 ############################################

maxscale-system-test/cnf/maxscale.cnf.template.mxs2521_double_exec

@@ -0,0 +1,26 @@
+[maxscale]
+threads=###threads###
+log_info=1
+
+###server###
+
+[MariaDB-Monitor]
+type=monitor
+module=mariadbmon
+servers=###server_line###
+user=maxskysql
+password=skysql
+monitor_interval=1000
+
+[RW-Split-Router]
+type=service
+router=readwritesplit
+servers=###server_line###
+user=maxskysql
+password=skysql
+
+[RW-Split-Listener]
+type=listener
+service=RW-Split-Router
+protocol=MySQLClient
+port=4006

maxscale-system-test/mxs2521_double_exec.cpp

@@ -0,0 +1,67 @@
+/**
+ * MXS-2521: COM_STMT_EXECUTE maybe return empty result
+ * https://jira.mariadb.org/browse/MXS-2521
+ */
+
+#include "testconnections.h"
+
+int main(int argc, char** argv)
+{
+    TestConnections test(argc, argv);
+    test.maxscales->connect();
+
+    auto conn = test.maxscales->conn_rwsplit[0];
+
+    test.try_query(conn, "DROP TABLE IF EXISTS double_execute;");
+    test.try_query(conn, "CREATE TABLE double_execute(a int);");
+    test.try_query(conn, "INSERT INTO double_execute VALUES (123), (456)");
+
+    auto stmt = mysql_stmt_init(conn);
+    std::string sql = "select a, @@server_id from double_execute where a = ?";
+    test.expect(mysql_stmt_prepare(stmt, sql.c_str(), sql.length()) == 0,
+                "Prepare should work: %s", mysql_error(conn));
+
+    int data[2] = {0, 0};
+    MYSQL_BIND my_bind[2] = {};
+    char is_null = 0;
+    my_bind[0].buffer_type = MYSQL_TYPE_LONG;
+    my_bind[0].buffer = &data[0];
+    my_bind[0].buffer_length = sizeof(data[0]);
+    my_bind[0].is_null = &is_null;
+    my_bind[1].buffer_type = MYSQL_TYPE_LONG;
+    my_bind[1].buffer = &data[1];
+    my_bind[1].buffer_length = sizeof(data[2]);
+    my_bind[1].is_null = &is_null;
+    data[1] = 123;
+    test.expect(mysql_stmt_bind_param(stmt, my_bind) == 0, "Bind: %s", mysql_stmt_error(stmt));
+
+    // The first execute is done on the master
+    test.try_query(conn, "BEGIN");
+
+    test.expect(mysql_stmt_execute(stmt) == 0, "First execute should work: %s", mysql_stmt_error(stmt));
+    data[0] = 0;
+
+    mysql_stmt_store_result(stmt);
+    test.expect(mysql_stmt_fetch(stmt) == 0, "First fetch of first execute should work");
+    test.expect(data[0] == 123, "Query should return one row with value 123: `%d`", data[0]);
+    test.expect(mysql_stmt_fetch(stmt) != 0, "Second fetch of first execute should NOT work");
+
+    test.try_query(conn, "COMMIT");
+
+    // The second execute goes to a slave, no new parameters are sent in it
+    data[0] = 123;
+    test.expect(mysql_stmt_execute(stmt) == 0, "Second execute should work: %s", mysql_stmt_error(stmt));
+    data[0] = 0;
+
+    mysql_stmt_store_result(stmt);
+
+    test.expect(mysql_stmt_fetch(stmt) == 0, "First fetch of second execute should work");
+    test.expect(data[0] == 123, "Query should return one row with value 123: `%d`", data[0]);
+    test.expect(mysql_stmt_fetch(stmt) != 0, "Second fetch of second execute should NOT work");
+
+    mysql_stmt_close(stmt);
+
+    test.try_query(conn, "DROP TABLE IF EXISTS double_execute;");
+
+    return test.global_result;
+}