MXS-1323: Fix crash on attempted retry of read

When a backend is waiting for a response but no statement is stored for
the session, the buffer where the stored statement is copied is not
modified. This means that it needs to be initialized to a NULL value.

Added a test that checks that the behavior works as expected even with
persistent connections. A second test reproduces the crash by executing
parallel SET commands while slaves are blocked.

There is still a behavioral problem in readwritesplit. If a session
command is being executed and it fails on a slave, an error is sent to the
client. In this case it would not be necessary to close the session if the
master is still alive.

maxscale-system-test/mxs1323_retry_read.cpp

@@ -0,0 +1,50 @@
+/**
+ * Test for MXS-1323.
+ * - Check that retried reads work with persistent connections
+ */
+
+#include "testconnections.h"
+
+void* async_block(void* data)
+{
+    TestConnections *test = (TestConnections*)data;
+    sleep(5);
+    test->tprintf("Blocking slave");
+    test->repl->block_node(1);
+    return NULL;
+}
+
+std::string do_query(TestConnections& test)
+{
+    MYSQL* conn = test.open_rwsplit_connection();
+
+    const char* query = "SELECT SLEEP(10), @@server_id";
+    char output[512] = "";
+
+    find_field(conn, query, "@@server_id", output);
+    mysql_close(conn);
+
+    return std::string(output);
+}
+
+int main(int argc, char *argv[])
+{
+    TestConnections test(argc, argv);
+
+    char server_id[2][1024];
+    test.repl->connect();
+    std::string master = test.repl->get_server_id_str(0);
+    std::string slave = test.repl->get_server_id_str(1);
+    test.repl->close_connections();
+
+    test.set_timeout(60);
+    test.add_result(do_query(test) != slave, "The slave should respond to the first query");
+
+    pthread_t thr;
+    pthread_create(&thr, NULL, async_block, &test);
+    test.add_result(do_query(test) != master, "The master should respond to the second query");
+    pthread_join(thr, NULL);
+    test.repl->unblock_node(1);
+
+    return test.global_result;
+}