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_stress.cpp

@@ -0,0 +1,81 @@
+/**
+ * Test for MXS-1323.
+ * - Check that retried reads work with persistent connections
+ */
+
+#include "testconnections.h"
+#include <sstream>
+
+static bool running = true;
+
+void* async_query(void* data)
+{
+    TestConnections *test = (TestConnections*)data;
+
+    while (running && test->global_result == 0)
+    {
+        MYSQL* conn = test->open_rwsplit_connection();
+
+        for (int i = 0; i < 50; i++)
+        {
+            const char* query = "SET @a = (SELECT SLEEP(1))";
+            test->try_query(conn, query);
+        }
+
+        mysql_close(conn);
+    }
+
+    return NULL;
+}
+
+#define NUM_THR 5
+
+int main(int argc, char *argv[])
+{
+    TestConnections test(argc, argv);
+    pthread_t query_thr[NUM_THR];
+    std::stringstream ss;
+
+    ss << "CREATE OR REPLACE TABLE test.t1 (id INT)";
+    test.connect_maxscale();
+    test.try_query(test.conn_rwsplit, ss.str().c_str());
+
+    ss.str("");
+    ss << "INSERT INTO test.t1 VALUES (0)";
+    for (int i = 1; i <= 10000; i++)
+    {
+        ss << ",(" << i << ")";
+    }
+    test.try_query(test.conn_rwsplit, ss.str().c_str());
+
+    test.close_maxscale_connections();
+
+    if (test.global_result)
+    {
+        return test.global_result;
+    }
+
+    for (int i = 0; i < NUM_THR; i++)
+    {
+        pthread_create(&query_thr[i], NULL, async_query, &test);
+    }
+
+    for (int i = 0; i < 3 && test.global_result == 0; i++)
+    {
+        test.tprintf("Round %d", i + 1);
+        test.repl->block_node(1);
+        sleep(5);
+        test.repl->unblock_node(1);
+        sleep(5);
+    }
+
+    running = false;
+
+    for (int i = 0; i < NUM_THR; i++)
+    {
+        test.set_timeout(10);
+        pthread_join(query_thr[i], NULL);
+    }
+
+    return test.global_result;
+}