MXS-2393 Reject statement that cannot be parsed completely

As the masking filter must reject statements that use columns to be
masked as arguments to functions, a statement that cannot be fully
parsed must be rejected. Unless a statement can be fully parsed we
cannot know whether such usage occurs.

server/modules/filter/masking/maskingfiltersession.cc

@@ -389,6 +389,8 @@ bool MaskingFilterSession::reject_if_function_used(GWBUF* pPacket)
         zHost = "";
     }
 
+    if (qc_parse(pPacket, QC_COLLECT_FIELDS | QC_COLLECT_FUNCTIONS) == QC_QUERY_PARSED)
+    {
         auto pred1 = [&sRules, zUser, zHost](const QC_FIELD_INFO& field_info) {
             const MaskingRules::Rule* pRule = sRules->get_rule_for(field_info, zUser, zHost);
 
@@ -425,6 +427,18 @@ bool MaskingFilterSession::reject_if_function_used(GWBUF* pPacket)
 
             rejected = true;
         }
+    }
+    else
+    {
+        std::stringstream ss;
+        ss << "The statement could not be fully parsed and will hence be "
+           << "rejected (masking filter).";
+
+        GWBUF* pResponse = modutil_create_mysql_err_msg(1, 0, 1141, "HY000", ss.str().c_str());
+        set_response(pResponse);
+
+        rejected = true;
+    }
 
     return rejected;
 }