MXS-4090: Add failsafe for unexpected responses

If the backend protocol itself is broken, as is seen in MXS-4090, an
unexpected response can still end up being routed to the routers. To
handle the cases where an unexpected result will cause a crash, we need
code in readwritesplit that kills the session if it was't expecting a
result. This should prevent the crash while also allowing better debug
information to be generated in the hopes that it will eventually be
fixed.

include/maxscale/backend.hh

@@ -161,6 +161,16 @@ public:
         return !m_responses.empty() && m_responses.front() == IGNORE_RESPONSE;
     }
 
+    /**
+     * @brief Check if a response is unexpected
+     *
+     * @return True if the result was not expected
+     */
+    bool unexpected_response() const
+    {
+        return m_responses.empty();
+    }
+
     /**
      * @brief Check if the backend is closed
      *

server/modules/routing/readwritesplit/rwsplitsession.cc

@@ -542,6 +542,18 @@ bool RWSplitSession::clientReply(GWBUF* writebuf, const mxs::ReplyRoute& down, c
 
     if (reply.is_complete())
     {
+        if (backend->unexpected_response())
+        {
+            MXS_ERROR("Unexpected response from '%s', closing session: %s",
+                      backend->name(), reply.describe().c_str());
+            m_pSession->dump_statements();
+            m_pSession->dump_session_log();
+            m_pSession->kill();
+            gwbuf_free(writebuf);
+            mxb_assert(!true);
+            return false;
+        }
+
         bool ignore_response = backend->should_ignore_response();
 
         if (ignore_response)