tests/qmp-test: Demonstrate QMP errors jumping the queue

When OOB is enabled, out-of-band commands are executed right away,
everything else is queued.  This lets out-of-band commands "jump the
queue".

However, certain errors are always reported right away, and therefore
can jump the queue even when the erroneous input does not request
out-of-band execution.  These errors are pretty unlikely to occur in
production, but it's wrong all the same.  Mark FIXME.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NPeter Xu <peterx@redhat.com>
Message-Id: <20180703085358.13941-17-armbru@redhat.com>

monitor.c

@@ -4339,6 +4339,7 @@ static void handle_qmp_command(JSONMessageParser *parser, GQueue *tokens)
     return;
 
 err:
+    /* FIXME overtakes queued in-band commands, wrong when !qmp_is_oob() */
     monitor_qmp_respond(mon, NULL, err, NULL);
     qobject_unref(req);
 }

tests/qmp-test.c

@@ -241,6 +241,13 @@ static void test_qmp_oob(void)
     unblock_blocked_cmd();
     recv_cmd_id(qts, "ib-blocks-1");
     recv_cmd_id(qts, "ib-quick-1");
+
+    /* FIXME certain in-band errors overtake slow in-band command */
+    send_cmd_that_blocks(qts, "blocks-2");
+    qtest_async_qmp(qts, "{ 'id': 'err-2' }");
+    recv_cmd_id(qts, NULL);
+    unblock_blocked_cmd();
+    recv_cmd_id(qts, "blocks-2");
     cleanup_blocking_cmd();
 
     qtest_quit(qts);