rootwrap daemon does not work properly

[xianchaobo]

When use rootwrap daemon mode, and it will fail to run command in the following case:
(1)Use daemon to run a command CMDA which does not install on the host. It will fail and raise an exception:
     Unserializable message: ('#ERROR', FilterMatchNotExecutable()) ---It's OK
(2)Then install CMDA on the host, and try to run command again through daemon(DO NOT restart daemon). It still fails and raise the same exception. --- I think this is not what we expect

I try to find what's wrong and find there is a BUG in the code:
https://github.com/openstack/oslo.rootwrap/blob/master/oslo_rootwrap/filters.py#L42

I think we do not need this line `self.real_exec = ""`

When first time to run cmd, real_exec is None.
And if we do not install the cmd on the host, it will be an empty string.
Then we install cmd on the host and run cmd again. It will return the empty string, because empty string is not None. Then it will not to check the cmd on the host forever.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/431516

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/431516
Committed: https://git.openstack.org/cgit/openstack/oslo.rootwrap/commit/?id=a76f5daad99de32bed3a5f901768c24fc869e2a5
Submitter: Jenkins
Branch: master

commit a76f5daad99de32bed3a5f901768c24fc869e2a5
Author: XianChaobo <email address hidden>
Date: Thu Feb 9 21:14:47 2017 +0800

    Always check cmd which does not exist

    In daemon mode, if run a cmd which does not exist,
    it will raise an Exception. But when we install the
    cmd later and run cmd, it still raise an Exception.
    The only work around right now is to restart the
    daemon, with this change, we can avoid restarting
    the daemon

    Closes-Bug: #1663216
    Change-Id: I6f6ff540ed103f0fec329d6264bdac26493c8fad

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/oslo.rootwrap 5.5.0 release.