[Bug 1825872] Re: python3-neutron dependency on python3-oslo.privsep dependency needs to run update-alternatives

[James Page]

Switching to python3 in cosmic or bionic/rocky is 100% manual in terms
of switching alternatives OR ensuring that the python-* modules are
purged or not installed in the first place.

For stein, the default is python3 so auto mode should do the right
things in terms of picking binaries, and for train python2 is being
completely dropped.

Which release are you having issues with?

** Changed in: neutron (Ubuntu)
       Status: New => Incomplete

** Changed in: neutron (Ubuntu)
   Importance: Undecided => Low

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Title:
  python3-neutron dependency on python3-oslo.privsep dependency needs to
  run update-alternatives

Status in neutron package in Ubuntu:
  Incomplete

Bug description:
  I'm not clear on how to trigger the set of installed packages that
  leads to this bug, but in any case, I ran into an issue where neutron-
  dhcp-agent would refuse to work. Logs looked like this:

  Apr 22 14:51:48 juju-df624b-4-lxd-10 neutron-dhcp-agent[1999891]: 2019-04-22 14:51:48.176 2000527 CRITICAL privsep [-] Unhandled error: ImportError: No module named neutron.privileged
  Apr 22 14:51:48 juju-df624b-4-lxd-10 neutron-dhcp-agent[1999891]: 2019-04-22 14:51:48.176 2000527 ERROR privsep Traceback (most recent call last):
  Apr 22 14:51:48 juju-df624b-4-lxd-10 neutron-dhcp-agent[1999891]: 2019-04-22 14:51:48.176 2000527 ERROR privsep   File "/usr/bin/privsep-helper", line 10, in <module>
  Apr 22 14:51:48 juju-df624b-4-lxd-10 neutron-dhcp-agent[1999891]: 2019-04-22 14:51:48.176 2000527 ERROR privsep     sys.exit(helper_main())
  Apr 22 14:51:48 juju-df624b-4-lxd-10 neutron-dhcp-agent[1999891]: 2019-04-22 14:51:48.176 2000527 ERROR privsep   File "/usr/lib/python2.7/dist-packages/oslo_privsep/daemon.py", line 480, in helper_main
  Apr 22 14:51:48 juju-df624b-4-lxd-10 neutron-dhcp-agent[1999891]: 2019-04-22 14:51:48.176 2000527 ERROR privsep     context = importutils.import_class(cfg.CONF.privsep_context)
  Apr 22 14:51:48 juju-df624b-4-lxd-10 neutron-dhcp-agent[1999891]: 2019-04-22 14:51:48.176 2000527 ERROR privsep   File "/usr/lib/python2.7/dist-packages/oslo_utils/importutils.py", line 30, in import_class
  Apr 22 14:51:48 juju-df624b-4-lxd-10 neutron-dhcp-agent[1999891]: 2019-04-22 14:51:48.176 2000527 ERROR privsep     __import__(mod_str)
  Apr 22 14:51:48 juju-df624b-4-lxd-10 neutron-dhcp-agent[1999891]: 2019-04-22 14:51:48.176 2000527 ERROR privsep ImportError: No module named neutron.privileged

  
  Looking at the neutron-dhcp-agent process, it was invoked using python3.6, whereas the traceback shows errors with python2.7. 

  With python3-neutron installed and an older python-oslo.privsep and
  newner python3-oslo.privsep set of packages, the wrapper defaults to
  the 2.7 version, which stops the agent from working. Again, I'm not
  sure what the process was that led up to the new python3-oslo.privsep
  being installed, but the cloud has been upgraded through various
  releases, so at some point neutron switched over to python3-neutron.

  The issue appears to be the upgrade did not run update-alternatives,
  which results in the following:

  sudo update-alternatives --config privsep-helper 
  There are 2 choices for the alternative privsep-helper (providing /usr/bin/privsep-helper).

    Selection    Path                             Priority   Status
  ------------------------------------------------------------
  * 0            /usr/bin/python2-privsep-helper   300       auto mode
    1            /usr/bin/python2-privsep-helper   300       manual mode
    2            /usr/bin/python3-privsep-helper   200       manual mode

  Switching the privsep-helper to python3-privsep-helper gets the
  neutron-dhcp-agent running again.

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