[Bug 1832075] Re: [19.04][Queens -> Rocky] python3-pymysql is not installed before use

[Corey Bryant]

I'll give this a test as well but need to mark as invalid for the
horizon package since horizon doesn't import pymysql.

Side note, in disco we dropped the ubuntu patch to sqlalchemy that
switched the default dialect from mysqldb to pymysql but it has not be
dropped for python-django so I think we should at least drop that patch
to python-django in eoan.

** Changed in: horizon (Ubuntu)
       Status: New => Invalid

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https://bugs.launchpad.net/bugs/1832075

Title:
  [19.04][Queens -> Rocky] python3-pymysql is not installed before use

Status in OpenStack openstack-dashboard charm:
  Incomplete
Status in horizon package in Ubuntu:
  Invalid

Bug description:
  Ran an upgrade from bionic-queens to bionic-rocky and encountered the
  following:

  /var/log/apt/term.log

  https://paste.ubuntu.com/p/n6WZJbDy6p/

    File "/usr/lib/python3/dist-packages/django/db/backends/mysql/base.py", line 25, in <module>
      import pymysql
  ModuleNotFoundError: No module named 'pymysql'
  dpkg: error processing package openstack-dashboard

  The problem is that python3-django only "suggests" that
  python3-pymysql is installed and does not depend on it while

  root at juju-f2e601-4-lxd-2:~# dpkg -l | grep pymysql
  ii  python-pymysql                   0.8.0-1                           all          Pure-Python MySQL driver - Python 2.x

  root at juju-f2e601-4-lxd-2:~# dpkg -l | grep -P 'python[3]?-django\s'
  ii  python-django                    1:1.11.11-1ubuntu1.3              all          High-level Python web development framework (Python 2 version)
  ii  python3-django                   1:1.11.11-1ubuntu1.3              all          High-level Python web development framework (Python 3 version)

  # python3-django does not depend on pymysql - only suggests it

  ```
  apt-rdepends -f Depends,PreDepends,Suggests python3-django | grep pymysql
  Reading package lists... Done
  Building dependency tree
  Reading state information... Done

  python3-pymysql
  python-pymysql-doc
  ```

  While during openstack-dashboard package setup there is a code path
  leading to an import of pymysql. See below the following line in the
  term.log:

  "Setting up openstack-dashboard (3:15.0.0-0ubuntu1~cloud0) ..."

  The dashboard charm contains python3-pymysql, however, and apt looks
  to be processing packages in the order they are specified (which leads
  to openstack-dashboard being set up before python3-pymysql as there is
  no direct dependency).

  BASE_PACKAGES = [
      'haproxy',
      'memcached',
      'openstack-dashboard',
      'openstack-dashboard-ubuntu-theme',
      'python-keystoneclient',
      'python-memcache',
      'python-novaclient',
  ]

  PY3_PACKAGES = [
      'python3-django-horizon',
      'python3-designate-dashboard',
      'python3-heat-dashboard',
      'python3-neutron-lbaas-dashboard',
      'python3-neutron-fwaas-dashboard',
      'python3-keystoneclient',
      'python3-novaclient',
      'python3-memcache',
      'python3-pymysql',
      'libapache2-mod-wsgi-py3',
  ]

  
  BASE_PACKAGES go first, then PY3_PACKAGES:
  # ...
  def determine_packages():
      """Determine packages to install"""
      packages = deepcopy(BASE_PACKAGES)
  # ...
      if release >= 'rocky':
          packages = [p for p in packages if not p.startswith('python-')]
          packages.extend(PY3_PACKAGES)

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