py3: add support for switching workloads from py2->py3

For migration of services to execution under Python 3 add support
for a Charm class to declare a set of packages to purge on upgrade.

This allows the target release Charm class to remove Python 2
modules installed as part of prior charm versions.

After the purge call, autoremove will also be called to purge any
other packages that where indirectly installed.

Add new python_version attribute for charm classes, supporting
conditional install of python{3}-memcache for token caching.

Change-Id: I17a93524fb7ec134b4a3a61cd0e86977daf49c4e

charms_openstack/charm/classes.py

@@ -67,6 +67,12 @@ class OpenStackCharm(BaseOpenStackCharm,
     # List of packages to install
     packages = []
 
+    # List of packages to purge
+    purge_packages = []
+
+    # Python version used to execute installed workload
+    python_version = 2
+
     # List of snaps to install
     snaps = []
 
@@ -232,7 +238,10 @@ class OpenStackAPICharm(OpenStackCharm):
         """
         packages = []
         if self.enable_memcache(release=release):
-            packages.extend(['memcached', 'python-memcache'])
+            if self.python_version == 2:
+                packages.extend(['memcached', 'python-memcache'])
+            else:
+                packages.extend(['memcached', 'python3-memcache'])
         return packages
 
     def get_amqp_credentials(self):

charms_openstack/charm/core.py

@@ -949,6 +949,17 @@ class BaseOpenStackCharmActions(object):
             packages=self.all_packages,
             options=dpkg_opts,
             fatal=True)
+        if self.purge_packages:
+            # NOTE(jamespage):
+            # Ensure packages that should be purged are actually installed
+            installed_packages = list(
+                set(self.purge_packages) -
+                set(fetch.filter_installed_packages(self.purge_packages))
+            )
+            if installed_packages:
+                fetch.apt_purge(packages=installed_packages,
+                                fatal=True)
+                fetch.apt_autoremove(purge=True, fatal=True)
         self.release = new_os_rel
 
     def do_openstack_upgrade_config_render(self, interfaces_list):

unit_tests/charms_openstack/charm/common.py

@@ -24,6 +24,7 @@ class MyOpenStackCharm(chm_classes.OpenStackCharm):
     release = 'icehouse'
     name = 'my-charm'
     packages = ['p1', 'p2', 'p3', 'package-to-filter']
+    purge_packages = ['python-obsolete', 'python-notinstalled']
     snaps = ['mysnap']
     version_package = 'p2'
     version_snap = 'mysnap'

unit_tests/charms_openstack/charm/test_core.py

@@ -789,6 +789,11 @@ class TestMyOpenStackCharm(BaseOpenStackCharmTest):
         self.patch_object(chm_core.charmhelpers.fetch, 'apt_update')
         self.patch_object(chm_core.charmhelpers.fetch, 'apt_upgrade')
         self.patch_object(chm_core.charmhelpers.fetch, 'apt_install')
+        self.patch_object(chm_core.charmhelpers.fetch, 'apt_purge')
+        self.patch_object(chm_core.charmhelpers.fetch, 'apt_autoremove')
+        self.patch_object(chm_core.charmhelpers.fetch,
+                          'filter_installed_packages',
+                          return_value=['python-notinstalled'])
         self.patch_object(chm_core.os_utils, 'snap_install_requested',
                           return_value=False)
         self.target.do_openstack_pkg_upgrade()
@@ -806,6 +811,12 @@ class TestMyOpenStackCharm(BaseOpenStackCharmTest):
                 '--option', 'Dpkg::Options::=--force-confnew', '--option',
                 'Dpkg::Options::=--force-confdef'],
             fatal=True)
+        self.apt_purge.assert_called_once_with(
+            packages=['python-obsolete'],
+            fatal=True)
+        self.apt_autoremove.assert_called_once_with(
+            purge=True,
+            fatal=True)
 
     def test_do_openstack_pkg_upgrade_snap(self):
         self.patch_target('config',