core: Add all supported releases - packages map

For principal - subordinate plugin type relations where the
principal Python payload imports code from packages managed by a
subordinate, upgrades can be problematic.

This helper will allow a subordinate charm to inform its principal
about all implemented release - packages combinations ahead of time.
With this information in place the principal can do the upgrade in
one operation without risk of charm relation RPC type processing at
a critical moment.

Related-Bug: #1806111
Change-Id: I455501c1c2cdb53e62b533be95d2493bf1a5027a

charms_openstack/charm/core.py

@@ -391,6 +391,40 @@ class BaseOpenStackCharm(object, metaclass=BaseOpenStackCharmMeta):
         # Note refers back to the Metaclass property for this charm.
         return self.__class__.singleton
 
+    @property
+    def releases_packages_map(self):
+        """Provide a map of all supported releases and their packages.
+
+        :returns: Map of release, package type and install / purge packages.
+            Example:
+            {
+                'mitaka': {
+                    'deb': {
+                        'install': ['python-ldappool'],
+                        'purge': []
+                    }
+                },
+                'rocky': {
+                    'deb': {
+                        'install': ['python3-ldap', 'python3-ldappool'],
+                        'purge': ['python-ldap', 'python-ldappool']}
+                }
+            }
+        :rtype: Dict[str,Dict[str,List[str]]]
+        """
+        return {
+            release: {
+                package_type: {
+                    'install': (instance.packages if package_type == 'deb'
+                                else instance.snaps),
+                    'purge': (instance.purge_packages if package_type == 'deb'
+                              else []),
+                },
+            }
+            for release, pkg_instance in _releases.items()
+            for package_type, instance in pkg_instance.items()
+        }
+
     def __init__(self, interfaces=None, config=None, release=None):
         """Instantiate an instance of the class.
 

unit_tests/charms_openstack/charm/test_core.py

@@ -127,6 +127,53 @@ class TestBaseOpenStackCharmMeta(BaseOpenStackCharmTest):
             class TestC2(chm_core.BaseOpenStackCharm):
                 release = 'liberty'
 
+    def test_releases_packages_map(self):
+        self.maxDiff = None
+        self.patch_object(chm_core, '_releases', new={})
+
+        class TestC1(chm_core.BaseOpenStackCharm):
+            release = 'liberty'
+            packages = ['l_inst_one', 'l_inst_two']
+            purge_packages = ['l_purge']
+
+        class TestC2(chm_core.BaseOpenStackCharm):
+            release = 'mitaka'
+            packages = ['m_inst_one', 'm_inst_two']
+            purge_packages = ['m_purge']
+
+        class TestC3(chm_core.BaseOpenStackCharm):
+            release = 'ocata'
+            package_type = 'snap'
+            snaps = ['o_snap_one', 'o_snap_two']
+
+        # from any charm release instance we see package_type / install_purge
+        # lists registered by all other charm release instances
+        for cls in (TestC1, TestC2, TestC3):
+            instance = cls()
+            self.assertDictEqual(
+                instance.releases_packages_map,
+                {
+                    'liberty': {
+                        'deb': {
+                            'install': ['l_inst_one', 'l_inst_two'],
+                            'purge': ['l_purge']
+                        }
+                    },
+                    'mitaka': {
+                        'deb': {
+                            'install': ['m_inst_one', 'm_inst_two'],
+                            'purge': ['m_purge']
+                        }
+                    },
+                    'ocata': {
+                        'snap': {
+                            'install': ['o_snap_one', 'o_snap_two'],
+                            'purge': []
+                        }
+                    }
+                }
+            )
+
 
 class TestFunctions(BaseOpenStackCharmTest):