Fix inventory cache interface (#50446)

* Replace InventoryFileCacheModule with a better developer-interface

Use new interface for inventory plugins with backwards compatibility

Auto-update the backing cache-plugin if the cache has changed after parsing the inventory plugin

* Update CacheModules to use the config system and add a deprecation warning if they are being imported directly rather than using cache_loader

* Fix foreman inventory caching

* Add tests

* Add integration test to check that fact caching works normally with cache plugins using ansible.constants and inventory caching provides a helpful error for non-compatible cache plugins

* Add some developer documentation for inventory and cache plugins

* Add user documentation for inventory caching

* Add deprecation docs

* Apply suggestions from docs review

* Add changelog

lib/ansible/plugins/cache/mongodb.py

@@ -27,6 +27,7 @@ DOCUMENTATION = '''
             section: defaults
       _prefix:
         description: User defined prefix to use when creating the DB entries
+        default: ansible_facts
         env:
           - name: ANSIBLE_CACHE_PLUGIN_PREFIX
         ini:
@@ -50,20 +51,33 @@ from contextlib import contextmanager
 from ansible import constants as C
 from ansible.errors import AnsibleError
 from ansible.plugins.cache import BaseCacheModule
+from ansible.utils.display import Display
 
 try:
     import pymongo
 except ImportError:
     raise AnsibleError("The 'pymongo' python module is required for the mongodb fact cache, 'pip install pymongo>=3.0'")
 
+display = Display()
+
 
 class CacheModule(BaseCacheModule):
     """
     A caching module backed by mongodb.
     """
     def __init__(self, *args, **kwargs):
-        self._timeout = int(C.CACHE_PLUGIN_TIMEOUT)
-        self._prefix = C.CACHE_PLUGIN_PREFIX
+        try:
+            super(CacheModule, self).__init__(*args, **kwargs)
+            self._connection = self.get_option('_uri')
+            self._timeout = int(self.get_option('_timeout'))
+            self._prefix = self.get_option('_prefix')
+        except KeyError:
+            display.deprecated('Rather than importing CacheModules directly, '
+                               'use ansible.plugins.loader.cache_loader', version='2.12')
+            self._connection = C.CACHE_PLUGIN_CONNECTION
+            self._timeout = int(C.CACHE_PLUGIN_TIMEOUT)
+            self._prefix = C.CACHE_PLUGIN_PREFIX
+
         self._cache = {}
         self._managed_indexes = False
 
@@ -94,7 +108,7 @@ class CacheModule(BaseCacheModule):
         This is a context manager for opening and closing mongo connections as needed. This exists as to not create a global
         connection, due to pymongo not being fork safe (http://api.mongodb.com/python/current/faq.html#is-pymongo-fork-safe)
         '''
-        mongo = pymongo.MongoClient(C.CACHE_PLUGIN_CONNECTION)
+        mongo = pymongo.MongoClient(self._connection)
         try:
             db = mongo.get_default_database()
         except pymongo.errors.ConfigurationError: