Provide slightly better msg on fact cache error (#18759)

If the configured fact_cache plugin (fact_caching config)
fails, raise a fatal error instead of failing mysteriously
later.

Fixes #18751

lib/ansible/plugins/cache/__init__.py

@@ -20,6 +20,7 @@ __metaclass__ = type
 from collections import MutableMapping
 
 from ansible import constants as C
+from ansible.errors import AnsibleError
 from ansible.plugins import cache_loader
 
 try:
@@ -33,6 +34,14 @@ class FactCache(MutableMapping):
 
     def __init__(self, *args, **kwargs):
         self._plugin = cache_loader.get(C.CACHE_PLUGIN)
+        if not self._plugin:
+            raise AnsibleError('Unable to load the facts cache plugin (%s)\n'
+                               'Check fact cache config options :\n'
+                               'Current values:\n'
+                               '    fact_caching: %s\n'
+                               '    fact_caching_connection: %s' %
+                               (C.CACHE_PLUGIN, C.CACHE_PLUGIN, C.CACHE_PLUGIN_CONNECTION))
+
         # Backwards compat: self._display isn't really needed, just import the global display and use that.
         self._display = display
 

test/units/plugins/cache/test_cache.py

@@ -20,6 +20,7 @@ from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type
 
 from ansible.compat.tests import unittest, mock
+from ansible.errors import AnsibleError
 from ansible.plugins.cache import FactCache
 from ansible.plugins.cache.base import BaseCacheModule
 from ansible.plugins.cache.memory import CacheModule as MemoryCache
@@ -56,6 +57,14 @@ class TestFactCache(unittest.TestCase):
         self.assertEqual(type(a_copy), dict)
         self.assertEqual(a_copy, dict(avocado='fruit', daisy='flower'))
 
+    def test_plugin_load_failure(self):
+        # See https://github.com/ansible/ansible/issues/18751
+        # Note no fact_connection config set, so this will fail
+        with mock.patch('ansible.constants.CACHE_PLUGIN', 'json'):
+            self.assertRaisesRegexp(AnsibleError,
+                                    "Unable to load the facts cache plugin.*json.*",
+                                    FactCache)
+
 
 class TestAbstractClass(unittest.TestCase):