Fixing bugs related to jfonfile cache plugin

* corrupt/invalid file causes tracebacks
* incorrect initialization of display/_display in BaseCacheModule class
* tweaking the way errors in get() on jsonfile caches work, to raise
  a proper AnsibleError in that situation so the playbook/task is stopped

Fixes #12708

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:

lib/ansible/plugins/cache/base.py

@@ -31,7 +31,7 @@ except ImportError:
 
 class BaseCacheModule(with_metaclass(ABCMeta, object)):
 
-    display = display
+    _display = display
 
     @abstractmethod
     def get(self, key):

lib/ansible/plugins/cache/jsonfile.py

@@ -68,9 +68,10 @@ class CacheModule(BaseCacheModule):
                 value = json.load(f)
                 self._cache[key] = value
                 return value
-            except ValueError:
+            except ValueError as e:
-                self._display.warning("error while trying to write to %s : %s" % (cachefile, str(e)))
+                self._display.warning("error while trying to read %s : %s. Most likely a corrupt file, so erasing and failing." % (cachefile, str(e)))
-                raise KeyError
+                self.delete(key)
+                raise AnsibleError("The JSON cache file %s was corrupt, or did not otherwise contain valid JSON data. It has been removed, so you can re-run your command now.")
         finally:
             f.close()
 
@@ -134,7 +135,10 @@ class CacheModule(BaseCacheModule):
                 pass
 
     def delete(self, key):
+        try:
             del self._cache[key]
+        except KeyError:
+            pass
         try:
             os.remove("%s/%s" % (self._cache_dir, key))
         except (OSError,IOError) as e: