Add a Singleton metaclass, use it with Display (#48935)

* Add a Singleton class, use it with Display

* update six import

* Move remaining failes to display singleton

* Fix rebase issues

* Singleton improvements

* Add code-smell for 'from __main__ import display'. ci_complete

* s/self/cls/g

* Add docs for no-main-display

* Address linting issues

* Add changelog fragment. ci_complete

* Implement reentrant lock for class instantiation in Singleton

* Add Display singleton porting guide

lib/ansible/plugins/inventory/__init__.py

@@ -33,12 +33,9 @@ from ansible.module_utils.common._collections_compat import Mapping
 from ansible.module_utils.parsing.convert_bool import boolean
 from ansible.module_utils.six import string_types
 from ansible.template import Templar
+from ansible.utils.display import Display
 
-try:
-    from __main__ import display
-except ImportError:
-    from ansible.utils.display import Display
-    display = Display()
+display = Display()
 
 _SAFE_GROUP = re.compile("[^A-Za-z0-9_]")
 

lib/ansible/plugins/inventory/aws_ec2.py

@@ -133,12 +133,7 @@ from ansible.module_utils.six import string_types
 from ansible.module_utils.ec2 import ansible_dict_to_boto3_filter_list, boto3_tag_list_to_ansible_dict
 from ansible.module_utils.ec2 import camel_dict_to_snake_dict
 from ansible.plugins.inventory import BaseInventoryPlugin, Constructable, Cacheable, to_safe_group_name
-try:
-    from __main__ import display
-except ImportError:
-    from ansible.utils.display import Display
-    display = Display()
-
+from ansible.utils.display import Display
 
 try:
     import boto3
@@ -146,6 +141,8 @@ try:
 except ImportError:
     raise AnsibleError('The ec2 dynamic inventory plugin requires boto3 and botocore.')
 
+display = Display()
+
 # The mappings give an array of keys to get from the filter name to the value
 # returned by boto3's EC2 describe_instances method.
 

lib/ansible/plugins/inventory/toml.py

@@ -100,6 +100,7 @@ from ansible.module_utils.common._collections_compat import MutableMapping, Muta
 from ansible.module_utils.six import string_types, text_type
 from ansible.parsing.yaml.objects import AnsibleSequence, AnsibleUnicode
 from ansible.plugins.inventory import BaseFileInventoryPlugin
+from ansible.utils.display import Display
 
 try:
     import toml
@@ -107,12 +108,7 @@ try:
 except ImportError:
     HAS_TOML = False
 
-try:
-    from __main__ import display
-except ImportError:
-    from ansible.utils.display import Display
-    display = Display()
-
+display = Display()
 
 WARNING_MSG = (
     'The TOML inventory format is marked as preview, which means that it is not guaranteed to have a backwards '