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

5 files changed, 38 insertions, 17 deletions

diff --git a/lib/ansible/utils/display.py b/lib/ansible/utils/display.py
index 2f21646467..51117761e1 100644
--- a/lib/ansible/utils/display.py
+++ b/lib/ansible/utils/display.py
@@ -36,7 +36,9 @@ from termios import TIOCGWINSZ
 from ansible import constants as C
 from ansible.errors import AnsibleError
 from ansible.module_utils._text import to_bytes, to_text
+from ansible.module_utils.six import with_metaclass
 from ansible.utils.color import stringc
+from ansible.utils.singleton import Singleton
 
 
 try:
@@ -77,7 +79,7 @@ b_COW_PATHS = (
 )
 
 
-class Display:
+class Display(with_metaclass(Singleton, object)):
 
     def __init__(self, verbosity=0):
 
diff --git a/lib/ansible/utils/encrypt.py b/lib/ansible/utils/encrypt.py
index 4445fb87c8..46029037b8 100644
--- a/lib/ansible/utils/encrypt.py
+++ b/lib/ansible/utils/encrypt.py
@@ -16,6 +16,7 @@ from ansible import constants as C
 from ansible.errors import AnsibleError, AnsibleAssertionError
 from ansible.module_utils.six import text_type
 from ansible.module_utils._text import to_text, to_bytes
+from ansible.utils.display import Display
 
 PASSLIB_AVAILABLE = False
 try:
@@ -27,11 +28,7 @@ try:
 except:
     pass
 
-try:
-    from __main__ import display
-except ImportError:
-    from ansible.utils.display import Display
-    display = Display()
+display = Display()
 
 __all__ = ['do_encrypt']
 
diff --git a/lib/ansible/utils/jsonrpc.py b/lib/ansible/utils/jsonrpc.py
index 794f1f4aea..d285cef688 100644
--- a/lib/ansible/utils/jsonrpc.py
+++ b/lib/ansible/utils/jsonrpc.py
@@ -9,13 +9,9 @@ import traceback
 
 from ansible.module_utils._text import to_text
 from ansible.module_utils.six import binary_type
+from ansible.utils.display import Display
 
-
-try:
-    from __main__ import display
-except ImportError:
-    from ansible.utils.display import Display
-    display = Display()
+display = Display()
 
 
 class JsonRpcServer(object):
diff --git a/lib/ansible/utils/plugin_docs.py b/lib/ansible/utils/plugin_docs.py
index c13c6529bc..30816462db 100644
--- a/lib/ansible/utils/plugin_docs.py
+++ b/lib/ansible/utils/plugin_docs.py
@@ -10,12 +10,9 @@ from ansible.module_utils._text import to_native
 from ansible.module_utils.common._collections_compat import MutableMapping, MutableSet, MutableSequence
 from ansible.parsing.plugin_docs import read_docstring, read_docstub
 from ansible.parsing.yaml.loader import AnsibleLoader
+from ansible.utils.display import Display
 
-try:
-    from __main__ import display
-except ImportError:
-    from ansible.utils.display import Display
-    display = Display()
+display = Display()
 
 
 # modules that are ok that they do not have documentation strings
diff --git a/lib/ansible/utils/singleton.py b/lib/ansible/utils/singleton.py
new file mode 100644
index 0000000000..4299403eae
--- /dev/null
+++ b/lib/ansible/utils/singleton.py
@@ -0,0 +1,29 @@
+# Copyright (c) 2017 Ansible Project
+# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
+
+# Make coding more python3-ish
+from __future__ import (absolute_import, division, print_function)
+__metaclass__ = type
+
+from threading import RLock
+
+
+class Singleton(type):
+    """Metaclass for classes that wish to implement Singleton
+    functionality.  If an instance of the class exists, it's returned,
+    otherwise a single instance is instantiated and returned.
+    """
+    def __init__(cls, name, bases, dct):
+        super(Singleton, cls).__init__(name, bases, dct)
+        cls.__instance = None
+        cls.__rlock = RLock()
+
+    def __call__(cls, *args, **kw):
+        if cls.__instance is not None:
+            return cls.__instance
+
+        with cls.__rlock:
+            if cls.__instance is None:
+                cls.__instance = super(Singleton, cls).__call__(*args, **kw)
+
+        return cls.__instance