Add docker_container_exec module (#105)

* Move some code from plugin_utils to module_utils.

* First version of docker_container_exec module.

* Linting.

* Avoid using display.

* Move common socket_handler code to module_utils.

* Add module support, use it in docker_container_exec.

* Add tests.

* Fix copyright lines.

* Add this and other 'new' modules to README.

* Apply suggestions from code review

Co-authored-by: Andrew Klychkov <aaklychkov@mail.ru>

* Update plugins/modules/docker_container_exec.py

Co-authored-by: Andrew Klychkov <aaklychkov@mail.ru>

plugins/module_utils/socket_helper.py

@@ -0,0 +1,53 @@
+# Copyright (c) 2019-2021, Felix Fontein <felix@fontein.de>
+# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
+
+from __future__ import (absolute_import, division, print_function)
+__metaclass__ = type
+
+
+import fcntl
+import os
+import os.path
+import socket as pysocket
+
+from ansible.module_utils.six import PY3
+
+
+def make_unblocking(sock):
+    if hasattr(sock, '_sock'):
+        sock._sock.setblocking(0)
+    elif hasattr(sock, 'setblocking'):
+        sock.setblocking(0)
+    else:
+        fcntl.fcntl(sock.fileno(), fcntl.F_SETFL, fcntl.fcntl(sock.fileno(), fcntl.F_GETFL) | os.O_NONBLOCK)
+
+
+def _empty_writer(msg):
+    pass
+
+
+def shutdown_writing(sock, log=_empty_writer):
+    if hasattr(sock, 'shutdown_write'):
+        sock.shutdown_write()
+    elif hasattr(sock, 'shutdown'):
+        try:
+            sock.shutdown(pysocket.SHUT_WR)
+        except TypeError as e:
+            # probably: "TypeError: shutdown() takes 1 positional argument but 2 were given"
+            log('Shutting down for writing not possible; trying shutdown instead: {0}'.format(e))
+            sock.shutdown()
+    elif PY3 and isinstance(sock, getattr(pysocket, 'SocketIO')):
+        sock._sock.shutdown(pysocket.SHUT_WR)
+    else:
+        log('No idea how to signal end of writing')
+
+
+def write_to_socket(sock, data):
+    if hasattr(sock, '_send_until_done'):
+        # WrappedSocket (urllib3/contrib/pyopenssl) doesn't have `send`, but
+        # only `sendall`, which uses `_send_until_done` under the hood.
+        return sock._send_until_done(data)
+    elif hasattr(sock, 'send'):
+        return sock.send(data)
+    else:
+        return os.write(sock.fileno(), data)