Docs: Make docsite rebuilds smarter/faster (#45062)

* Make the following scripts  idempotent so that we only have to rebuild changed docs, not all docs:

  * plugin_formatter
  * generate_man
  * dump_keywords.py
  * dump_config.py
  * testing_formatter.sh

lib/ansible/utils/_build_helpers.py

@@ -0,0 +1,38 @@
+# Copyright: (c) 2018, Ansible Project
+# 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
+
+"""
+This file contains common code for building ansible. If you want to use code from here at runtime,
+it needs to be moved out of this file and the implementation looked over to figure out whether API
+should be changed before being made public.
+"""
+
+import os.path
+
+
+def update_file_if_different(filename, b_data):
+    '''
+    Replace file content only if content is different.
+
+    This preserves timestamps in case the file content has not changed.  It performs multiple
+    operations on the file so it is not atomic and may be slower than simply writing to the file.
+
+    :arg filename: The filename to write to
+    :b_data: Byte string containing the data to write to the file
+    '''
+    try:
+        with open(filename, 'rb') as f:
+            b_data_old = f.read()
+    except IOError as e:
+        if e.errno != 2:
+            raise
+        # File did not exist, set b_data_old to a sentinel value so that
+        # b_data gets written to the filename
+        b_data_old = None
+
+    if b_data_old != b_data:
+        with open(filename, 'wb') as f:
+            f.write(b_data)

lib/ansible/utils/plugin_docs.py

@@ -1,22 +1,6 @@
-# (c) 2012, Jan-Piet Mens <jpmens () gmail.com>
-#
-# This file is part of Ansible
-#
-# Ansible is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-#
-# Ansible is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-#
-# You should have received a copy of the GNU General Public License
-# along with Ansible.  If not, see <http://www.gnu.org/licenses/>.
-#
+# Copyright: (c) 2012, Jan-Piet Mens <jpmens () gmail.com>
+# 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