Add module 'write-files-deferred' executed in stage 'final' (#916)

The main idea is to introduce a second module that takes care of
writing files, but in the 'final' stage.

While the introduction of a second module would allow for choosing
the appropriate place withing the order of modules (and stages),
there is no addition top-level directive being added to the cloud
configuration schema. Instead, 'write-files' schema is being extended
to include a 'defer' attribute used only by the 'write-deffered-files'
modules.

The new module 'write-deferred-files' reuses as much as
possible of the 'write-files' functionality.

2 files changed, 92 insertions, 4 deletions

diff --git a/cloudinit/config/cc_write_files.py b/cloudinit/config/cc_write_files.py
index 8601e707..41c75fa2 100644
--- a/cloudinit/config/cc_write_files.py
+++ b/cloudinit/config/cc_write_files.py
@@ -21,6 +21,7 @@ frequency = PER_INSTANCE
 
 DEFAULT_OWNER = "root:root"
 DEFAULT_PERMS = 0o644
+DEFAULT_DEFER = False
 UNKNOWN_ENC = 'text/plain'
 
 LOG = logging.getLogger(__name__)
@@ -90,6 +91,24 @@ schema = {
         # Create an empty file on the system
         write_files:
         - path: /root/CLOUD_INIT_WAS_HERE
+        """),
+        dedent("""\
+        # Defer writing the file until after the package (Nginx) is
+        # installed and its user is created alongside
+        write_files:
+        - path: /etc/nginx/conf.d/example.com.conf
+          content: |
+            server {
+                server_name example.com;
+                listen 80;
+                root /var/www;
+                location / {
+                    try_files $uri $uri/ $uri.html =404;
+                }
+            }
+          owner: 'nginx:nginx'
+          permissions: '0640'
+          defer: true
         """)],
     'frequency': frequency,
     'type': 'object',
@@ -151,6 +170,15 @@ schema = {
                             ``path`` exists. Default: **false**.
                         """),
                     },
+                    'defer': {
+                        'type': 'boolean',
+                        'default': DEFAULT_DEFER,
+                        'description': dedent("""\
+                            Defer writing the file until 'final' stage, after
+                            users were created, and packages were installed.
+                            Default: **{defer}**.
+                        """.format(defer=DEFAULT_DEFER)),
+                    },
                 },
                 'required': ['path'],
                 'additionalProperties': False
@@ -163,13 +191,18 @@ __doc__ = get_schema_doc(schema)  # Supplement python help()
 
 
 def handle(name, cfg, _cloud, log, _args):
-    files = cfg.get('write_files')
-    if not files:
+    validate_cloudconfig_schema(cfg, schema)
+    file_list = cfg.get('write_files', [])
+    filtered_files = [
+        f for f in file_list if not util.get_cfg_option_bool(f,
+                                                             'defer',
+                                                             DEFAULT_DEFER)
+    ]
+    if not filtered_files:
         log.debug(("Skipping module named %s,"
                    " no/empty 'write_files' key in configuration"), name)
         return
-    validate_cloudconfig_schema(cfg, schema)
-    write_files(name, files)
+    write_files(name, filtered_files)
 
 
 def canonicalize_extraction(encoding_type):
diff --git a/cloudinit/config/cc_write_files_deferred.py b/cloudinit/config/cc_write_files_deferred.py
new file mode 100644
index 00000000..0c75aa22
--- /dev/null
+++ b/cloudinit/config/cc_write_files_deferred.py
@@ -0,0 +1,55 @@
+# Copyright (C) 2021 Canonical Ltd.
+#
+# This file is part of cloud-init. See LICENSE file for license information.
+
+"""Defer writing certain files"""
+
+from textwrap import dedent
+
+from cloudinit.config.schema import validate_cloudconfig_schema
+from cloudinit import util
+from cloudinit.config.cc_write_files import (
+    schema as write_files_schema, write_files, DEFAULT_DEFER)
+
+
+schema = util.mergemanydict([
+    {
+        'id': 'cc_write_files_deferred',
+        'name': 'Write Deferred Files',
+        'title': dedent("""\
+            write certain files, whose creation as been deferred, during
+            final stage
+        """),
+        'description': dedent("""\
+            This module is based on `'Write Files' <write-files>`__, and
+            will handle all files from the write_files list, that have been
+            marked as deferred and thus are not being processed by the
+            write-files module.
+
+            *Please note that his module is not exposed to the user through
+            its own dedicated top-level directive.*
+        """)
+    },
+    write_files_schema
+])
+
+# Not exposed, because related modules should document this behaviour
+__doc__ = None
+
+
+def handle(name, cfg, _cloud, log, _args):
+    validate_cloudconfig_schema(cfg, schema)
+    file_list = cfg.get('write_files', [])
+    filtered_files = [
+        f for f in file_list if util.get_cfg_option_bool(f,
+                                                         'defer',
+                                                         DEFAULT_DEFER)
+    ]
+    if not filtered_files:
+        log.debug(("Skipping module named %s,"
+                   " no deferred file defined in configuration"), name)
+        return
+    write_files(name, filtered_files)
+
+
+# vi: ts=4 expandtab