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.

1 files changed, 55 insertions, 0 deletions

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