remote/config: support drop-in files

Pass "dropins" variable to a template being rendered containing a list of
*.yaml files in a directory named after the template being rendered with
".d" appended. For example when rendering /etc/labgrid/configuration.yaml
a list of files matching /etc/labgrid/configuration.yaml.d/*.yaml will be
available. Then configuration.yaml may be as simple as

{% for f in drop_ins %}
{% include f %}
{% endfor %}

And adding a target is possible by simply creating a file in
/etc/labgrid/configuration.yaml.d directory.

Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>

Author: Łukasz Stelmach

doc/configuration.rst

@@ -3995,12 +3995,31 @@ something like ``{{ env['FOOBAR'] }}`` to insert the content of environment
 variable ``FOOBAR``.
 In addition to ``env`` the template also has access to the following variables:
 
-isolated
-  ``True`` or ``False``, depending on the :code:`--isolated` command line option.
+drop_ins
+  An alphabetically sorted list of files in a directory named after the
+  currently processed template with ".d" appended. For example if the template
+  is named ``/etc/labgrid/configuration.yaml`` the list will contain files
+  matching ``/etc/labgrid/configuration.yaml.d/*.yaml`` pattern. The paths are
+  relative to the directory containing the template (i.e. ``/etc/labgrid``
+  in our example). These files can be included using the ``include`` directive.
+  A template loading all its drop-ins at would look like this
+
+.. code-block:: yaml
+   # for f in drop_ins
+   {% include f %}
+   # endfor
+
+.. warning::
+   Use the ``{% include f %}`` form instead of ``# include f``, otherwise
+   Jinja2 would strip newline characters before concatenation, and thus break
+   the YAML formatting.
 
 hostname
   The hostname of the exporter host. Can be used to e.g. construct URLs to the
   current host (``http://{{ hostname }}/``).
 
+isolated
+  ``True`` or ``False``, depending on the :code:`--isolated` command line option.
+
 name
   The name of the exporter.

labgrid/remote/config.py

@@ -1,5 +1,6 @@
 from pprint import pprint
 
+import glob
 import os
 
 import attr
@@ -15,15 +16,21 @@ class ResourceConfig:
     template_env = attr.ib(default=attr.Factory(dict), validator=attr.validators.instance_of(dict))
 
     def __attrs_post_init__(self):
+        _dirname = os.path.dirname(self.filename)
         env = jinja2.Environment(
-            loader=jinja2.FileSystemLoader(os.path.dirname(self.filename)),
+            loader=jinja2.FileSystemLoader(_dirname),
             line_statement_prefix="#",
             line_comment_prefix="##",
         )
         try:
             template = env.get_template(os.path.basename(self.filename))
         except jinja2.TemplateNotFound:
             raise NoConfigFoundError(f"{self.filename} could not be found")
+        drop_ins = [
+            os.path.relpath(p, _dirname) for p in sorted(glob.glob(os.path.join(self.filename + '.d','*.yaml')))
+        ]
+
+        self.template_env["drop_ins"] = drop_ins
         rendered = template.render(self.template_env)
         pprint(("rendered", rendered))
         self.data = load(rendered)