help: Fix failing setup when manual directory is not available

- It is reported on the mailing list that first setup operation on the help app
fails. The logs indicate that /usr/share/freedombox/manual directory is not
present on the system. Although this situation does not occur on a normal
installation, catch, log and ignore this error to be safe.

- Also ensure that this static files component is added in post_init so that
basic setup is not at all affected by it.

Tests:

- In the development directory, move doc/manual to doc/manual.bak. Observe that
the error message is printed during initialization but the process continues.

- With the manual directory properly restored, the full help manual is displayed
properly with images.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/help/__init__.py

@@ -3,6 +3,7 @@
 FreedomBox app for help pages.
 """
 
+import logging
 import os
 
 from django.utils.translation import gettext_lazy as _
@@ -11,6 +12,8 @@ from django.utils.translation import pgettext_lazy
 from plinth import app as app_module
 from plinth import cfg, menu, web_server
 
+logger = logging.getLogger(__name__)
+
 app = None
 
 
@@ -54,12 +57,19 @@ class HelpApp(app_module.App):
                               order=100)
         self.add(menu_item)
 
+    def post_init(self):
+        """Perform post initialization operations."""
         directory_map = {}
-        langs = os.listdir(os.path.join(cfg.doc_dir, 'manual'))
-        for lang in langs:
-            manual_dir = os.path.join(cfg.doc_dir, 'manual', lang, 'images')
-            manual_url = f'/help/manual/{lang}/images'
-            directory_map[manual_url] = manual_dir
+        try:
+            langs = os.listdir(os.path.join(cfg.doc_dir, 'manual'))
+            for lang in langs:
+                manual_dir = os.path.join(cfg.doc_dir, 'manual', lang,
+                                          'images')
+                manual_url = f'/help/manual/{lang}/images'
+                directory_map[manual_url] = manual_dir
+        except FileNotFoundError:
+            logger.error('Unable to find manual directory. Ensure that '
+                         'freedombox-doc-* packages are installed')
 
         static_files = web_server.StaticFiles('static-files-help',
                                               directory_map)