Rewrite the index view to avoid risk of path traversal

I've simplified this view by removing the custom HTTP error handlers, Python 3.5
exception handling and adding documentation.

conservancy/tests.py

@@ -0,0 +1,36 @@
+import datetime
+
+from django.http import Http404
+from django.test import RequestFactory, TestCase
+
+from . import views
+from conservancy.fundgoal.models import FundraisingGoal
+
+
+class ContentTest(TestCase):
+    def setUp(self):
+        self.factory = RequestFactory()
+        FundraisingGoal.objects.create(
+            fundraiser_code_name='cy2023-end-year-match',
+            fundraiser_goal_amount=0,
+            fundraiser_so_far_amount=0,
+            fundraiser_donation_count=0,
+            fundraiser_donation_count_disclose_threshold=0,
+            fundraiser_endtime=datetime.datetime(2000, 1, 1)
+        )
+
+    def test_about_page_served(self):
+        request = self.factory.get('/about/')
+        with self.assertTemplateUsed('about/index.html'):
+            response = views.index(request).render()
+        self.assertContains(response, 'Conservancy is a nonprofit organization')
+
+    def test_annual_report_file_served(self):
+        request = self.factory.get('/docs/conservancy_annual-report_fy-2011.pdf')
+        response = views.index(request)
+        self.assertEqual(response.headers['Content-Type'], 'application/pdf')
+
+    def test_path_traversal_404s(self):
+        request = self.factory.get('/about/../../settings.py')
+        with self.assertRaises(Http404):
+            views.index(request)

conservancy/urls.py

@@ -40,8 +40,6 @@ urlpatterns = [
     url(r'^news/', include('conservancy.news.urls')),
     url(r'^blog/', include('conservancy.blog.urls')),
     # formerly static templated things... (dirs with templates)
-    url(r'^error/(40[134]|500)(?:/index\.html|/|)$', static_views.handler),
-    url(r'^error', static_views.index),
     url(r'^about', static_views.index),
     url(r'^activities', static_views.index),
     url(r'^donate', static_views.index),

conservancy/views.py

@@ -1,61 +1,51 @@
 import mimetypes
-import os.path
 
-from django.http import HttpResponse
+from django.conf import settings
+from django.http import Http404
+from django.http import FileResponse
 from django.template.response import TemplateResponse
 
 from .local_context_processors import fundgoal_lookup
 
-# This component doesn't actually have anything to do with Django's
-# "staticfiles" app. It's only that its templates happen to be located in the
-# STATIC_ROOT directory. They probably shouldn't be there.
-STATIC_ROOT = os.path.abspath(os.path.dirname(__file__))
-FILESYSTEM_ENCODING = 'utf-8'
-
-def handler(request, errorcode):
-    path = os.path.join('error', str(errorcode), 'index.html')
-    fullpath = os.path.join(STATIC_ROOT, path)
-    if not os.path.exists(fullpath):
-        return HttpResponse("Internal error: " + path, status=int(errorcode))
-    else:
-        return TemplateResponse(request, path, status=int(errorcode))
-
-def handler401(request):
-    return handler(request, 401)
-
-def handler403(request):
-    return handler(request, 403)
-
-def handler404(request):
-    return handler(request, 404)
-
-def handler500(request):
-    return handler(request, 500)
-
 def index(request, *args, **kwargs):
+    """Faux CMS: bulk website content stored in templates and document files.
+
+    Rationale: Many websites have a CMS and store the majority of their website
+    content in a relational database eg. WordPress or Wagtail. That's useful
+    because various people can easily be given access to edit the website. The
+    downside is that is application complexity - the management of who change
+    what, when it changed and what changed becomes an application concern. At
+    the other end of the spectrum, we have files that are checked into a Git
+    repository - we get the precise who/what/when out of the box with Git, but
+    require you to have some technical knowledge and appropriate access to
+    commit. Since you're committing to a code repository, this also opens up the
+    possibility to break things you couldn't break via a CMS.
+
+    This view serves most of the textual pages and documents on
+    sfconservancy.org. It works a little like Apache serving mixed PHP/static
+    files - it looks at the URL and tries to find a matching file on the
+    filesystem. If it finds a template, it renders it via Django's template
+    infrastructure. If it finds a file but it's not a template, it will serve
+    the file as-is.
+    """
+    # The name "static" has no connection to Django staticfiles.
+    base_path = settings.BASE_DIR / 'static'
     path = request.path.lstrip('/')
     if path.endswith('/'):
         path += 'index.html'
-    fullpath = os.path.join(STATIC_ROOT, 'static', path)
-    try:
-        # Junk URLs in production (Python 3.5) are causing UnicodeEncodeErrors
-        # here. Can't reproduce in development in Python 3.9 - only Python 2.7.
-        if not os.path.exists(fullpath):
-            return handler404(request)
-    except UnicodeEncodeError:
-        return handler404(request)
-    content_type, _ = mimetypes.guess_type(path)
-    if content_type != 'text/html':
-        return HttpResponse(open(fullpath, 'rb'), content_type)
+    full_path = (base_path / path).resolve()
+    safe_from_path_traversal = full_path.is_relative_to(base_path)
+    if not full_path.exists() or not safe_from_path_traversal:
+        raise Http404()
+    is_template = mimetypes.guess_type(full_path)[0] == 'text/html'
+    if not is_template:
+        return FileResponse(open(full_path, 'rb'))
     else:
         context = kwargs.copy()
         try:
             context['fundgoal'] = fundgoal_lookup(kwargs['fundraiser_sought'])
         except KeyError:
             pass
+        # Maybe this should open() the template file directly so that these
+        # don't have to be included in the global template TEMPLATES.DIRS?
         return TemplateResponse(request, path, context)
-
-def debug(request):
-    path = request.get_full_path()
-    path = path.lstrip('/')
-    return HttpResponse("Hello, static world: " + path)