Catch errors from badly formatted date strings

We’ve been seeing attempts to access regulation versions with the format YEAR-DAY-MONTH. This isn’t a valid date string, and raises a `ValidationError`, which we don’t catch, so the user sees a 500. This change catches that `ValidationError` and raises a 404.

cfgov/regulations3k/models/pages.py

@@ -6,6 +6,7 @@
 from functools import partial
 from urllib.parse import urljoin
 
+from django.core.exceptions import ValidationError
 from django.core.paginator import InvalidPage, Paginator
 from django.db import models
 from django.http import Http404, HttpResponse, JsonResponse
@@ -283,11 +284,15 @@ def get_effective_version(self, request, date_str=None):
         if not draft_permission:
             query_filter['draft'] = False
 
-        effective_version = self.regulation.versions.filter(
-            **query_filter
-        ).order_by(
-            '-effective_date'
-        ).first()
+        try:
+            effective_version = self.regulation.versions.filter(
+                **query_filter
+            ).order_by(
+                '-effective_date'
+            ).first()
+        except ValidationError:
+            # This can be raised by an invalid date string
+            raise Http404
 
         if effective_version is None:
             raise Http404

cfgov/regulations3k/tests/test_pages.py

@@ -87,3 +87,11 @@ def test_redirect_uppercase(self):
         self.assertEqual(
             response.get('location'),
             '/reg-landing/1002/interp-2/')
+
+    def test_invalid_effective_date(self):
+        # Try to fetch a date string that's incorrectly formatted.
+        # It should 404.
+        response = self.client.get(
+            '/reg-landing/1002/2014-18-01/'
+        )
+        self.assertEqual(response.status_code, 404)