fix(logs): a 404 is not written to the app log

Django logs every 4xx as a django.request WARNING; each 404 of a scanner
or an old link was a line in the app log, burying the ones that matter.
404s are left out; 403s and errors stay (a refused page also has a Logs
row).

webvirtcloud/log_context.py

@@ -22,3 +22,11 @@ class RequestFilter(logging.Filter):
         except Exception:  # the user may be read from the database here; a failure must not lose the line
             pass
         return True
+
+
+class NotFoundFilter(logging.Filter):
+    """Leaves out django.request's line for a 404: scanners and old links
+    would bury the lines that matter."""
+
+    def filter(self, record):
+        return getattr(record, "status_code", None) != 404

webvirtcloud/settings.py.template

@@ -256,6 +256,7 @@ LOGGING = {
     },
     'filters': {
         'request': {'()': 'webvirtcloud.log_context.RequestFilter'},
+        'not_found': {'()': 'webvirtcloud.log_context.NotFoundFilter'},
     },
     'handlers': {
         'mail_admins': {
@@ -282,6 +283,7 @@ LOGGING = {
             "handlers": ["mail_admins"],
             "level": "ERROR",
             "propagate": True,
+            "filters": ["not_found"],
         }
     },
 }

webvirtcloud/tests/test_logging.py

@@ -72,3 +72,14 @@ class RequestIdTestCase(TestCase):
     def test_an_id_sent_by_a_client_cannot_write_into_the_log(self):
         response = self.client.get("/accounts/login/", HTTP_X_REQUEST_ID="x\n[forged] line")
         self.assertRegex(response["X-Request-ID"], r"^[0-9a-f]{32}$")
+
+
+class NotFoundTestCase(TestCase):
+    def test_a_404_is_not_logged_and_a_403_is(self):
+        self.client.force_login(User.objects.create_user("plain"))
+        with self.assertLogs("django.request", "WARNING") as logs:
+            self.client.get("/no-such-page/")
+            self.client.get("/admin/users/")
+            logging.getLogger("django.request").warning("end")  # assertLogs needs one line
+        self.assertEqual([line.split(":")[0] for line in logs.output], ["WARNING", "WARNING"])
+        self.assertIn("Forbidden", logs.output[0])