Set no-cache flag when serving a file

Previously, a file served through FileServerMiddleware could be re-used by
a client browser without asking the server whether there's a new
version of the file. Now the client is told to always ask whether the file
was changed instead of just using the cached file.

This is necessary, e.g. if the contest admins make changes to a task
statement during the contest, so contestants won't still get served the
previous version they've already downloaded because the browser assumes
it's a persistent file.

cms/server/file_middleware.py

@@ -2,6 +2,7 @@
 
 # Contest Management System - http://cms-dev.github.io/
 # Copyright © 2018 Luca Wehrstedt <luca.wehrstedt@gmail.com>
+# Copyright © 2021 Manuel Gundlach <manuel.gundlach@gmail.com>
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU Affero General Public License as
@@ -23,9 +24,6 @@
 from cms.db.filecacher import FileCacher, TombstoneError
 
 
-SECONDS_IN_A_YEAR = 365 * 24 * 60 * 60
-
-
 class FileServerMiddleware:
     """Intercept requests wanting to serve files and serve those files.
 
@@ -105,7 +103,7 @@ def wsgi_app(self, environ, start_response):
             response.headers.add(
                 "Content-Disposition", "attachment", filename=filename)
         response.set_etag(digest)
-        response.cache_control.max_age = SECONDS_IN_A_YEAR
+        response.cache_control.no_cache = True
         response.cache_control.private = True
         response.response = \
             wrap_file(environ, fobj, buffer_size=FileCacher.CHUNK_SIZE)