Don't set Last-Modified header if mtime is zero

Some filesystems (e.g. certain FUSE mounts) incorrecly report all mtimes
as zero which causes WhiteNoise to set a Last-Modified header at the
UNIX epoch and then to consider all files as unmodified.

We now ignore zero-valued mtimes as, for all practical purposes, these
are guaranteed to be incorrect.

Closes #222

tests/test_whitenoise.py

@@ -7,13 +7,16 @@
 except ImportError:
     from urlparse import urljoin
 import shutil
+import stat
 import sys
 import warnings
+from wsgiref.headers import Headers
 from wsgiref.simple_server import demo_app
 
 from .utils import TestServer, Files
 
 from whitenoise import WhiteNoise
+from whitenoise.responders import StaticFile
 
 
 # Update Py2 TestCase to support Py3 method names
@@ -287,3 +290,18 @@ def test_directory_path_can_be_pathlib_instance(self):
         root = Path(Files('root').directory)
         # Check we can construct instance without it blowing up
         WhiteNoise(None, root=root, autorefresh=True)
+
+    def test_last_modified_not_set_when_mtime_is_zero(self):
+        class FakeStatEntry(object):
+            st_mtime = 0
+            st_size = 1024
+            st_mode = stat.S_IFREG
+        stat_cache = {
+            __file__: FakeStatEntry()
+        }
+        responder = StaticFile(__file__, [], stat_cache=stat_cache)
+        response = responder.get_response('GET', {})
+        response.file.close()
+        headers_dict = Headers(response.headers)
+        self.assertNotIn('Last-Modified', headers_dict)
+        self.assertNotIn('ETag', headers_dict)

whitenoise/responders.py

@@ -8,6 +8,7 @@
 import os
 import re
 import stat
+from time import mktime
 try:
     from urllib.parse import quote
 except ImportError:
@@ -133,10 +134,16 @@ def get_headers(self, headers_list, files):
             headers['Vary'] = 'Accept-Encoding'
         if 'Last-Modified' not in headers:
             mtime = main_file.stat.st_mtime
-            headers['Last-Modified'] = formatdate(mtime, usegmt=True)
+            # Not all filesystems report mtimes, and sometimes they report an
+            # mtime of 0 which we know is incorrect
+            if mtime:
+                headers['Last-Modified'] = formatdate(mtime, usegmt=True)
         if 'ETag' not in headers:
-            headers['ETag'] = '"{:x}-{:x}"'.format(
-                    int(main_file.stat.st_mtime), main_file.stat.st_size)
+            last_modified = parsedate(headers['Last-Modified'])
+            if last_modified:
+                timestamp = int(mktime(last_modified))
+                headers['ETag'] = '"{:x}-{:x}"'.format(
+                        timestamp, main_file.stat.st_size)
         return headers
 
     @staticmethod
@@ -170,6 +177,8 @@ def is_not_modified(self, request_headers):
         previous_etag = request_headers.get('HTTP_IF_NONE_MATCH')
         if previous_etag is not None:
             return previous_etag == self.etag
+        if self.last_modified is None:
+            return False
         try:
             last_requested = request_headers['HTTP_IF_MODIFIED_SINCE']
         except KeyError: