wsgi: Make response headers RFC7231-compliant.

- 405 (Method Not Allowed) response contains the list of methods allowed on
  current resource
- all responses contain a Date header

caucase/test.py

@@ -1778,10 +1778,21 @@ class CaucaseTest(unittest.TestCase):
       'PATH_INFO': '/cau/crl/123',
       'REQUEST_METHOD': 'GET',
     })[0], 404)
-    self.assertEqual(request({
+
+    min_date = round(time.time())
+    status, _, header_dict, _ = request({
       'PATH_INFO': '/cau/crl',
       'REQUEST_METHOD': 'PUT',
-    })[0], 405)
+    })
+    max_date = round(time.time())
+    self.assertEqual(status, 405)
+    self.assertItemsEqual(
+      [x.strip() for x in header_dict['Allow'].split(',')],
+      ['GET', 'OPTIONS'],
+    )
+    response_date = utils.IMFfixdate2timestamp(header_dict['Date'])
+    self.assertGreaterEqual(response_date, min_date)
+    self.assertLessEqual(response_date, max_date)
 
     self.assertEqual(request({
       'PATH_INFO': '/cau/csr/123/456',

caucase/utils.py

@@ -22,8 +22,10 @@ Small-ish functions needed in many places.
 """
 from __future__ import absolute_import, print_function
 from binascii import a2b_base64, b2a_base64
+import calendar
 from collections import defaultdict
 import datetime
+import email
 import json
 import os
 import threading
@@ -443,6 +445,25 @@ def datetime2timestamp(value):
   """
   return (value - EPOCH).total_seconds()
 
+def timestamp2IMFfixdate(value):
+  """
+  Convert a timestamp into an IMF-fixdate string following RFC7231.
+  """
+  return email.utils.formatdate(
+    value,
+    localtime=False,
+    usegmt=True,
+  )
+
+def IMFfixdate2timestamp(value):
+  """
+  Convert an IMF-fixdate string following RFC7231 into a timestamp.
+  """
+  result = email.utils.parsedate(value)
+  if result is None:
+    return None
+  return calendar.timegm(result)
+
 class SleepInterrupt(KeyboardInterrupt):
   """
   A sleep was interrupted by a KeyboardInterrupt

caucase/wsgi.py

@@ -147,6 +147,12 @@ class BadMethod(ApplicationError):
   """
   status = _getStatus(httplib.METHOD_NOT_ALLOWED)
 
+  def __init__(self, allowed_list):
+    super(BadMethod, self).__init__(allowed_list)
+    self._response_headers = [
+      ('Allow', ', '.join(allowed_list)),
+    ]
+
 class Conflict(ApplicationError):
   """
   HTTP conflict
@@ -543,7 +549,7 @@ class Application(object):
               # CORS headers added, add more
               self._optionAddCORSHeaders(method_dict, cors_header_list)
           else:
-            raise BadMethod
+            raise BadMethod(method_dict.keys() + ['OPTIONS'])
         else:
           subpath = action_dict.get('subpath', SUBPATH_FORBIDDEN)
           if (
@@ -592,6 +598,7 @@ class Application(object):
     # Note: header_list and cors_header_list are expected to contain
     # distinct header sets. This may not always stay true for "Vary".
     header_list.extend(cors_header_list)
+    header_list.append(('Date', utils.timestamp2IMFfixdate(time.time())))
     start_response(status, header_list)
     return result