Provide retry-after guidance on throttled requests

Fixes bug 942874

Guide the caller's redrive strategy with a Retry-After hint in
the 413 response emitted when rate limits are exceeded.

The simplest format of Retry-After is used, i.e. number of seconds
as opposed to a HTTP time string.

Change-Id: I8b1a28f964a111008b1a88d9c0f96c5a0abd8314

2 files changed, 15 insertions, 1 deletions

diff --git a/nova/api/openstack/wsgi.py b/nova/api/openstack/wsgi.py
index 8f0259024..047b68956 100644
--- a/nova/api/openstack/wsgi.py
+++ b/nova/api/openstack/wsgi.py
@@ -18,6 +18,8 @@
 import inspect
 from xml.dom import minidom
 from xml.parsers import expat
+import math
+import time
 
 from lxml import etree
 import webob
@@ -1075,7 +1077,8 @@ class OverLimitFault(webob.exc.HTTPException):
         """
         Initialize new `OverLimitFault` with relevant information.
         """
-        self.wrapped_exc = webob.exc.HTTPRequestEntityTooLarge()
+        hdrs = OverLimitFault._retry_after(retry_time)
+        self.wrapped_exc = webob.exc.HTTPRequestEntityTooLarge(headers=hdrs)
         self.content = {
             "overLimitFault": {
                 "code": self.wrapped_exc.status_int,
@@ -1084,6 +1087,13 @@ class OverLimitFault(webob.exc.HTTPException):
             },
         }
 
+    @staticmethod
+    def _retry_after(retry_time):
+        delay = int(math.ceil(retry_time - time.time()))
+        retry_after = delay if delay > 0 else 0
+        headers = {'Retry-After': '%d' % retry_after}
+        return headers
+
     @webob.dec.wsgify(RequestClass=Request)
     def __call__(self, request):
         """
diff --git a/nova/tests/api/openstack/compute/test_limits.py b/nova/tests/api/openstack/compute/test_limits.py
index 26e0094c0..eb4ab00c4 100644
--- a/nova/tests/api/openstack/compute/test_limits.py
+++ b/nova/tests/api/openstack/compute/test_limits.py
@@ -315,6 +315,10 @@ class LimitMiddlewareTest(BaseLimitTestSuite):
         response = request.get_response(self.app)
         self.assertEqual(response.status_int, 413)
 
+        self.assertTrue('Retry-After' in response.headers)
+        retry_after = int(response.headers['Retry-After'])
+        self.assertAlmostEqual(retry_after, 60, 1)
+
         body = json.loads(response.body)
         expected = "Only 1 GET request(s) can be made to * every minute."
         value = body["overLimitFault"]["details"].strip()