Fix HEAD request response when request not given to response.

If a middleware (swift3, I'm looking at you), doesn't pass a Request
object into the Response constructor, Response._response_iter cannot
know to send zero bytes in the body of the HEAD response.

This patch fixes this usage of swob by making Response.__call__
helpfully reify self.request from env if it wasn't already set by the
Response object's constructor.

This fixes a bug in swift3 + swob-enabled-Swift where HEAD requests to
swift3 resulted in a response with a body in violation of the relevant
RFC and confusing clients.

Thanks to kostecky for finding the bug and describing it accurately.

Change-Id: I2bdb098052b161e1cddf1e4e482ab4dfafeb18c0

swift/common/swob.py

@@ -997,6 +997,8 @@ class Response(object):
         return self.host_url() + self.location
 
     def __call__(self, env, start_response):
+        if not self.request:
+            self.request = Request(env)
         self.environ = env
         app_iter = self._response_iter(self.app_iter, self._body)
         if 'location' in self.headers:

test/unit/common/test_swob.py

@@ -462,6 +462,24 @@ class TestResponse(unittest.TestCase):
         resp.body = u'\N{SNOWMAN}'
         self.assertEquals(resp.body, u'\N{SNOWMAN}'.encode('utf-8'))
 
+    def test_call_reifies_request_if_necessary(self):
+        """
+        The actual bug was a HEAD response coming out with a body because the
+        Request object wasn't passed into the Response object's constructor.
+        The Response object's __call__ method should be able to reify a
+        Request object from the env it gets passed.
+        """
+        def test_app(environ, start_response):
+            start_response('200 OK', [])
+            return ['hi']
+        req = swift.common.swob.Request.blank('/')
+        req.method = 'HEAD'
+        status, headers, app_iter = req.call_application(test_app)
+        resp = swift.common.swob.Response(status=status, headers=dict(headers),
+                                          app_iter=app_iter)
+        output_iter = resp(req.environ, lambda *_: None)
+        self.assertEquals(list(output_iter), [''])
+
     def test_location_rewrite(self):
         def start_response(env, headers):
             pass