Iterates swift response earlier to get the correct status

We must iterate the next middleware app until we get a response
before asking for the headers and status.

Closes-bug: #1326250

Change-Id: I8947bc88754db93e3ebe01ea864c4b489d1bd870

ceilometer/objectstore/swift_middleware.py

@@ -93,14 +93,22 @@ class CeilometerMiddleware(object):
             start_response_args[0] = (status, list(headers), exc_info)
 
         def iter_response(iterable):
+            iterator = iter(iterable)
+            try:
+                chunk = iterator.next()
+                while not chunk:
+                    chunk = iterator.next()
+            except StopIteration:
+                chunk = ''
+
             if start_response_args[0]:
                 start_response(*start_response_args[0])
             bytes_sent = 0
             try:
-                for chunk in iterable:
-                    if chunk:
+                while chunk:
                     bytes_sent += len(chunk)
                     yield chunk
+                    chunk = iterator.next()
             finally:
                 try:
                     self.publish_sample(env,

ceilometer/tests/objectstore/test_swift_middleware.py

@@ -39,13 +39,15 @@ class FakeApp(object):
         self.body = body
 
     def __call__(self, env, start_response):
+        yield
         start_response('200 OK', [
             ('Content-Type', 'text/plain'),
             ('Content-Length', str(sum(map(len, self.body))))
         ])
         while env['wsgi.input'].read(5):
             pass
-        return self.body
+        for line in self.body:
+            yield line
 
 
 class TestSwiftMiddleware(tests_base.BaseTestCase):