Wait for a non-empty chunk in WSGIContext._app_call

We're functioning as a WSGI server here, so this bit from PEP-3333 seems
to apply:

> The start_response callable must not actually transmit the response
> headers. Instead, it must store them for the server or gateway to
> transmit only after the first iteration of the application return
> value that yields a non-empty bytestrin ... . In other words, response
> headers must not be sent until there is actual body data available, or
> until the application's returned iterable is exhausted.

Plus, it mirrors what swob.Request.call_application does.

Change-Id: I1e8501f8ce91ea912780db64fee1c56bef809a98

swift/common/wsgi.py

@@ -42,7 +42,7 @@ from swift.common.swob import Request
 from swift.common.utils import capture_stdio, disable_fallocate, \
     drop_privileges, get_logger, NullLogger, config_true_value, \
     validate_configuration, get_hub, config_auto_int_value, \
-    CloseableChain
+    reiterate
 
 # Set maximum line size of message headers to be accepted.
 wsgi.MAX_HEADER_LINE = constraints.MAX_HEADER_SIZE
@@ -1053,16 +1053,11 @@ class WSGIContext(object):
         self._response_headers = None
         self._response_exc_info = None
         resp = self.app(env, self._start_response)
-        # if start_response has been called, just return the iter
-        if self._response_status is not None:
-            return resp
-        resp = iter(resp)
-        try:
-            first_chunk = next(resp)
-        except StopIteration:
-            return iter([])
-        else:  # We got a first_chunk
-            return CloseableChain([first_chunk], resp)
+        # if start_response has not been called, iterate until we've got a
+        # non-empty chunk, by which time the app *should* have called it
+        if self._response_status is None:
+            resp = reiterate(resp)
+        return resp
 
     def _get_status_int(self):
         """

test/unit/common/test_wsgi.py

@@ -1242,6 +1242,8 @@ class TestWSGIContext(unittest.TestCase):
     def test_app_iter_is_closable(self):
 
         def app(env, start_response):
+            yield ''
+            yield ''
             start_response('200 OK', [('Content-Length', '25')])
             yield 'aaaaa'
             yield 'bbbbb'