Raising error in HTTP11Response if created with illegal attributes

An HTTP response must either specify a content-length header, specify
'close' for a connection header to signal that the connection will be
closed after the response, or be a chunked response.

If none of these conditions are true, we raise a detailed ValueError,
instead of the plain assertion.

hyper/http11/response.py

@@ -58,11 +58,10 @@ def __init__(self, code, reason, headers, sock, connection=None):
             b'chunked' in self.headers.get(b'transfer-encoding', [])
         )
 
-        # One of the following must be true: we must expect that the connection
-        # will be closed following the body, or that a content-length was sent,
-        # or that we're getting a chunked response.
-        # FIXME: Remove naked assert, replace with something better.
-        assert self._expect_close or self._length is not None or self._chunked
+        if (not self._expect_close) and (not self._chunked) and (self._length is None):
+            raise ValueError('A response must either specify a content-length, be a chunked response, '
+                             'or specify that the connection be closed after the response. '
+                             'None of these conditions were met.')
 
         # This object is used for decompressing gzipped request bodies. Right
         # now we only support gzip because that's all the RFC mandates of us.

test/test_http11.py

@@ -837,6 +837,18 @@ def test_closing_chunked_reads_dont_call_close_callback(self):
         assert r._sock is None
         assert connection.close.call_count == 1
 
+    def test_regular_response_with_no_content_length_and_no_connection_close_raises_error(self):
+        headers = {}
+        sock = DummySocket()
+        connection = mock.MagicMock()
+
+        with pytest.raises(ValueError) as exc_info:
+            HTTP11Response(200, 'OK', headers, sock, connection)
+        assert 'A response must either specify a content-length, be a chunked response, ' \
+               'or specify that the connection be closed after the response. ' \
+               'None of these conditions were met.' \
+               in str(exc_info)
+
 
 class DummySocket(object):
     def __init__(self):