WSGIHandler is not threadsafe

[colin@…]

The call method of WSGIHandler is called from multiple threads, however there code that checks for the middleware being setup is not thread safe.

{{{if self._request_middleware is None:

self.load_middleware()}}}

Multiple threads can launch into load_middleware at once, leading to corrupt lists.

The attached patch uses a lock to protect the middleware loading if it's required.

[Malcolm Tredinnick]

(In [5868]) Fixed #4909 -- Fixed a race condition with middleware initialisation in multi-threaded setups. Thanks, colin@….