[Python-Dev] wsgi validator with asynchronous handlers/servers

[Luca Sbardella]

Hi,

I have an asynchronous wsgi application handler which yields empty bytes
before it is ready to yield the response body and, importantly, to call
start_response.

Something like this:

def wsgi_handler(environ, start_response):
        body = generate_body(environ)
        body = maybe_async(body)
        while is_async(body):
            yield b''
        start_response(...)
        ...

I started using wsgiref.validator recently, nice little gem in the standard
lib, and I discovered that the above handler does not validate! Disaster.
Reading pep 3333

"the application *must* invoke the start_response() callable before the
iterable yields its first body bytestring, so that the server can send the
headers before any body content. However, this invocation *may* be
performed by the iterable's first iteration, so servers *must not* assume
that start_response() has been called before they begin iterating over the
iterable."

The pseudocode above does yields bytes before start_response, but they are
not *body* bytes, they are empty bytes so that the asynchronous wsgi server
releases the eventloop and call back at the next eventloop iteration.

I'm I misinterpreting the pep, or the wsgi validator should be
fixed accordingly?

Regards,
Luca
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