Trying to replicate dgtlmoon/changedetection.io#2053 which is also living at eventlet/eventlet#862

$ python3 -m venv  ./venv
$ source  venv/bin/activate
$ pip3 install -r requirements.txt
$ ./app.py

then (you may need to edit the etag and mod-if dates)

curl -vvv 'http://localhost:5000/static' \
    -H 'User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:121.0) Gecko/20100101 Firefox/121.0' \
    -H 'Accept: image/avif,image/webp,*/*' \
    -H 'Accept-Language: de,en-US;q=0.7,en;q=0.3' \
    -H 'Accept-Encoding: gzip, deflate, br' \
    -H 'Connection: keep-alive' \
    -H 'If-Modified-Since: Fri, 22 Dec 2023 10:14:33 GMT' \
    -H 'If-None-Match: "1703239999.0048695-13861-933627416"' \
    -H 'TE: trailers'

and now you should be able to see (because of the RFC violation)

* Excess found: excess = 5 url = /static (zero-length body)
* Connection #0 to host localhost left intact

This bad behaviour is enabled by using eventlet wsgi as a wrapper

    s_type = socket.AF_INET
    eventlet.wsgi.server(eventlet.listen(("127.0.0.1", int(5000)), s_type), app)

BUT

This does NOT show 'Excess found' (because we dont use eventlet in this way)

$ pip3 install -r requirements.txt
$ flask --app app run

So why when adding the eventlet wrapper we see the problem