Chunked response from function is blocked

Dear maintainers,

I wrote my template that is similar to https://github.com/openfaas-incubator/python-flask-template/tree/master/template/python3-flask.

The different from the original is that my template uses stream response (or, chunked response) as follows.

@app.route("/", defaults={"path": ""}, methods=["POST", "GET"])
@app.route("/<path:path>", methods=["POST", "GET"])
def main_route(path):
    ...
    def gen():
        yield "1"
        time.sleep(1)
        yield "2"
        time.sleep(1)
        yield "3"

    return Response(gen())

When I invoke a function from the template, the function does not return response chunk by chunk. It blocks 3 seconds, and then, returns whole the response (1, 2 and 3) all at once.

Expected Behaviour

Function returns response chunk by chunk.

Current Behaviour

Function blocks 3 seconds, and then, returns whole the response all at once.

Possible Solution

The following code blocks until whole the response returned.

https://github.com/openfaas-incubator/of-watchdog/blob/bae373954932a07d89ab926457d237f03f6c60dc/executor/http_runner.go#L173

And openfaas/faas's following code also blocks, maybe.

https://github.com/openfaas/faas/blob/04d240aac26acdaf49863dbab768b4f5cb311d84/gateway/handlers/forwarding_proxy.go#L130

Steps to Reproduce (for bugs)

fetch this template https://github.com/openfaas-incubator/python-flask-template/tree/master/template/python3-flask
edit the template as described above
create new function from the template
build, push, deploy and invoke the function

Your Environment

Docker version 18.09.1, build 4c52b90 Docker swarm Linux (vagrant, vm.box = "bento/centos-7.4")

Thank you.

openfaas / of-watchdog