Uvicorn inserted HTTP headers into CSV responses with the sanic framework

Checklist

• The bug is reproducible against the latest release and/or master.
• There are no similar issues or pull requests to fix it yet.

Describe the bug

I am running uvicorn for an app written with the sanic framework. If i run the app directly to download a CSV file, the file gets returned normally. But when I run it with uvicorn, it has inserted HTTP header information.

To reproduce

app.py (this example is straight from the sanic docs: https://sanic.readthedocs.io/en/latest/sanic/streaming.html#response-streaming)

from sanic import Sanic
from sanic.response import stream

app = Sanic(__name__)


@app.route("/")
async def test(request):
    async def sample_streaming_fn(response):
        await response.write('foo,')
        await response.write('bar')

    return stream(sample_streaming_fn, content_type='text/csv')

Expected behavior

Running sanic directly as a module should give the same result as running the app with uvicorn.

python -m sanic app.app

 $ curl http://127.0.0.1:8000
foo,bar

Actual behavior

Using uvicorn to run the app:

uvicorn app:app

 $ curl http://127.0.0.1:8000
HTTP/1.1 200 OK
Transfer-Encoding: chunked
Content-Type: text/csv
Connection: close

4
foo,
3
bar
0

Debugging material

Environment

• Running uvicorn 0.11.8 with CPython 3.8.1 on Darwin
• uvicorn app:app

Additional context

I am using 0.11.8 here but I can reproduce with 0.12.2 also.

[euri10]

Tanks @ahopkins !

[ahopkins]

This issue can be closed.