An error occurred when publishing a static file using uvloop

[hoto17296]

🐞 Describe the bug
An error occurred when publishing a static file using aiohttp 3.7.x and uvloop.

💡 To Reproduce

# Dockerfile

FROM python:3.8.6

WORKDIR /work

RUN pip install aiohttp==3.7.1 uvloop==0.14.0

ADD app.py .

RUN mkdir static && echo "/* DUMMY */" > static/index.css

CMD ["python", "app.py"]

# app.py

import uvloop
from aiohttp import web

uvloop.install()

app = web.Application()
app.add_routes([web.static("/static", "/work/static")])

if __name__ == "__main__":
    web.run_app(app)

Then, build and run server.

$ docker build . -t tmp
$ docker run --rm -it -p 8080:8080 tmp

Finally, try to access http://localhost:8080/static/index.css .

When accessed from a browser, the following error is displayed in the developer console.

net::ERR_CONTENT_LENGTH_MISMATCH 200 (OK)

When accessed from a cURL, the following error is displayed.

curl: (18) transfer closed with 12 bytes remaining to read

It looks like the connection was closed before sending the response body.

💡 Expected behavior
No error occurs.

📋 Logs/tracebacks
No messages are displayed in the Python log at all.

📋 Your version of the Python

$ docker run --rm tmp python --version       
Python 3.8.6

📋 Your version of the aiohttp/yarl/multidict distributions

$ docker run --rm tmp python -m pip show aiohttp
Name: aiohttp
Version: 3.7.1
Summary: Async http client/server framework (asyncio)
Home-page: https://github.com/aio-libs/aiohttp
Author: Nikolay Kim
Author-email: fafhrd91@gmail.com
License: Apache 2
Location: /usr/local/lib/python3.8/site-packages
Requires: chardet, multidict, attrs, async-timeout, typing-extensions, yarl
Required-by:

$ docker run --rm tmp python -m pip show multidict
Name: multidict
Version: 5.0.0
Summary: multidict implementation
Home-page: https://github.com/aio-libs/multidict
Author: Andrew Svetlov
Author-email: andrew.svetlov@gmail.com
License: Apache 2
Location: /usr/local/lib/python3.8/site-packages
Requires: 
Required-by: yarl, aiohttp

$ docker run --rm tmp python -m pip show yarl     
Name: yarl
Version: 1.6.2
Summary: Yet another URL library
Home-page: https://github.com/aio-libs/yarl/
Author: Andrew Svetlov
Author-email: andrew.svetlov@gmail.com
License: Apache 2
Location: /usr/local/lib/python3.8/site-packages
Requires: idna, multidict
Required-by: aiohttp

📋 Additional context

• When aiphttp==3.6.*, it works fine
• When uvloop disabled, it works fine

[asvetlov]

Thanks for the very cool reproducible example for the bug!
I wish all bug reports look like this :)

I'm working on an investigation. As a quick workaround you can disable sendfile support for static files by setting AIOHTTP_NOSENDFILE=1 environment variable.

[asvetlov]

Aha, I see the reason.
uvloop.sendfile() exists but it raises NotImplementedError -- which is 100% correct, it works exactly as I designed asyncio sendfile implementation.
Unfortunately, in aiohttp we forgot to handle this error properly by falling back to a regular loop over file.read() and transport.write().

The thing is not as easy as it should be; earlier aiohttp versions used a hack with sock.dup() and os.sendfile to work around the fact that asyncio did not support native sendfile until Python 3.7. This hack should send the whole message over the copy (dup) of the socket to avoid the corrupted internal state of the asyncio transport.

But the code cannot figure out is the native sendfile supported or not until loop.sendfile() call! Thus, the first transport.write() for sending HTTP headers conflicts with sock.dup() approach, the sent data is broken.

The solution is dropping custom sock.dup() + os.sendfile() workaround for the sake of standard loop.sendfile() and transport.write() fallback. `

loop.sendfile() works pretty well on Python 3.7+, the fallback is called for Python 3.6, compressed data, systems without native os.sendfile (I cannot name them, all Windows, Linux, and MacOS have the native sendfile).

Ironically, uvloop uses the slow fallback until it supports the native loop.sendfile() itself.

This is the plan. Implementation can take a day or two -- depending on my daily job. If somebody wants to pick the issue up and prepare a fix himself -- you are welcome.

[greshilov]

Hey! I've created a PR with fixes, following your plan. I hope I got it right :)