Using web.FileResponse to send an empty file causes an error

[ddurham2]

Describe the bug

If a web.FileResponse() is used to send an empty file, then the following stack trace is raise...

  File \"/usr/local/lib/python3.8/dist-packages/aiohttp/web_protocol.py\", line 485, in start
    resp, reset = await task
  File \"/usr/local/lib/python3.8/dist-packages/aiohttp/web_protocol.py\", line 440, in _handle_request
    reset = await self.finish_response(request, resp, start_time)
  File \"/usr/local/lib/python3.8/dist-packages/aiohttp/web_protocol.py\", line 591, in finish_response
    await prepare_meth(request)
  File \"/usr/local/lib/python3.8/dist-packages/aiohttp/web_fileresponse.py\", line 241, in prepare
    return await self._sendfile(request, fobj, offset, count)
  File \"/usr/local/lib/python3.8/dist-packages/aiohttp/web_fileresponse.py\", line 96, in _sendfile
    await loop.sendfile(transport, fobj, offset, count)
  File \"/usr/lib/python3.8/asyncio/base_events.py\", line 1120, in sendfile
    return await self._sendfile_native(transport, file,
  File \"/usr/lib/python3.8/asyncio/selector_events.py\", line 578, in _sendfile_native
    return await self.sock_sendfile(transp._sock, file, offset, count,
  File \"/usr/lib/python3.8/asyncio/base_events.py\", line 836, in sock_sendfile
    self._check_sendfile_params(sock, file, offset, count)
  File \"/usr/lib/python3.8/asyncio/base_events.py\", line 889, in _check_sendfile_params
    raise ValueError(

So, it's actually an error to call asyncio's sendfile() function with 0 bytes.. which seems like something they would have short-circuited on purpose, but apparently have not.

Nevertheless, it was easily fixed in web.file_response.py ~96 just to add an if count > 0 around calling sendfile. And that resulted in the expected 200 response with content-length of 0.

To Reproduce

return web.FileResponse(path="/dev/null")
or
return web.FileResponse(path="/path/to/requested-resource/that-happens/to-have-zero-size")

Expected behavior

correct 200 response just with 0 length.

Logs/tracebacks

File \"/usr/local/lib/python3.8/dist-packages/aiohttp/web_protocol.py\", line 485, in start
    resp, reset = await task
  File \"/usr/local/lib/python3.8/dist-packages/aiohttp/web_protocol.py\", line 440, in _handle_request
    reset = await self.finish_response(request, resp, start_time)
  File \"/usr/local/lib/python3.8/dist-packages/aiohttp/web_protocol.py\", line 591, in finish_response
    await prepare_meth(request)
  File \"/usr/local/lib/python3.8/dist-packages/aiohttp/web_fileresponse.py\", line 241, in prepare
    return await self._sendfile(request, fobj, offset, count)
  File \"/usr/local/lib/python3.8/dist-packages/aiohttp/web_fileresponse.py\", line 96, in _sendfile
    await loop.sendfile(transport, fobj, offset, count)
  File \"/usr/lib/python3.8/asyncio/base_events.py\", line 1120, in sendfile
    return await self._sendfile_native(transport, file,
  File \"/usr/lib/python3.8/asyncio/selector_events.py\", line 578, in _sendfile_native
    return await self.sock_sendfile(transp._sock, file, offset, count,
  File \"/usr/lib/python3.8/asyncio/base_events.py\", line 836, in sock_sendfile
    self._check_sendfile_params(sock, file, offset, count)
  File \"/usr/lib/python3.8/asyncio/base_events.py\", line 889, in _check_sendfile_params
    raise ValueError(

Python Version

$ python --version
Python 3.8.10

aiohttp Version

$ python -m pip show aiohttp
Name: aiohttp
Version: 3.8.4
Summary: Async http client/server framework (asyncio)
Home-page: https://github.com/aio-libs/aiohttp
Author: None
Author-email: None
License: Apache 2
Location: /usr/local/lib/python3.8/dist-packages
Requires: charset-normalizer, frozenlist, attrs, async-timeout, multidict, aiosignal, yarl
Required-by: pyht, keycloak-api, htdm, aiohttp-jinja2

multidict Version

$ python -m pip show multidict
Name: multidict
Version: 6.0.4
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/dist-packages
Requires: 
Required-by: yarl, aiohttp

yarl Version

$ python -m pip show yarl
Name: yarl
Version: 1.8.2
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/dist-packages
Requires: multidict, idna
Required-by: aiohttp

OS

uname -a
Linux myhost 5.10.0-0.deb10.17-amd64 #1 SMP Debian 5.10.136-1~deb10u3 (2022-09-06) x86_64 x86_64 x86_64 GNU/Linux

Related component

Server

Additional context

No response

Code of Conduct

• I agree to follow the aio-libs Code of Conduct

[Dreamsorcerer]

If you could pop that fix into a PR with a test, that'd be great.

[yoelbassin]

From what I see this issue was fixed in #5380 with a great test that matches the issue you described.

    filepath = pathlib.Path(__file__).parent / "data.zero_bytes"

    async def handler(request):
        asyncio.set_event_loop(loop_with_mocked_native_sendfile)
        return web.FileResponse(filepath)

Could you provide more detailed instructions to reproduce your issue?

[ddurham2]

It does look like that'll fix it, I was not on the latest release. It's working now. Thx