Fix RuntimeError in _sendfile_cb

[awarecan]

What do these changes do?

Under some circumstances, a Runtime Error RuntimeError: File descriptor 36 is used by transport <ReadUnixTransport closed=False reading=True 0x7f9c51da9d48> will be raise.

This change try to fix it by check if task has been canceled before remove stream writer.

I have never met this issue personally, so I won't be able to write a case to reproduce issue. However this patch has been published in home-assistant/core#12832 (comment) and got positive responses from users who had this issue.

Are there changes in behavior for the user?

Related issue number

fixes #1893

Checklist

• I think the code is well written
• Unit tests for the changes exist
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[asvetlov]

Could you go further and rewrite the code by using add_done_callback like https://github.com/python/cpython/blob/master/Lib/asyncio/unix_events.py#L319-L420 ?

asyncio code is overcomplicated a little to update filepos properly, aiohttp can skip this logic. But you can borrow the cancellation callback and its registration.

[awarecan]

I feel little uncomfortable to do such refactoring since I won't be able to reproduce the issue.

[codecov-io]

Codecov Report

Merging #3370 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3370      +/-   ##
==========================================
- Coverage   97.97%   97.93%   -0.04%     
==========================================
  Files          44       44              
  Lines        8482     8482              
  Branches     1378     1378              
==========================================
- Hits         8310     8307       -3     
- Misses         73       75       +2     
- Partials       99      100       +1

Impacted Files	Coverage Δ
aiohttp/web_fileresponse.py	97.51% <100%> (ø)	⬆️
aiohttp/tcp_helpers.py	90% <0%> (-6.67%)	⬇️
aiohttp/client_reqrep.py	97.28% <0%> (-0.16%)	⬇️

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[asvetlov]

Superseded by #3383

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