Use loop.sendfile() for file sending on Python 3.7+ #4269

[alex-kostirin]

What do these changes do?

Implemented loop.sendfile() for file sending on Python 3.7+

Are there changes in behavior for the user?

None

Related issue number

#4269

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[codecov-io]

Codecov Report

Merging #4328 into master will decrease coverage by 0.28%.
The diff coverage is 58.82%.

@@            Coverage Diff             @@
##           master    #4328      +/-   ##
==========================================
- Coverage   97.55%   97.26%   -0.29%     
==========================================
  Files          43       43              
  Lines        8862     8873      +11     
  Branches     1389     1389              
==========================================
- Hits         8645     8630      -15     
- Misses         93      120      +27     
+ Partials      124      123       -1

Impacted Files	Coverage Δ
aiohttp/web_fileresponse.py	85.56% <58.82%> (-12.17%)	⬇️
aiohttp/worker.py	94.11% <0%> (-2.53%)	⬇️

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[alex-kostirin]

It should be also noted that I kept implementation of _sendfile_fallback from aiohttp. Asyncio's _sendfile_fallback implementation uses file reading directly without ThreadPoolExecutor. Othervise aiohttp's implementation is with ThreadPoolExecutor

[asvetlov]

asyncio does use run_in_executor for file reading:
https://github.com/python/cpython/blob/master/Lib/asyncio/base_events.py#L890

[asvetlov]

Fixed by #4517