FileResponse works with files asynchronously

[Tolmachofof]

What do these changes do?

The web FileResponse works with files asynchronously

Are there changes in behavior for the user?

No

Related issue number

#3313

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
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  • 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
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    • .feature: Signifying a new feature.
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  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[asvetlov]

filepath.stat() and filepath.open() in def prepare() can block too.
Please run them in an executor as well.

[codecov-io]

Codecov Report

Merging #3476 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3476      +/-   ##
==========================================
+ Coverage   97.93%   97.93%   +<.01%     
==========================================
  Files          44       44              
  Lines        8559     8561       +2     
  Branches     1383     1383              
==========================================
+ Hits         8382     8384       +2     
  Misses         74       74              
  Partials      103      103

Impacted Files	Coverage Δ
aiohttp/web_fileresponse.py	96.55% <100%> (+0.04%)	⬆️

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[asvetlov]

thanks!

[aamalev]

@Tolmachofof blocking close of the fobj

[Tolmachofof]

@aamalev thanks for your comment!
I think that something like _FileContextManager would solve this problem.
Simplified example:

class _FileContextManager:

    def __init__(self, filepath, flags, start):
        self._filepath = filepath
        self._flags = flags
        self._start = start
        self._fobj = None

    async def __aenter__(self):
        loop = asyncio.get_event_loop()
        self._fobj = await loop.run_in_executor(
            None, self._open, self._filepath, self._start
        )
        return self._fobj

    def _open(self, filepath, start):
        fobj = filepath.open(self._flags)
        if start:
            fobj.seek(start)
        return fobj

    async def __aexit__(self, exc_type, exc_val, exc_tb):
        loop = asyncio.get_event_loop()
        await loop.run_in_executor(None, self._fobj.close)
        self._fobj = None

If this acceptable i can make a PR

[aamalev]

I think the conversion with to try..finally is more suitable. _FileContextManager need members loop and executor, and this is too much for a one-time code

[Tolmachofof]

You are right. try..finally statement sounds good. I will make a PR.