small optimization of BytesIOPayload.size

[arthurdarcet]

When posting a large body that you have as a bytes object, as per #2127 , the body should be passed as a io.BytesIO object.

But when this BytesIO is initialised on a bytes object that might be used somewhere else, calling getbuffer needs to copy the underlying data to return a writable memoryview.

python -m timeit -s "import io; a = b'0' * 2 ** 30" 'io.BytesIO(a).getbuffer()'
10 loops, best of 3: 627 msec per loop
python -m timeit -s "import io; a = b'0' * 2 ** 30" 'io.BytesIO(a).getvalue()'
1000000 loops, best of 3: 0.247 usec per loop
python -m timeit -s "import io; a = b'0' * 2 ** 30" 'io.BytesIO(a)'
10000000 loops, best of 3: 0.186 usec per loop

(the instantiating the BytesIO in the loop to avoid the caching done by BytesIO)

This is problematic beyond wasting 500ms when making a request: the event loop is actually locked during this memory copy, so an underlying aiohttp server won't be able to process any other pending requests.

But, calling getvalue instead of getbuffer will make some (weird) use-cases way worse than now:

python -m timeit -s "import io; d = io.BytesIO(b'0' * 2 ** 30)" i = d.getbuffer()" 'd.getvalue()'
10 loops, best of 3: 624 msec per loop

Calling seek(0, SEEK_END) avoid all this

[codecov-io]

Codecov Report

Merging #2129 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2129      +/-   ##
==========================================
+ Coverage   97.07%   97.07%   +<.01%     
==========================================
  Files          38       38              
  Lines        7720     7723       +3     
  Branches     1346     1346              
==========================================
+ Hits         7494     7497       +3     
  Misses        102      102              
  Partials      124      124

Impacted Files	Coverage Δ
aiohttp/payload.py	98.86% <100%> (+0.01%)	⬆️

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[fafhrd91]

lgtm

@asvetlov comments?

[asvetlov]

looks good

[asvetlov]

Thank you

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