Skip middleware code when there are no user middlewares installed

[pfreixes]

What do these changes do?

Fixes performance issue introduced by #2577, boosting from 8K req/sec
to almost 9K req/sec.

If there are no middlewares installed by the user the attribute
request._match_info.current_app already looks at to the right app,
this is by design.

Are there changes in behavior for the user?

No

[pfreixes]

Test numbers

$ python3 benchmark.py --reference=origin/master --wrk-path=/home/ec2-user/wrk/wrk /home/ec2-user/aiohttp
[origin/master] Benchmark req/sec 8206

$ python3 benchmark.py --reference=origin/fix_degradation_performance_middleware --wrk-path=/home/ec2-user/wrk/wrk /home/ec2-user/aiohttp-pfreixes
[origin/fix_degradation_performance_middleware] Benchmark req/sec 9049

I've used this benchmark [1]

[1] https://github.com/pfreixes/aiohttp_regression_performance_test

[codecov-io]

Codecov Report

Merging #2629 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2629      +/-   ##
==========================================
+ Coverage   97.92%   97.92%   +<.01%     
==========================================
  Files          38       38              
  Lines        7261     7265       +4     
  Branches     1260     1261       +1     
==========================================
+ Hits         7110     7114       +4     
  Misses         47       47              
  Partials      104      104

Impacted Files	Coverage Δ
aiohttp/web_app.py	98.84% <100%> (+0.02%)	⬆️

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[asvetlov]

The line is cryptic a little.
Please replace with self._run_middlewares = self._run_middlewares or subapp._run_middlewares
The change respects middlewares from deep nested subapps: app -> subapp1 -> subapp2.

[pfreixes]

Fair enough, done. Added also a bit of internal documentation about this flag and the root cause of its usage.

[asvetlov]

Please merge when green

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new