Preserve handler function attributes across middlewares

[gjcarneiro]

What do these changes do?

When using middlewares (such as the hidden middleware implicitly added by subapps), it fixes the functools.partial() usage on the handler view, to copy any attributes from the view function.

Are there changes in behavior for the user?

Now any attributes added to a view handler function, e.g. by a decorator, are seen by all the middlewares in the chain.

Before this change, attributes we only seen by a middleware in the case of the last middleware (first to run), and only for the main Application, for subapps that wasn't true.

Related issue number

#4174

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
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[codecov-io]

Codecov Report

Merging #4195 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4195      +/-   ##
==========================================
- Coverage   97.57%   97.56%   -0.02%     
==========================================
  Files          43       43              
  Lines        8825     8825              
  Branches     1381     1381              
==========================================
- Hits         8611     8610       -1     
- Misses         92       93       +1     
  Partials      122      122

Impacted Files	Coverage Δ
aiohttp/web_app.py	97.3% <100%> (ø)	⬆️
aiohttp/web_fileresponse.py	97.72% <0%> (-0.57%)	⬇️

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[gjcarneiro]

I modified a unit test to check for this.

I don't know whether this behaviour should be documented explicitly. Or else developers will find that it just works, and needs no documentation as such.

[asvetlov]

The updated test is for application+middleware.

Please provide a test that checks sub-app handler attributes in parent-app middleware.

[gjcarneiro]

Pushed a new test_middleware_subapp test.

[asvetlov]

LGTM
I'll merge it soon; when will have time to generate backport PR as well

[gjcarneiro]

Thanks! Well, for me, personally, there is no need to backport it, necessarily. I have an ugly workaround in the code, it can wait a few months.

[asvetlov]

Thanks.
The PR can be backported easily, there is no reason to postpone the feature delivery.