Fix static _handler pickling error.

[ashleysommer]

Moves the subfunction _handler out to a module-level function, and parameterizes it with functools.partial().
Fixes the case when picking a sanic app which has a registered static route handler. This is usually encountered when attempting to use multiprocessing or auto_reload on OSX or Windows.
Fixes #1774

[codecov]

Codecov Report

Merging #1842 into master will increase coverage by 0.00%.
The diff coverage is 92.15%.

@@           Coverage Diff           @@
##           master    #1842   +/-   ##
=======================================
  Coverage   92.17%   92.17%           
=======================================
  Files          23       23           
  Lines        2312     2314    +2     
  Branches      424      424           
=======================================
+ Hits         2131     2133    +2     
  Misses        141      141           
  Partials       40       40           

Impacted Files	Coverage Δ
sanic/static.py	93.93% <92.15%> (+0.18%)	⬆️

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[Tronic]

This fixes static files but unfortunately there are also problems with route decorators:

# @app.get(...)
_pickle.PicklingError: Can't pickle <function my_handler at 0x10b2129d0>: it's not the same object as __main__.my_handler

# @app.websocket(...)
AttributeError: Can't pickle local object 'Sanic.websocket.<locals>.response.<locals>.websocket_handler'

This can be tried on UNIX by changing sanic/server.py:930 to

    mp = multiprocessing.get_context("spawn")

[ashleysommer]

Ah, ok. I was testing on Linux and not seeing that problem. Thanks for the tip @Tronic

[ashleysommer]

This can probably be still merged, as it does fix the static _handler pickle error, and adds a new test for that problem.
The route decorator pickle problem can probably be created as a secondary issue and receive an additional fix.

[Tronic]

Fixes a problem and adds a test for it.

Further changes to be done in other PRs (don't pickle app, or fix the route decorators as well, or enable fork mode multiprocessing with 19.12LTS on MacOS py38). Windows cannot use fork mode but also has multiple worker support completely disabled for now.

[ashleysommer]

I was hoping to get this change into the (belated) 20.03 release. Looks like it didn't make it. Still says one more review is required?

[ashleysommer]

Oh, when I saw the 20.3 PR go through yesterday, I thought we had done the 20.3 release. We should merge this now and it might still make the release.

[ashleysommer]

@Tronic I've made a fix for the websocket_handler pickle error, I will open a PR with that shortly. However I still cannot reproduce the route decorator pickling error.
I've changed the mp context to 'spawn' as you suggested, and even force-picking the whole app with decorated routes, and cannot get it to fail with Can't pickle <function handler at 0x0>: it's not the same object as __main__.handler error.

Are you able to share some code which reproduces the pickling error with route decorators?

[ahopkins]

This can me merged?

[ashleysommer]

Yes, it's finished and approved and is ready to be merged.