Pass scrapy arguments to ScrapyProcessProtocol

[dfrank-a]

Pass scrapy arguments along to ScrapyProcessProtocol.

This will allow users to use this data in extensions of the minimal admin UI in whatever way they see fit.

[codecov]

Codecov Report

Merging #272 into master will decrease coverage by 0.08%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #272      +/-   ##
==========================================
- Coverage   68.37%   68.29%   -0.09%     
==========================================
  Files          16       16              
  Lines         819      820       +1     
  Branches       96       96              
==========================================
  Hits          560      560              
- Misses        230      231       +1     
  Partials       29       29

Impacted Files	Coverage Δ
scrapyd/launcher.py	43.2% <50%> (-0.55%)	⬇️

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[Digenis]

Hi @dfrank-a
welcome to scrapyd.

This feature is something we want to add eventually.
I only didn't open an issue
because of worries I express in the following 2 comments of issue 123
(sorry for the mess, I didn't have the time to write an issue)
#123 (comment)
#123 (comment)
To sum them up, one day we may need to write a new webservice
to replace schedule.json with one that nests spider arguments & settings
in separate objects, so that their names don't collide with special arguments
for the webservice or other scrapyd components.
Will merging this PR make it harder to replace the webservice later?
I really haven't thought much about it and don't have the time now.
What's your opinion on this?

[Digenis]

Actually I just took a look and I think it won't make it harder.
This however only means us. It won't make it harder for us.
If users start writing custom components that use the process protocol's args
their components will break when we reorganize the object into something more nested.
Perhaps we should include it as an experimental change
with a warning about the structure of the args attribute.

[dfrank-a]

That is something I hadn't considered. How do you feel about declaring a public interface to the data with property methods? This should allow for flexibility in our implementation while maintaining consistency in the external interface.

[Digenis]

I see what you mean, it makes sense
but I confused the spider arguments with the scrapy process's args.
Since it's process's args you are adding to the proc. protocol,
the compatibility concerns don't apply here.
To avoid confusion in the future (e.g. if we decide to add the spider arguments)
perhaps args should be named cmdline
both in _spawn_process and ProcessProtocol.

Also, the commit message should fit in a single line up to 50 characters.

[jpmckinney]

Re-done in #474.