Allow specifying the desired IO selector backend for nio4r

[jcmfernandes]

Description

nio4r will soon have experimental support for io_uring via libev. The brave might want to try it out :) regardless of that, allowing users to explicitly set the IO selector backend seems like a good idea to me.

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[nateberkopec]

Thanks! Left some comments, lmk what you think.

[jcmfernandes]

Thanks! Left some comments, lmk what you think.

Thank you! I updated the docs.

[dentarg]

Should we rescue the ArgumentError coming from nio4r and raise a better one?

It is not that clear that it is coming from nio4r:

$ grep io_selector_backend test/config/app.rb
io_selector_backend :ruby

Puma starting in single mode...
* Puma version: 5.1.1 (ruby 2.7.1-p83) ("At Your Service")
*  Min threads: 0
*  Max threads: 5
*  Environment: development
*          PID: 15846
* Listening on http://0.0.0.0:57461
bundler: failed to load command: bin/puma (bin/puma)
Traceback (most recent call last):
  23: from /Users/dentarg/.gem/ruby/2.7.1/bin/bundle:23:in `<main>'
  22: from /Users/dentarg/.gem/ruby/2.7.1/bin/bundle:23:in `load'
  21: from /Users/dentarg/.rubies/ruby-2.7.1/lib/ruby/gems/2.7.0/gems/bundler-2.2.4/exe/bundle:37:in `<top (required)>'
  20: from /Users/dentarg/.rubies/ruby-2.7.1/lib/ruby/site_ruby/2.7.0/bundler/friendly_errors.rb:130:in `with_friendly_errors'
  19: from /Users/dentarg/.rubies/ruby-2.7.1/lib/ruby/gems/2.7.0/gems/bundler-2.2.4/exe/bundle:49:in `block in <top (required)>'
  18: from /Users/dentarg/.rubies/ruby-2.7.1/lib/ruby/site_ruby/2.7.0/bundler/cli.rb:24:in `start'
  17: from /Users/dentarg/.rubies/ruby-2.7.1/lib/ruby/site_ruby/2.7.0/bundler/vendor/thor/lib/thor/base.rb:485:in `start'
  16: from /Users/dentarg/.rubies/ruby-2.7.1/lib/ruby/site_ruby/2.7.0/bundler/cli.rb:30:in `dispatch'
  15: from /Users/dentarg/.rubies/ruby-2.7.1/lib/ruby/site_ruby/2.7.0/bundler/vendor/thor/lib/thor.rb:392:in `dispatch'
  14: from /Users/dentarg/.rubies/ruby-2.7.1/lib/ruby/site_ruby/2.7.0/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
  13: from /Users/dentarg/.rubies/ruby-2.7.1/lib/ruby/site_ruby/2.7.0/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
  12: from /Users/dentarg/.rubies/ruby-2.7.1/lib/ruby/site_ruby/2.7.0/bundler/cli.rb:494:in `exec'
  11: from /Users/dentarg/.rubies/ruby-2.7.1/lib/ruby/site_ruby/2.7.0/bundler/cli/exec.rb:28:in `run'
  10: from /Users/dentarg/.rubies/ruby-2.7.1/lib/ruby/site_ruby/2.7.0/bundler/cli/exec.rb:63:in `kernel_load'
   9: from /Users/dentarg/.rubies/ruby-2.7.1/lib/ruby/site_ruby/2.7.0/bundler/cli/exec.rb:63:in `load'
   8: from bin/puma:10:in `<top (required)>'
   7: from /Users/dentarg/src/puma/lib/puma/cli.rb:80:in `run'
   6: from /Users/dentarg/src/puma/lib/puma/launcher.rb:181:in `run'
   5: from /Users/dentarg/src/puma/lib/puma/single.rb:53:in `run'
   4: from /Users/dentarg/src/puma/lib/puma/server.rb:241:in `run'
   3: from /Users/dentarg/src/puma/lib/puma/server.rb:241:in `new'
   2: from /Users/dentarg/src/puma/lib/puma/reactor.rb:23:in `initialize'
   1: from /Users/dentarg/src/puma/lib/puma/reactor.rb:23:in `new'
/Users/dentarg/src/puma/lib/puma/reactor.rb:23:in `initialize': unsupported backend: :ruby (ArgumentError)

[jcmfernandes]

I hope I addressed all feedback, thank you! Let me know what you think.

[nateberkopec]

Great work from everyone - @jcmfernandes and reviewers 👍

[jcmfernandes]

Thank you @nateberkopec and all the reviewers! 🙌