Prevent multiple values being set to run_via

[y-yagi]

When executing the test via rake, since rake is set for run_via, ruby should not be set.
Related 2cb6c27

[rails-bot]

r? @arthurnn

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[kaspth]

Think this means we can just remove the run_via[:rails] = true lines. And that storing the run_via as a hash was a poor decision on my part. How about:

def run_via
  @@run_via
end

def run_via=(runner)
  if @@run_via
    raise ArgumentError, 'run_via already assigned'
  else
    @@run_via = RunVia.new(runner)
  end
end

class RunVia < Struct.new(:runner)
  def ruby?
    runner == :ruby
  end

  def rake?
    runner == :rake
  end
end

(Could be cleaned up with an ActiveSupport::SymbolInquirer but that's for another time).

[y-yagi]

Looks great. I updated.

I have one concern. There was using Minitest.run_via in test script for rails plugin.
For that reason, I made it possible to set it by [] = for compatibility. What do you think?
(By the way, it was not good to call the method directly in test script, I will modify it to use a wrapper file.)

[kaspth]

Nice call on the backwardscompatibility! Agreed a wrapper file would be nicer, as right now we're exposing a private API in a public file, but you know that 😊

[kaspth]

Why do we need this guard?

[y-yagi]

This is because rake_run is called multiple times when running multiple tests in one rake command(e.g. bin/rake test:models test:controllers).

[kaspth]

Won't this add the []= method to nil? I don't see how it can provide backwardscompatibility reliably?

[kaspth]

How about revising the surface to:

mattr_reader(:run_via) { RunVia.new }

def self.run_via=(runner)
  if run_via.set?
    raise # …
  else
    run_via.runner = runner
  end
end

class RunVia
  attr_accessor :runner
  alias set? runner

  # Backwardscompatibility with Rails 5.0 generated plugin test scripts.
  alias []= runner=

  def ruby?
    @runner == :ruby
  end

  def rake?
    @runner == :rake
  end
end

Would also prevent NoMethodError's in cases where run_via.rake? is called without first having something assigned (albeit an unlikely scenario).

[y-yagi]

👍 Great! I updated.

[kaspth]

Do we have any tasks akin to rails app:update for plugins that'll help updating this file and let us avoid the backwardscompatibility line?

[y-yagi]

Currently there is no update task for plugin. Also, plugin currently does not load the task provided by rails. Therefore, if provide a task similar to rails app:update for plugin, first need to update the plugin's Rakefile.
However, it is inconvenient that there is no task for updating, so I will add it with another PR.

[kaspth]

Sweet! ❤️