Return instead of yield from Concurrent::Promise (Ruby 3.1 compat)

[casperisfine]

So i have to admit I don't fully understand what's going nor how it even worked until now.

While testing ruby 3.1.0-dev on a Rails app using sprockets 4.0.2, I get the following error:

Minitest::UnexpectedError: ActionView::Template::Error: undefined method `exception' for nil:NilClass
 
        reason.exception(*args)
              ^^^^^^^^^^
    /tmp/bundle/ruby/3.1.0/gems/concurrent-ruby-1.1.9/lib/concurrent-ruby/concurrent/concern/obligation.rb:128:in `exception'
    /tmp/bundle/ruby/3.1.0/gems/concurrent-ruby-1.1.9/lib/concurrent-ruby/concurrent/concern/obligation.rb:87:in `raise'
    /tmp/bundle/ruby/3.1.0/gems/concurrent-ruby-1.1.9/lib/concurrent-ruby/concurrent/concern/obligation.rb:87:in `block in wait!'
    <internal:kernel>:90:in `tap'
    /tmp/bundle/ruby/3.1.0/gems/concurrent-ruby-1.1.9/lib/concurrent-ruby/concurrent/concern/obligation.rb:87:in `wait!'
    /tmp/bundle/ruby/3.1.0/gems/sprockets-4.0.2/lib/sprockets/manifest.rb:130:in `each'
    /tmp/bundle/ruby/3.1.0/gems/sprockets-4.0.2/lib/sprockets/manifest.rb:130:in `find'
    /tmp/bundle/ruby/3.1.0/gems/sprockets-4.0.2/lib/sprockets/manifest.rb:142:in `each'
    /tmp/bundle/ruby/3.1.0/gems/sprockets-4.0.2/lib/sprockets/manifest.rb:142:in `find_sources'

I dug into concurrent-ruby and somehow the wait! on the promise is supposed to return multiple values, but simply return nil.

I think it's caused by the fact that we yield when we're supposed to return, so we're somehow interupting the flow.

But looking at the original code I don't even get how it's supposed to work in the first place:

    def find(*args)
      unless environment
        raise Error, "manifest requires environment for compilation"
      end

      return to_enum(__method__, *args) unless block_given?

      environment = self.environment.cached
      promises = args.flatten.map do |path|
        Concurrent::Promise.execute(executor: executor) do
          environment.find_all_linked_assets(path) do |asset|
            yield asset
          end
        end
      end
      promises.each(&:wait!)

      nil
    end

The block passed to Concurrent::Promise.execute executes in a background thread, so there's no-one to yield to.

What's weird is that the thread is like immediately killed, there's no backtraces or anything, so that might be a Ruby bug of some sort.

Either way, we should return rather than yield.