first_to_win.rb

require 'concurrent'

class Observer
  attr_reader :key
  def initialize(event, key)
    @event = event
    @key = key
    @finished = false
  end

  def finished?
    @finished == true
  end

  def update(time, value, reason)
    @finished = true
    @event.set
  end
end

foo = Concurrent::Future.new do
  sleep(5)
  :foo
end

bar = Concurrent::Future.new do
  sleep(1)
  :bar
end

group = { foo: foo, bar: bar }

group_observer = Concurrent::Future.new do
  event = Concurrent::Event.new

  observers = group.map do |key, future|
    Observer.new(event, key).tap { |o| future.add_observer(o) }
  end

  event.wait
  group[observers.select(&:finished?).first.key].value
end

[*group.values, group_observer].map(&:execute)
group_observer.value