limit total num threads

limit_threads.rb

THREAD_LIMIT = ENV["THREAD_LIMIT"].to_i || 10
THREAD = {count: 0}

class TooManyThreadsError < StandardError; end

class Thread
  def new(*args, &blk)
    too_many_threads_error if THREAD[:count] > THREAD_LIMIT
    THREAD["count"] += 1
    super(*args, &->{blk.call; THREAD["count"] -= 1})
  end
end

10.times { Thread.new { sleep 1 } }
begin
  Thread.new { }
rescue TooManyThreadsError
  puts "rescued as expected"
end

sleep 1

Thread.new { sleep 1 }