class Puma::Reactor

The implementation uses a Queue to synchronize adding new objects from the internal select loop.
and wakes up any object when its timeout elapses.
This class additionally tracks a timeout for every added object,
‘wakeup’ any IO object that receives data.
Java NIO or just plain IO#select). The call to ‘NIO::Selector#select` will
The waiting/wake up is performed with nio4r, which will use the appropriate backend (libev,
any monitored object either receives data or times out, or when the Reactor shuts down.
Monitors a collection of IO objects, calling a block whenever

def add(client)

Returns false if the reactor is already shut down.
The object must respond to #timeout and #timeout_at.
Add a new client to monitor. 
def add(client)
  @input << client
  @selector.wakeup
  true
rescue ClosedQueueError, IOError # Ignore if selector is already closed
  false
end

def initialize(backend, &block)

its timeout elapses, or when the Reactor shuts down.
The provided block will be invoked when an IO has data available to read,
Create a new Reactor to monitor IO objects added by #add. 
def initialize(backend, &block)
  require 'nio'
  valid_backends = [:auto, *::NIO::Selector.backends]
  unless valid_backends.include?(backend)
    raise ArgumentError.new("unsupported IO selector backend: #{backend} (available backends: #{valid_backends.join(', ')})")
  end
  @selector = ::NIO::Selector.new(NIO::Selector.backends.delete(backend))
  @input = Queue.new
  @timeouts = []
  @block = block
end

def register(client)

Start monitoring the object. 
def register(client)
  @selector.register(client.to_io, :r).value = client
  @timeouts << client
rescue ArgumentError
  # unreadable clients raise error when processed by NIO
end

def run(background=true)

Run the internal select loop, using a background thread by default. 
def run(background=true)
  if background
    @thread = Thread.new do
      Puma.set_thread_name "reactor"
      select_loop
    end
  else
    select_loop
  end
end

def select_loop 

def select_loop
  close_selector = true
  begin
    until @input.closed? && @input.empty?
      # Wakeup any registered object that receives incoming data.
      # Block until the earliest timeout or Selector#wakeup is called.
      timeout = (earliest = @timeouts.first) && earliest.timeout
      @selector.select(timeout) {|mon| wakeup!(mon.value)}
      # Wakeup all objects that timed out.
      timed_out = @timeouts.take_while {|t| t.timeout == 0}
      timed_out.each { |c| wakeup! c }
      unless @input.empty?
        until @input.empty?
          client = @input.pop
          register(client) if client.io_ok?
        end
        @timeouts.sort_by!(&:timeout_at)
      end
    end
  rescue StandardError => e
    STDERR.puts "Error in reactor loop escaped: #{e.message} (#{e.class})"
    STDERR.puts e.backtrace
    # NoMethodError may be rarely raised when calling @selector.select, which
    # is odd.  Regardless, it may continue for thousands of calls if retried.
    # Also, when it raises, @selector.close also raises an error.
    if NoMethodError === e
      close_selector = false
    else
      retry
    end
  end
  # Wakeup all remaining objects on shutdown.
  @timeouts.each(&@block)
  @selector.close if close_selector
end

def shutdown

Shutdown the reactor, blocking until the background thread is finished. 
def shutdown
  @input.close
  begin
    @selector.wakeup
  rescue IOError # Ignore if selector is already closed
  end
  @thread&.join
end

def wakeup!(client)

Stop monitoring the object if the block returns `true`.
'Wake up' a monitored object by calling the provided block. 
def wakeup!(client)
  if @block.call client
    @selector.deregister client.to_io
    @timeouts.delete client
  end
end