Pry session over telnet. Minimal and forking (plus a thread for the TCPServer).

gistfile1.rb

drbig@swordfish:pts/16 ~/P/pry-tcprepl> cat pry-tcprepl.rb 
require 'pry'
require 'pty'

class PryTcpRepl
  attr_reader :thread

  def initialize(where, host = '127.0.0.1', port = '9123')
    @where = where
    @host = host
    @port = port

    @thread = false
  end

  def run!
    @thread ||= Thread.new do
      TCPServer.open(@host, @port) do |s|
        loop do
          c = s.accept
          pid = fork do
            puts 'Client fork in.'

            m, s = PTY.open
            m.raw!
            s.raw!
            c.sync = m.sync = s.sync = true

            old_ios = Array.new
            [STDIN, STDOUT, STDERR].each do |i|
              old_ios.push(i.dup)
              i.reopen(s)
            end

            Thread.new { IO.copy_stream(c, m) }
            Thread.new { IO.copy_stream(m, c) }
            
            Pry.start(@where)

            [STDIN, STDOUT, STDERR].each_with_index do |i, x|
              i.reopen(old_ios[x])
            end
            
            s.close
            m.close
            c.close_write
            c.close_read
            
            puts 'Client fork out.'
            exit(0)
          end
          Process.detach(pid)
          puts "New client from #{c.peeraddr(:numeric).last} - pid #{pid}."
        end
      end
    end
  end
end

drbig@swordfish:pts/16 ~/P/pry-tcprepl> cat test.rb 
require 'sinatra/base'
load 'pry-tcprepl.rb'

class MyApp < Sinatra::Base
  get '/' do
    'Hello world!'
  end
end

repl = PryTcpRepl.new(MyApp)
repl.run!
MyApp.run!