http://blog.darkhax.com/2009/06/01/gtk-and-ruby-threading-issues

gtk-mod.rb

require "thread"

module Gtk
  PENDING_CALLS_MUTEX = Mutex.new
  PENDING_CALLS = []

  def self.thread_protect(&proc)
    if Thread.current == Thread.main
      proc.call
    else
      PENDING_CALLS_MUTEX.synchronize do
        PENDING_CALLS << proc
      end
    end
  end

  def self.thread_flush
    if PENDING_CALLS_MUTEX.try_lock
      PENDING_CALLS.each { |closure| closure.call }
      PENDING_CALLS.clear
      PENDING_CALLS_MUTEX.unlock
    end
  end

  def self.init_thread_protect
    Gtk.timeout_add(100) do
      PENDING_CALLS_MUTEX.synchronize do
        PENDING_CALLS.each { |closure| closure.call }
        PENDING_CALLS.clear
      end
      true
    end
  end
end

gtk.rb

require "thread"

module Gtk
  # Thread-safety stuff.
  # Loosely based on booh, by Guillaume Cottenceau.

  PENDING_CALLS_MUTEX = Mutex.new
  PENDING_CALLS = []

  def self.thread_protect(&proc)
    if Thread.current == Thread.main
      proc.call
    else
      PENDING_CALLS_MUTEX.synchronize do
        PENDING_CALLS << proc
      end
    end
  end

  def self.thread_flush
    if PENDING_CALLS_MUTEX.try_lock
      for closure in PENDING_CALLS
        closure.call
      end
      PENDING_CALLS.clear
      PENDING_CALLS_MUTEX.unlock
    end
  end

  def self.init_thread_protect
    Gtk.timeout_add(100) do
      PENDING_CALLS_MUTEX.synchronize do
        for closure in PENDING_CALLS
          closure.call
        end
        PENDING_CALLS.clear
      end
      true
    end
  end
end