require 'fiddle'
require 'fiddle/import'

class NS
  extend Fiddle::Importer
  dlload Fiddle.dlopen( nil)
  SetNS_Function = Fiddle::Function.new handler['setns'], [Fiddle::TYPE_VOIDP, Fiddle::TYPE_VOIDP], Fiddle::TYPE_INT

  class <<self
    def setns fd, t
      r = SetNS_Function.call fd.to_i, t.to_i
      SystemCallError.new "setns(#{fd.to_i})", Fiddle.last_error  if -1 == r
      r
    end

    def _open_nsfd pid, type
      File.open "/proc/%d/ns/%s" % [pid, type]
    end

    def open pid, *types, &exe
      ns = new pid, *types
      if block_given?
        begin yield ns
        ensure ns.close
        end
      else ns
      end
    end

    def change pid, *types, &exe
      open pid, *types do |ns|
        ns.change &exe
      end
    end
  end

  attr_reader :pid, :fds, :types

  def initialize pid, *types
    @pid, @fds = pid, {}
    types.map {|t| @fds[t] = self.class._open_nsfd pid, t }
  end

  def close
    @fds.each {|t, fd| fd.close }
  end

  def change &block
    @owns = @fds.map {|t, _f| Own[t] }
    begin
      @fds.each {|type, fd| self.class.setns fd, 0 }
      yield
    ensure
      @owns.each {|fd| self.class.setns fd, 0 }
    end
  end

  Own = Hash.new {|h, ns| h[ns] = _open_nsfd $$, ns }
end