The self-pipe trick in Ruby

about.md

An example implementation of the self-pipe trick in Ruby, by Paul Annesley (found here: http://pastebin.com/1SZprwZm)

Further reading:

• D.J. Berstein - The self-pipe trick - http://cr.yp.to/docs/selfpipe.html
• Alex Pennace - Safe UNIX Signal Handling Tips - http://osiris.978.org/~alex/safesignalhandling.html
• Ryan Tomayko - I like Unicorn because it's Unix - http://tomayko.com/writings/unicorn-is-unix
• Unicorn Sourcecode - http://bogomips.org/unicorn.git/tree/lib/unicorn/http_server.rb
• man 2 select

self_pipe.rb

#!/usr/bin/env ruby

# Contrived example of self-pipe preventing signal race condition prior to select()
# @see http://cr.yp.to/docs/selfpipe.html
# @author Paul Annesley

SELF_READ, SELF_WRITE = IO.pipe
@run = true

trap :INT do
  @run = false
  SELF_WRITE.putc(0)
end

# send SIGINT mid-sleep
Process.fork { sleep 1; Process.kill :INT, Process.ppid; exit }

r,w = IO.pipe
while @run do
  sleep 0.2 # .. amplify race-condition; exists until select()
  w.putc(".") if @run # .. nothing written if SIGINT handler has disabled @run
  selected = IO.select [r, SELF_READ]
  break if selected.first.include? SELF_READ
  print r.getc
end

puts 'without self-pipe, select() would have blocked'