From: Joel VanderWerf <joelvanderwerf@...>
Date: 2010-09-07T06:53:13+09:00
Subject: Re: trouble with mkfifo and forks

On 09/06/2010 01:10 PM, Josh wrote:
> # In a nutshell:
> 1) Create a pipe
> 2) Fork twice
> 3) Each for sends 10 strings over the pipe
> 4) See what comes out the other side

I'm probably naive about something, but fifos almost never work right 
for what I want to do, and I usually end up using (unix domain) sockets 
(which also makes it easier to move to tcp sockets if you need to 
distribute across hosts).

Anyway, in your code, it seems that the repeated closing of the *read* 
end of the pipe causes some data to be lost. If you can modify your read 
loop to keep using the same IO object, it seems to work (see below). 
Also, I found I needed to watch for #gets returning nil (though that 
doesn't happen reliably).

Perhaps someone who knows more about pipes will "pipe up" on this thread.

class WaitListTest
   NUMBER_OF_ORDERS = 2
   PIPE = File.expand_path("~/tmp/wl_pipe")

   def setup
     `rm -f #{PIPE}`
     `mkfifo #{PIPE}`
   end

   def test_registering_last_race
     term_count = 0
     responses = []
     pids = []

     NUMBER_OF_ORDERS.times do |i|
       pids << Process.fork do
         sleep 0.1 + (0.1 * i)
         10.times do |a|
           File.open(PIPE, "w") do |pipe|
             sleep 0.1
             pipe.puts "Fork #{i} - #{a}"
           end
         end
         File.open(PIPE, "w") do |pipe|
           sleep 0.1
           pipe.puts "DONE"
         end
       end
     end

     pipe = File.open(PIPE, "r")
     while term_count < NUMBER_OF_ORDERS
#      File.open(PIPE, "r") do |pipe|
         val = pipe.gets
         case val
         when /DONE/
           term_count += 1
         when nil
           puts "pipe closed" # need this too
         else
           responses << val
         end
#      end
     end

     pids.each do |pid|
       Process.waitpid(pid)
     end

     puts responses.sort
     puts responses.count
   end
end

wlt = WaitListTest.new
wlt.setup
wlt.test_registering_last_race