Open3 throws exception when preceded by fork

I am attempting to create a daemon that (when required) spawns off
child processes to perform “jobs”. It is important that this daemon
does not block so it can start concurrent jobs, and that it knows when
jobs have completed. Some of the jobs are written in C++ and send
their output over standard error (stderr).

Here’s an outline of my daemon program.

#!/usr/bin/ruby
require ‘open3’
stop = false
Signal.trap(‘CLD’) do
child = Process.wait
puts “child #{child} closed.”
stop = true
end
child = Kernel.fork()
if(child == nil)
stdin, stdout, stderr =
Open3.popen3(‘program_that_sends_output_to_stderr’)
stderr.each do |line|
puts line
end
Kernel.exit!
end
while(! stop)
sleep 5
end
puts “finished”

When tested separately the fork and popen3 calls seem to be just what
I need to meet my objectives, but when they are run sequentially the
popen3 call throws an exception:
Error: “/usr/lib/ruby/1.8/open3.rb:75:in `waitpid’: No child processes
(Errno::ECHILD)”

Is this a limitation of Open3? Any alternative approaches would be
gratefully received.
Thank you.

On Jul 14, 2008, at 1:05 PM, trickyvail wrote:

s this a limitation of Open3? Any alternative approaches would be
gratefully received.

require ‘rubygems’
require ‘slave’ # gem install slave

class Server
def run job
system job
end
end

slave = Slave.new{ Sever.new }
server = slave.object

server.run ‘echo “this is in another process you do not have to ever
worry about”’

a @ http://codeforpeople.com/

On Jul 14, 2:52 pm, “ara.t.howard” [email protected] wrote:

server = slave.object

server.run ‘echo “this is in another process you do not have to ever
worry about”’

Thank you for you reply. Unfortunately I am unable to utilize the
slave ruby gem (and open4 gem also) due to some rubygems package
corruption? on my Ubuntu machine. I’ve given up fighting with it and
come up with a different solution.

daemon:
#!/usr/bin/ruby
stop = false
Signal.trap(‘CLD’) do
child = Process.wait
puts “child #{child} closed.”
stop = true
end
exec(’./job.rb’) if fork.nil?
while(! stop)
puts ‘not finished, do some other stuff’
sleep 5
end
puts “finished”

job:
#!/usr/bin/ruby
require ‘open3’
stdin, stdout, stderr = Open3.popen3(‘real_job_binary’)
stderr.each do |line|
puts line
end

This way the calls to fork and open3 are isolated in different ruby
interpreter instances where they can’t interfere with each other.

On Jul 14, 2008, at 5:50 PM, trickyvail wrote:

Thank you for you reply. Unfortunately I am unable to utilize the
slave ruby gem (and open4 gem also) due to some rubygems package
corruption? on my Ubuntu machine. I’ve given up fighting with it and
come up with a different solution.

the tgz files are on rubyforge. just unpack and run

‘sudo ruby install.rb’

cheers.

a @ http://codeforpeople.com/

This forum is not affiliated to the Ruby language, Ruby on Rails framework, nor any Ruby applications discussed here.

| Privacy Policy | Terms of Service | Remote Ruby Jobs