a.md

daemon scripts (example)

(▰╹◡╹) time ruby daemon.rb
daemon: bye
daemon: bye (stderr)
ruby daemon.rb  0.08s user 0.04s system 2% cpu 5.136 total

(▰╹◡╹) time ruby daemon-starter.rb
loop 0
loop 1
start
started 93847
ruby daemon-starter.rb  0.08s user 0.04s system 5% cpu 2.141 total
(▰╹◡╹) daemon: bye
daemon: bye (stderr)

run.rb waits IO EOF, so it waits grandchild

(▰╹◡╹) time ruby run.rb
94129
#<Process::Status: pid 94129 exit 0>
daemon: bye (stderr)
"loop 0\nloop 1\nstart\nstarted 94139\ndaemon: bye\n"
ruby run.rb  0.15s user 0.05s system 2% cpu 7.292 total

run2.rb reads child output progressive, and quits when the first child exited

(▰╹◡╹) time ruby run2.rb
94350
"loop 0\nloop 1\nstart\n"
"started 94351\n"
ruby run2.rb  0.15s user 0.04s system 8% cpu 2.215 total
(▰╹◡╹) daemon: bye (stderr)
(▰╹◡╹)

daemon-starter.rb

2.times do |x|
  puts "loop #{x}"
sleep 1
end

puts "start"

pid = spawn("ruby", "daemon.rb")

puts "started #{pid}"

daemon.rb

sleep 5
puts "daemon: bye"
$stderr.puts "daemon: bye (stderr)"
exit

run.rb

Dir.chdir File.dirname(__FILE__)

child_i, child_o = IO.pipe

pid = spawn("ruby", "./daemon-starter.rb", out: child_o, err: $stderr)
p pid

child_o.close

pid, stat = Process.waitpid2(pid)

p stat
p child_i.read

run2.rb

Dir.chdir File.dirname(__FILE__)

begin
  quit_i,  quit_o  = IO.pipe
  child_i, child_o = IO.pipe
  
  th = Thread.new do
    begin
      loop do
        readable_ios, = IO.select([quit_i, child_i])
  
        if readable_ios.include?(child_i)
          p child_i.read_nonblock(1000)
        end
  
        if readable_ios.include?(quit_i)
          p :th_quit
          break
        end
      end
    rescue EOFError
      p :th_quit_eof
    ensure
      quit_i.close unless quit_i.closed?
      child_i.close unless child_i.closed?
    end
  end

  th.abort_on_exception = true

  pid = spawn("ruby", "./daemon-starter.rb", out: child_o, err: $stderr)
  p pid
  
  child_o.close
  
  pid, stat = Process.waitpid2(pid)
  
  begin
    quit_o.syswrite 1
  rescue Errno::EPIPE
  end
ensure
  quit_o.close
end