From: John Carter <john.carter@...>
Date: 2008-10-06T11:11:26+09:00
Subject: Ruby lacks atfork : The evil that lives in fork...

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Consider this simple usage of Thread and Process....

I use a mutex to block access to the $state variable when it is in
an "inconsistent" state.

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D
require 'thread'
require 'pp'
Thread.abort_on_exception =3D true
$state =3D "Uninited"

def state(m)
    print "#{caller(0)[1]}: #{Time.now} #{$state} "
    if m.locked?
       puts "Mutex locked"
    else
       puts "Mutex unlocked"
    end
end

m =3D Mutex.new
state(m)
$state =3D "Good"
t =3D Thread.new do
    begin
       state(m)
       m.synchronize do
          $state =3D "Inconsistent"
          state(m)
          sleep 10
          $state =3D "Good Again"
          state(m)
       end
    ensure
       state(m)
    end
end

state(m)
sleep 2

state(m)

pid =3D Process.fork do
    state(m)

    sleep 2

    state(m)
end

state(m)
pp Process.waitpid2(pid)

t.join

state(m)
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D
This is what it outputs...

ruby -v;ruby -w evil_fork.rb
ruby 1.8.7 (2008-06-20 patchlevel 22) [i686-linux]
evil_fork.rb:16: Mon Oct 06 14:51:17 +1300 2008 Uninited Mutex unlock=
ed
evil_fork.rb:20: Mon Oct 06 14:51:17 +1300 2008 Good Mutex unlocked
evil_fork.rb:23: Mon Oct 06 14:51:17 +1300 2008 Inconsistent Mutex lo=
cked
evil_fork.rb:33: Mon Oct 06 14:51:17 +1300 2008 Inconsistent Mutex lo=
cked
evil_fork.rb:36: Mon Oct 06 14:51:19 +1300 2008 Inconsistent Mutex lo=
cked
evil_fork.rb:39: Mon Oct 06 14:51:19 +1300 2008 Inconsistent Mutex un=
locked
evil_fork.rb:46: Mon Oct 06 14:51:19 +1300 2008 Inconsistent Mutex lo=
cked
evil_fork.rb:43: Mon Oct 06 14:51:21 +1300 2008 Inconsistent Mutex un=
locked
[5082, #<Process::Status: pid=3D5082,exited(0)>]
evil_fork.rb:26: Mon Oct 06 14:51:27 +1300 2008 Good Again Mutex lock=
ed
evil_fork.rb:29: Mon Oct 06 14:51:27 +1300 2008 Good Again Mutex unlo=
cked
evil_fork.rb:51: Mon Oct 06 14:51:27 +1300 2008 Good Again Mutex unlo=
cked

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D

Oh dear!

When I Process.fork'ed I saw this....
evil_fork.rb:39: Mon Oct 06 14:51:19 +1300 2008 Inconsistent Mutex un=
locked

ie. I could be accessing $state when it is in an inconsistent state
and the Mutex doesn't protect me.

=46rom the fork man page...

        * The child process is created with a single thread =E2=80=
=94 the one
          that called fork().  The entire virtual address space of
          the parent is replicated in the child, including the states
          of mutexes, condition variables, and other pthreads objects=
;
          the use of pthread_atfork(3) may be helpful for dealing wit=
h
          problems that this can cause.

Unfortunately Ruby doesn't provide an atfork facility.



John Carter                             Phone : (64)(3) 358 6639
Tait Electronics                        Fax   : (64)(3) 359 4632
PO Box 1645 Christchurch                Email : john.carter@tait.co.n=
z
New Zealand

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