From: "ara.t.howard" <ara.t.howard@...>
Date: 2007-08-19T06:03:34+09:00
Subject: Re: Slave not removing slave_proc_* files


On Aug 18, 2007, at 12:54 PM, Jos Backus wrote:

>     Hi Ara,

how do.

>>
>> what is this code supposed to do exactly?
>
> Mongrel::Runner.new.run(args) will start running relative to the  
> parent,
> meaning that (since we are talking about the swiftiplied Mongrel  
> here) it will
> connect to the swiftiply proxy and handle any requests sent by it.  
> The parent
> isn't interested in making any method calls to the slaves; the  
> slaves are
> meant to run forever.

does mongrel fork to accomplish that?  if not why not simply

   threads << Thread.new{ Mongrel::Runner.new.run(args) }

>
> So my guess that I am using Slave wrong appears correct. The above  
> explains
> why the sockets remain. Since Slave.object returns a thread I  
> figured I could
> just join them to block the parent.
>

you could *if* mongrel returned an object.  the fact that i never  
returns leaves slave half baked...


> My  idea is to have a simple way to host a bunch of Rails apps/ 
> multiple
> instances of the same Rails app.  So I am trying to start N  
> mongrel_rails
> processes managed by swiftiply_mongrel_rails which in turn will be  
> managed by
> daemontools.  To accomplish this, I added the change in the post below
> mentioned to mongrel_rails.rb so that rather than using backticks I  
> can just
> call this Mongrel::Runner.run method (maybe that is the wrong  
> approach and I
> should just use backticks/Kernel.system()).
>
> Slave seemed like an easy way to manage the children and avoid  
> zombies. Is
> there a correct way of doing this with Slave? Or is it the wrong  
> tool for the
> job? In that case I'll use fork/exec, Process.waitall and a SIGTERM  
> handler in
> the parent which kills the children, etc. to get the job done.
>

i think the LifeLine class of slave is exactly what you need and that  
you can re-use it.  basically the idea is this

socket = Socket.pair


cid = fork

if cid
   stuff
else
   Thread.new{ socket.read rescue exit }
   stuff
end

in summary, the child reads from a pipe to the parent.  if the read  
ever returns the parent has died - so exit.  this is the 'zombie'  
preventer of slave.rb.  if you look at slave.rb and grep for LifeLine  
and @lifeline you'll see the usage.  summary is:

lifeline = LifeLine.new

cid = fork

unless cid # child
   @lifeline.catch
   @lifeline.cling
else
   @lifeline.throw
end


and the poor man's version:



cfp:~ > cat a.rb
r, w = IO.pipe
cid = fork

unless cid ### child
   w.close

   Thread.new{
     begin
       r.read
     ensure
       STDERR.puts 'parent died... exiting!'
       Kernel.exit
     end
   }

   sleep and 'pretend some processing is going on...'
else
   r.close
   sleep 2 and 'pretend the parent exited'
end



cfp:~ > ruby a.rb
parent died... exiting!


> There are no references to SystemExit in swiftiply-0.6.1:
>
>     # pwd
>     /usr/lib/ruby/gems/1.8/gems/swiftiply-0.6.1
>     # grep -r SystemExit .
>     #
>

right.  but there *should* be *if* blanket Exceptions are rescued.

> Thanks for your help, Ara.

always good to have testers. er, i mean users ;-)

a @ http://drawohara.com/
--
we can deny everything, except that we have the possibility of being  
better. simply reflect on that.
h.h. the 14th dalai lama