From: Caleb Clausen <vikkous@...>
Date: 2010-10-04T11:29:29+09:00
Subject: Re: having problems with open4 and stuck forked processes

On 10/3/10, Tim Uckun <timuckun@gmail.com> wrote:
> Hey guys I want to revist this issue because I can't seem to find any
> documentation on how to do this.
>
> What I want to do seem simple enough. I want to shell out to a process
> which sometimes gets stuck. It won't return at all. It just sits there
> taking up 100% of the CPU (one of the cores anyway).  I just want to
> make sure that if the process does not end in a reasonable amount of
> time I want to kill it.
>
> So far I have tried wrapping it in a timeout block but that doesn't
> always trigger for some reason. I have plenty of error handling and
> have an ensure block which says to kill the process if it exists but
> nothing I do seems to work. Sooner or later I get a stuck process that
> hangs around forever till I kill it by hand.

Timeout::timeout is kind of a hack. It's probably better to avoid it.

> Surely there is a simple way to do this.
>
> Here is the code I have so far.
>
> http://gist.github.com/609119

Your problem may be that you're sending signal 0; you should pass
"TERM" or (if that won't work) "KILL" as the first parameter to
Signal.kill. signal 0 just queries if the process can receive signals
or not...

If you want to use select instead of timeout, then instead of this:

    Timeout::timeout(seconds) {

      @pid, @stdin, @stdout, @stderr = Open4.popen4(cmd)

      ignored, @status = Process::waitpid2 @pid

      if @status.exitstatus != 0
        raise "Exit Status not zero"
      end
    }

You should use something like this: (UNTESTED)

      @pid, @stdin, @stdout, @stderr = Open4.popen4(cmd)

      if IO::select([@stdout],nil,nil,seconds)
        Util.kill_process_if_exists? @pid
      else
        fail 'unexpected data on stdout'
      end

      ignored, @status = Process::waitpid2 @pid

      if @status.exitstatus != 0
        raise "Exit Status not zero"
      end

Except, if the external process actually prints something to stdout,
then you need to call select in a loop until select returns nil, with
decreasing timeouts depending on how much time has passed.

Unfortunately, 'ri Kernel#select'  seems to be broken... it just
refers you back to Kernel#select. I hope somebody fixes that. Check
what it says in the pickaxe instead. (There's a free version available
online if you don't own a copy yourself.)