From: Robert Klemme <shortcutter@...>
Date: 2010-09-23T00:33:36+09:00
Subject: Re: having problems with open4 and stuck forked processes

On Wed, Sep 22, 2010 at 2:31 PM, Tim Uckun <timuckun@gmail.com> wrote:
> I am running a batch process which uses the wkhtmltoimage-i386 binary
> to make screenshots of urls.  Unfortunately this is in beta and it
> frequently hangs up and takes up 100% of one of the CPUs on the
> machine.
>
> I have the following code to try and detect the hung process and kill
> it but it doesn't always work and I was wondering if anybody has a
> better idea of how to do this.  When I run it by testing simple
> commands like sleep it works perfectly. In production with this binary
> it doesn't seem to always work.

What do you mean by that?  Goes the timeout undetected?  Can't you
kill the process?  Are there any unexpected error messages /
exceptions?

> def Util.shell_with_timeout(cmd, seconds = 3600)
>    #the default timeout is an hour. That's probably way too long
>
>    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
>    }
>
>    @stdout ? @stdout.read.strip : ''
>  rescue Timeout::Error
>    Process.detach @pid
>    Process.kill 'SIGKILL', @pid
>    raise "Process Timed out"
>  rescue => e
>    msg =  @stderr ?  @stderr.read.strip : ''
>    msg +=   e.to_s
>    raise "Error during execution of command #{cmd}\n #{msg}"
>  end

A frequent problem with #popen methods is to not read file descriptors
which can make the client hang (i.e. if it writes more than fits into
a pipe).  That could be something to check since you are not reading
any of the streams.

Kind regards

robert

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