From: Joe Van Dyk <joevandyk@...>
Date: 2006-09-27T07:28:58+09:00
Subject: Re: forking processes and writing to log file

On 9/26/06, Joe Van Dyk <joevandyk@gmail.com> wrote:
> On 9/26/06, Gennady Bystritsky <Gennady.Bystritsky@quest.com> wrote:
> > Joe Van Dyk wrote:
> > > On 9/26/06, Joe Van Dyk <joevandyk@gmail.com> wrote:
> > >> Hello,
> > >>
> > >> So, on linux, I'm forking a new Ruby process.  I redirect stdout and
> > >> stderr into a file and then I execute another program via 'system'.
> > >
> > > I should probably mention that the other program that's being executed
> > > is rsh, which executes a command on a different machine.
> >
> > You either did not redirect stdout/stderr to a file properly, or
> > something you execute tries to output directly to the controlling
> > terminal (/dev/tty). You can disassosiate your spawned process from a
> > controlling terminal, just to be sure (not with 'system' though):
> >
> > logfile = "my_log_file.log"
> > pid = fork do
> >   File.open(logfile, 'w') do |_io|
> >     $stdout.reopen(_io)
> >     $stderr.reopen(_io)
> >   end
> >
> >   Process.setsess # In this scenario disassociates the controlling
> >                   # terminal until another tty device is open (will
> >                   # become new controlling tty).
> >
> >   exec "the_command_line_used_to_be_launched_by_system"
> > end
> >
> > Process.waitpid pid
>
> I assume you mean Process.setsid?
>
> I have this function called at the beginning of the fork:
> # Redirect stdout and stderr to a file and flush the buffers on a write
> def redirect_all_output_to_file file
>    log = File.new(file, "w+")
>    STDOUT.reopen log;  STDERR.reopen log
>    STDOUT.sync = true
> end
>
> When I add Process.setsid to the end of that function, when I
> background the ruby process, stuff keeps writing to the log file, but
> my controlling terminal gets all messed up.  The only keys on my
> keyboard that work are 'e', 's', the space bar, and the backspace key,
> and they only work half the time.  The terminal works normally once
> the ruby process is killed.

Ah, this appears to be an rsh issue, not a Ruby one.  (can't use ssh
here because people are too lazy to set up ssh keys or something)

 $ rsh machine some_command &
That seems to need to wait for input, even if some_command doesn't
read from stdin.

  $ ssh machine some_command &
works as expected.

My workaround is to create a file with half a dozen blank lines and do
  $ rsh machine some_command < file_with_blank_lines &

But that seems icky.