Re: Pausing POE::Wheel::FollowTail?

[ Top posting corrected ]

On Mon, Oct 06, 2008 at 04:24:40PM -0500, Votava, Craig M (Craig) wrote:
> > On Mon, Oct 06, 2008 at 10:11:42AM -0500, Votava, Craig M (Craig) wrote:
> > > I wrote a simple perl application that uses
> > > POE::Wheel::FollowTail to get entries out of a logfile, and puts
> > > them up in a Tk Text window.
> > > 
> > > I want to add a pause/resume toggle button at the bottom of the
> > > Text window, so that the user can stop and start the incoming
> > > data whenever they want. When the input is paused, I would like
> > > to have minimal impact on the CPU. If the pause lasts long
> > > enough to where the file is reset, then the user would get the
> > > data starting at the beginning of the current file once they
> > > unpause. What is the best way to do this?
> > > 
> > > I wonder if it would make sense to add a pause method to this
> > > module? If the general consensus is yes, I can volunteer to take
> > > a stab at it. If there is a better/easier/faster way to do it
> > > with the existing methods, please let me know.
> > 
> > I am new to POE, but it seems to me you could either
> > 
> >   - use tell() to get the current position
> >   - destroy the wheel
> >   - recreate it when you're ready and set the Seek to the previous
> >     position
> > 
> >   (There's actually an example of this in the
> >   POE::Wheel::FollowTail manpage under tell().)
> > 
> > or
> > 
> >   - Alter (or subclass) FollowTail so that you can change the
> >     PollInterval dynamically.
> > 
> > Neither would be very hard.
> 
> Thanks for the response! Either of your solutions could be best,
> but I'm not sure about a couple of things:
> 
> 1.) Say I save the current state then destroy the wheel. When I
> start up a new wheel, how do I know whether to use the saved data
> to index back to where I left off or (if the file has been reset)
> to just start from the beginning?

That's a good question, and not one I thought of.  :)

I could think of a couple of strategies (check the inode?), but mostly
I think you'd want to dig into the wheel and either do what it does,
or figure out how to get it to *track* the file without actually
reading the data and sending it to you.

I guess if you go with that strategy, you really would need "pause"
functionality, as you initially started out asking for.

> 2.) If I change PollInterval, what value would I change it to, so
> that it would never fire?

I'd probably go with 32e6 seconds, which is just over a year.  Or
there's always 2**32, which is ~136 years.

But as mentioned above, you probably really do want a "pause"
function.  Also, if you fake a "pause" by setting a long interval, as
I initially suggested, I realize now that a) you'd obviously need some
way to get the ball rolling again, and b) those year-later events
would hang around in the queue, and who wants that?

So I guess I'd recommend:

- dig into the wheel; add a flag that tells the read code to skip the
  actual "read the file" part of what it does, but it continues the
  polling at the normal cycle.
- add methods to the wheel to allow you to set or reset the flag

The wheel has a _define_timer_states function that defines how it
deals with files (as opposed to sockets, which is
_define_select_states).  That function looks kind of like this:

    sub _define_timer_states {
      # ... stuff ...
      $poe_kernel->state(
	  $$state_read,
	  sub {
	    # check the inode, read the file, etc
	  } );

      $poe_kernel->delay($$state_read, 0);
    }

I guess I'd try something vaguely similar to:

    my $running = 1;

    sub pause {
      $running = 0;
    }

    sub unpause {
      $running = 1;
    }

    sub _define_timer_states {
      # ... stuff ...
      $poe_kernel->state(
	  $$state_read,
	  sub {
	    if (!$running) {
	      # ... requeue the $$state_read event for $interval seconds from now ...
	      return;
	    }

	    # check the inode, read the file, etc
	  } );

      $poe_kernel->delay($$state_read, 0);
    }

... except you'd put the $running flag into the wheel object, of
course, otherwise you've paused *all* the wheels in the system!  :)

This is obviously untested and may make demons fly out your nose.  But
I hope it helped anyway.  :)

-- Larry