Extend the tailer utility.

Extend the tailer utility.

When looking at the pyjuju debug-log, there was a replay
function that allowed getting the log from the start.
There was no way to tell the tailer to start from the
beginning of the file.

I changed the meaning of NewTailer to be one that doesn't
do any seeking.  The NewTailerBacktrack does seek to the end,
and then goes back a certain number of lines.

There was a bug in the backtrack tailer when zero lines
were passed in.  This is fixed and a test added.

In order to provide a synchronization mechanism for other
tests, the Tailer has a public function param that is called
after the backtracking, and prior to starting tailing.

The number of lines to look back is now a unit, as negative
values make no sense at all.

https://code.launchpad.net/~thumper/juju-core/tailer-tweaks/+merge/214150

(do not edit description out of merge proposal)

[thumper, 2014-04-04 03:42:04]

Please take a look.

[axw, 2014-04-04 04:21:34]

On 2014/04/04 03:42:04, thumper wrote:
> Please take a look.

Looks fine in general. As discussed on IRC, I would prefer if we didn't expose
StartedTailing for testing, but instead you override Seek for your testing
purposes.

I'm not crazy about NewTailer doing this seeking business. It seems like that
should be pulled out, and just have NewTailer do one thing.
Just a suggestion, but how about something like:

    func SeekLastMatching(r io.ReadSeeker, n int, filter TailerFilterFunc) error
    func NewTailer(r io.Reader, writer io.Writer, filter TailerFilterFunc)
*Tailer

I see that Tailer does use Seek, but I'm not sure that it needs to.

[thumper, 2014-04-04 04:30:50]

On 2014/04/04 04:21:34, axw wrote:
> On 2014/04/04 03:42:04, thumper wrote:
> > Please take a look.
> 
> Looks fine in general. As discussed on IRC, I would prefer if we didn't expose
> StartedTailing for testing, but instead you override Seek for your testing
> purposes.
> 
> I'm not crazy about NewTailer doing this seeking business. It seems like that
> should be pulled out, and just have NewTailer do one thing.
> Just a suggestion, but how about something like:
> 
>     func SeekLastMatching(r io.ReadSeeker, n int, filter TailerFilterFunc)
error
>     func NewTailer(r io.Reader, writer io.Writer, filter TailerFilterFunc)
> *Tailer
> 
> I see that Tailer does use Seek, but I'm not sure that it needs to.

I'd rather keep the Tailer as unchanged as possible for now.

I've removed the StartedTailing testing callback

[thumper, 2014-04-04 04:33:10]

Please take a look.

[axw, 2014-04-04 04:34:25]

On 2014/04/04 04:33:10, thumper wrote:
> Please take a look.

LGTM, thanks.