Add logging options to config

[GeertJohan]

The library generates logging output when files are closed or recreated. I didn't want to display this to the user of my program so I added extra options to modify the way tail logs.

There's Config.Logger, allowing you to set your own Logger (using logfile, or other prefix or flags).
And there's Config.DisableLogging, which disables logging all together..
This doesn't change anything for existing users, as Config.Logger is set to the log package's default logger (Log.std) when it's nil.

I hope this code will be useful for anyone else out there 😄

[srid]

i understand the intent to suppress logging from a library, however do we really want to add two more config options? i.e., why not avoid DisableLogging and exclusively use Config.Logger - which if set to nil or tail.DEFAULT_LOGGER (the default; or a custom logger), makes the tail library log to it, else (if explicitly set to tail.NO_LOGGER) totally suppresses logging?

[GeertJohan]

What would tail.NO_LOGGER be? an io.Writer to /dev/null ?

[srid]

yes. perhaps:

NIL_LOGGER=log.New(ioutil.Discard, "", log.LstdFlags)
...// suppress logging
tail.Config{Logger: tail.NIL_LOGGER...

[GeertJohan]

It feels a bit like using cpu-time for nothing to log to ioutil.Discard. But I can see the simplicity it adds to the configuration of tail. Also, tail doesn't log that much, so there's not that much wasted cputime..

Working on a patch now.

[GeertJohan]

Done.
I also exposed DefaultLogger (used when Config.Logger == nil)