SetHandler() sets handler for all logs in all packages importing same log15 path

[sbward]

If a package importing "gopkg.in/inconshreveable/log15.v2" imports another package which also uses "gopkg.in/inconshreveable/log15.v2", then any calls to l.SetHandler(...) for any logger will overwrite the handler in all loggers across both of those packages.

The reason is that there's a shared root in the package which always references the same underlying log.Handler even after calling l.New() on it.

Your documentation suggests initializing logging like this:

package yourlib

import "gopkg.in/inconshreveable/log15.v2"

var Log = log.New()

func init() {
    Log.SetHandler(log.DiscardHandler())
}

I tried this method and got burned because both my main package and my library imported "gopkg.in/inconshreveable/log15.v2". Suddenly my logs were gone because the library changed the singular root logger's handler to log.DiscardHandler.

I recommend changing the semantics of log15.New() to do a completely new object without a shared Root at the bottom, while allowing Logger.New() to perform shared state chaining.

[inconshreveable]

Sorry for the slow response.

You're right. This is very broken. The internal model I had for this in my head was:

A logger should inherit the handler of its parent until a call to SetHandler changes it.

Does that seem like a reasonable answer? It's a little bit different than what you propose in your PR

[sbward]

No worries, and yes, that does sound reasonable to me.

[inconshreveable]

Actual fix is here, the first one would have broken something else:

da1a931