consul/vendor/github.com/hashicorp/go-hclog/global.go

package hclog

import (
	"sync"
)

var (
	protect sync.Once
	def     Logger

	// DefaultOptions is used to create the Default logger. These are read
	// only when the Default logger is created, so set them as soon as the
	// process starts.
	DefaultOptions = &LoggerOptions{
		Level:  DefaultLevel,
		Output: DefaultOutput,
	}
)

// Default returns a globally held logger. This can be a good starting
// place, and then you can use .With() and .Name() to create sub-loggers
// to be used in more specific contexts.
// The value of the Default logger can be set via SetDefault() or by
// changing the options in DefaultOptions.
//
// This method is goroutine safe, returning a global from memory, but
// cause should be used if SetDefault() is called it random times
// in the program as that may result in race conditions and an unexpected
// Logger being returned.
func Default() Logger {
	protect.Do(func() {
		// If SetDefault was used before Default() was called, we need to
		// detect that here.
		if def == nil {
			def = New(DefaultOptions)
		}
	})

	return def
}

// L is a short alias for Default().
func L() Logger {
	return Default()
}

// SetDefault changes the logger to be returned by Default()and L()
// to the one given. This allows packages to use the default logger
// and have higher level packages change it to match the execution
// environment. It returns any old default if there is one.
//
// NOTE: This is expected to be called early in the program to setup
// a default logger. As such, it does not attempt to make itself
// not racy with regard to the value of the default logger. Ergo
// if it is called in goroutines, you may experience race conditions
// with other goroutines retrieving the default logger. Basically,
// don't do that.
func SetDefault(log Logger) Logger {
	old := def
	def = log
	return old
}