Update nlopes/slack to 4.1-dev (#595)

vendor/github.com/nlopes/slack/logger.go

@@ -2,52 +2,59 @@ package slack
 
 import (
 	"fmt"
-	"sync"
 )
 
-// SetLogger let's library users supply a logger, so that api debugging
-// can be logged along with the application's debugging info.
-func SetLogger(l logProvider) {
-	loggerMutex.Lock()
-	logger = ilogger{logProvider: l}
-	loggerMutex.Unlock()
-}
-
-var (
-	loggerMutex = new(sync.Mutex)
-	logger      logInternal // A logger that can be set by consumers
-)
-
-// logProvider is a logger interface compatible with both stdlib and some
-// 3rd party loggers such as logrus.
-type logProvider interface {
+// logger is a logger interface compatible with both stdlib and some
+// 3rd party loggers.
+type logger interface {
 	Output(int, string) error
 }
 
-// logInternal represents the internal logging api we use.
-type logInternal interface {
+// ilogger represents the internal logging api we use.
+type ilogger interface {
+	logger
 	Print(...interface{})
 	Printf(string, ...interface{})
 	Println(...interface{})
-	Output(int, string) error
 }
 
-// ilogger implements the additional methods used by our internal logging.
-type ilogger struct {
-	logProvider
+type debug interface {
+	Debug() bool
+
+	// Debugf print a formatted debug line.
+	Debugf(format string, v ...interface{})
+	// Debugln print a debug line.
+	Debugln(v ...interface{})
+}
+
+// internalLog implements the additional methods used by our internal logging.
+type internalLog struct {
+	logger
 }
 
 // Println replicates the behaviour of the standard logger.
-func (t ilogger) Println(v ...interface{}) {
+func (t internalLog) Println(v ...interface{}) {
 	t.Output(2, fmt.Sprintln(v...))
 }
 
 // Printf replicates the behaviour of the standard logger.
-func (t ilogger) Printf(format string, v ...interface{}) {
+func (t internalLog) Printf(format string, v ...interface{}) {
 	t.Output(2, fmt.Sprintf(format, v...))
 }
 
 // Print replicates the behaviour of the standard logger.
-func (t ilogger) Print(v ...interface{}) {
+func (t internalLog) Print(v ...interface{}) {
 	t.Output(2, fmt.Sprint(v...))
 }
+
+type discard struct{}
+
+func (t discard) Debug() bool {
+	return false
+}
+
+// Debugf print a formatted debug line.
+func (t discard) Debugf(format string, v ...interface{}) {}
+
+// Debugln print a debug line.
+func (t discard) Debugln(v ...interface{}) {}