set default logger level to error, and give controll method

The output of default log are noisy, set to error default.

Give an method to controll at program startup time. To set level to info,
set enviroment like this:
    export RPCX_LOG_LEVEL=4
To turn everything on,
    export RPCX_LOG_LEVEL=99
the value of levels are:
	- lvPicni = 0
	- lvFatal = 1
	- lvError = 2
	- lvWarn = 3
	- lvInfo = 4
	- lvDebug = 5

This trick works on my company, you may interested. If not, just ignore
and drop this pr.

log/default_logger.go

@@ -2,55 +2,110 @@ package log
 
 import (
 	"fmt"
+	"io"
 	"log"
 	"os"
+	"strconv"
 
 	"github.com/fatih/color"
 )
 
+// The default logger output at level error, to turn everything on like this:
+// 	export RPCX_LOG_LEVEL=99
+
+// type of log level
+type Level int
+
+// log level constant
+const (
+	LvPanic Level = iota
+	LvFatal
+	LvError
+	LvWarn
+	LvInfo
+	LvDebug
+
+	LvMax
+)
+
+type outputFn func(calldepth int, s string) error
+
+func dropOutput(calldepth int, s string) error {
+	return nil
+}
+
 type defaultLogger struct {
 	*log.Logger
+	out [LvMax]outputFn
+}
+
+func NewDefaultLogger(out io.Writer, prefix string, flag int, lv Level) *defaultLogger {
+	// set default level to error
+	level := lv
+
+	// read level from system enviroment to override
+	levelStr := os.Getenv("RPCX_LOG_LEVEL")
+	if len(levelStr) != 0 {
+		if vl, err := strconv.Atoi(levelStr); err == nil {
+			level = Level(vl)
+		}
+	}
+
+	l := &defaultLogger{}
+	l.Logger = log.New(out, prefix, flag)
+
+	for i := int(LvFatal); i < int(LvMax); i++ {
+		// always enable fatal and panic
+
+		// enable only lv <= RPCX_LOG_LEVEL
+		if i <= int(level) {
+			l.out[i] = l.Output
+		} else {
+			l.out[i] = dropOutput
+		}
+	}
+	return l
 }
 
 func (l *defaultLogger) Debug(v ...interface{}) {
-	_ = l.Output(calldepth, header("DEBUG", fmt.Sprint(v...)))
+	_ = l.out[int(LvDebug)](calldepth, header("DEBUG", fmt.Sprint(v...)))
 }
 
 func (l *defaultLogger) Debugf(format string, v ...interface{}) {
-	_ = l.Output(calldepth, header("DEBUG", fmt.Sprintf(format, v...)))
+	_ = l.out[int(LvDebug)](calldepth, header("DEBUG", fmt.Sprintf(format, v...)))
 }
 
 func (l *defaultLogger) Info(v ...interface{}) {
-	_ = l.Output(calldepth, header(color.GreenString("INFO "), fmt.Sprint(v...)))
+	_ = l.out[int(LvInfo)](calldepth, header(color.GreenString("INFO "), fmt.Sprint(v...)))
 }
 
 func (l *defaultLogger) Infof(format string, v ...interface{}) {
-	_ = l.Output(calldepth, header(color.GreenString("INFO "), fmt.Sprintf(format, v...)))
+	_ = l.out[int(LvInfo)](calldepth, header(color.GreenString("INFO "), fmt.Sprintf(format, v...)))
 }
 
 func (l *defaultLogger) Warn(v ...interface{}) {
-	_ = l.Output(calldepth, header(color.YellowString("WARN "), fmt.Sprint(v...)))
+	_ = l.out[int(LvWarn)](calldepth, header(color.YellowString("WARN "), fmt.Sprint(v...)))
 }
 
 func (l *defaultLogger) Warnf(format string, v ...interface{}) {
-	_ = l.Output(calldepth, header(color.YellowString("WARN "), fmt.Sprintf(format, v...)))
+	_ = l.out[int(LvWarn)](calldepth, header(color.YellowString("WARN "), fmt.Sprintf(format, v...)))
 }
 
 func (l *defaultLogger) Error(v ...interface{}) {
-	_ = l.Output(calldepth, header(color.RedString("ERROR"), fmt.Sprint(v...)))
+	_ = l.out[int(LvError)](calldepth, header(color.RedString("ERROR"), fmt.Sprint(v...)))
 }
 
 func (l *defaultLogger) Errorf(format string, v ...interface{}) {
-	_ = l.Output(calldepth, header(color.RedString("ERROR"), fmt.Sprintf(format, v...)))
+	_ = l.out[int(LvError)](calldepth, header(color.RedString("ERROR"), fmt.Sprintf(format, v...)))
 }
 
 func (l *defaultLogger) Fatal(v ...interface{}) {
-	_ = l.Output(calldepth, header(color.MagentaString("FATAL"), fmt.Sprint(v...)))
+	_ = l.Logger.Output(calldepth, header(color.MagentaString("FATAL"), fmt.Sprint(v...)))
 	os.Exit(1)
 }
 
 func (l *defaultLogger) Fatalf(format string, v ...interface{}) {
-	_ = l.Output(calldepth, header(color.MagentaString("FATAL"), fmt.Sprintf(format, v...)))
+	_ = l.Logger.Output(calldepth, header(color.MagentaString("FATAL"), fmt.Sprintf(format, v...)))
 	os.Exit(1)
 }
 

log/logger.go

@@ -9,7 +9,7 @@ const (
 	calldepth = 3
 )
 
-var l Logger = &defaultLogger{log.New(os.Stdout, "", log.LstdFlags|log.Lshortfile)}
+var l Logger = NewDefaultLogger(os.Stdout, "", log.LstdFlags|log.Lshortfile, LvError)
 
 type Logger interface {
 	Debug(v ...interface{})