Fix concurrent data race problem

backend.go

@@ -4,8 +4,17 @@
 
 package logging
 
+import (
+	"sync"
+)
+
 // defaultBackend is the backend used for all logging calls.
 var defaultBackend LeveledBackend
+var defaultBackendMutex sync.Mutex
+
+func init() {
+	defaultBackendMutex = sync.Mutex{}
+}
 
 // Backend is the interface which a log backend need to implement to be able to
 // be used as a logging backend.
@@ -23,7 +32,9 @@ func SetBackend(backends ...Backend) LeveledBackend {
 		backend = MultiLogger(backends...)
 	}
 
+	defaultBackendMutex.Lock()
 	defaultBackend = AddModuleLevel(backend)
+	defaultBackendMutex.Unlock()
 	return defaultBackend
 }
 

level.go

@@ -70,6 +70,7 @@ type moduleLeveled struct {
 	backend   Backend
 	formatter Formatter
 	once      sync.Once
+	mutex     sync.Mutex
 }
 
 // AddModuleLevel wraps a log backend with knobs to have different log levels
@@ -81,13 +82,15 @@ func AddModuleLevel(backend Backend) LeveledBackend {
 		leveled = &moduleLeveled{
 			levels:  make(map[string]Level),
 			backend: backend,
+			mutex:   sync.Mutex{},
 		}
 	}
 	return leveled
 }
 
 // GetLevel returns the log level for the given module.
 func (l *moduleLeveled) GetLevel(module string) Level {
+	l.mutex.Lock()
 	level, exists := l.levels[module]
 	if exists == false {
 		level, exists = l.levels[""]
@@ -96,12 +99,15 @@ func (l *moduleLeveled) GetLevel(module string) Level {
 			level = DEBUG
 		}
 	}
+	l.mutex.Unlock()
 	return level
 }
 
 // SetLevel sets the log level for the given module.
 func (l *moduleLeveled) SetLevel(level Level, module string) {
+	l.mutex.Lock()
 	l.levels[module] = level
+	l.mutex.Unlock()
 }
 
 // IsEnabledFor will return true if logging is enabled for the given module.

logger.go

@@ -159,7 +159,9 @@ func (l *Logger) log(lvl Level, format string, args ...interface{}) {
 		return
 	}
 
+	defaultBackendMutex.Lock()
 	defaultBackend.Log(lvl, 2+l.ExtraCalldepth, record)
+	defaultBackendMutex.Unlock()
 }
 
 // Fatal is equivalent to l.Critical(fmt.Sprint()) followed by a call to os.Exit(1).