Support customer logger (#330)

When martian is used as package, it needs to work along with the host's
logging infrastructure. By default, it retains current logging, provides an
option to change it if needed.

log/log.go

@@ -34,10 +34,24 @@ const (
 
 // Default log level is Error.
 var (
-	level = Error
-	lock  sync.Mutex
+	level      = Error
+	lock       sync.Mutex
+	currLogger Logger = &logger{}
 )
 
+type Logger interface {
+	Infof(format string, args ...interface{})
+	Debugf(format string, args ...interface{})
+	Errorf(format string, args ...interface{})
+}
+
+// SetLogger changes the default logger. This must be called very first,
+// before interacting with rest of the martian package. Changing it at
+// runtime is not supported.
+func SetLogger(l Logger) {
+	currLogger = l
+}
+
 // SetLevel sets the global log level.
 func SetLevel(l int) {
 	lock.Lock()
@@ -48,6 +62,22 @@ func SetLevel(l int) {
 
 // Infof logs an info message.
 func Infof(format string, args ...interface{}) {
+	currLogger.Infof(format, args...)
+}
+
+// Debugf logs a debug message.
+func Debugf(format string, args ...interface{}) {
+	currLogger.Debugf(format, args...)
+}
+
+// Errorf logs an error message.
+func Errorf(format string, args ...interface{}) {
+	currLogger.Errorf(format, args...)
+}
+
+type logger struct{}
+
+func (l *logger) Infof(format string, args ...interface{}) {
 	lock.Lock()
 	defer lock.Unlock()
 
@@ -63,8 +93,7 @@ func Infof(format string, args ...interface{}) {
 	log.Println(msg)
 }
 
-// Debugf logs a debug message.
-func Debugf(format string, args ...interface{}) {
+func (l *logger) Debugf(format string, args ...interface{}) {
 	lock.Lock()
 	defer lock.Unlock()
 
@@ -80,8 +109,7 @@ func Debugf(format string, args ...interface{}) {
 	log.Println(msg)
 }
 
-// Errorf logs an error message.
-func Errorf(format string, args ...interface{}) {
+func (l *logger) Errorf(format string, args ...interface{}) {
 	lock.Lock()
 	defer lock.Unlock()