fix(leak): Avoid mutating logger (#97)

* Avoid mutating logger

* Change test to cover this scenario

* better comment

* lint

logger.go

@@ -137,14 +137,16 @@ func SetLogger(opts ...Option) gin.HandlerFunc {
 			}
 		}
 
+		// Use a separate logger to save to the context, as we want to avoid mutating the original logger.
+		contextLogger := rl
 		if track {
-			l = l.With().
+			contextLogger = rl.With().
 				Str("method", c.Request.Method).
 				Str("path", path).
 				Str("ip", c.ClientIP()).
 				Str("user_agent", c.Request.UserAgent()).Logger()
 		}
-		c.Set(loggerKey, l)
+		c.Set(loggerKey, contextLogger)
 
 		c.Next()
 

logger_test.go

@@ -180,9 +180,10 @@ func TestCustomLoggerIssue68(t *testing.T) {
 	buffer := new(concurrentBuffer)
 	gin.SetMode(gin.ReleaseMode)
 	r := gin.New()
-	l := zerolog.New(buffer)
+	// Use JSON logger as it will explicitly print keys multiple times if they are added multiple times,
+	// which may happen if there are mutations to the logger.
 	r.Use(SetLogger(
-		WithLogger(func(*gin.Context, zerolog.Logger) zerolog.Logger { return l }),
+		WithLogger(func(_ *gin.Context, l zerolog.Logger) zerolog.Logger { return l.Output(buffer).With().Logger() }),
 		WithDefaultLevel(zerolog.DebugLevel),
 		WithClientErrorLevel(zerolog.ErrorLevel),
 		WithServerErrorLevel(zerolog.FatalLevel),