feat(logger): ability to skip logs based on user-defined logic (#3593)

* log skipper

* do not call time.now() if logging should be skipped

* do not ignore skip func delay in latency calculation

* write docs

* write test

docs/doc.md

@@ -508,6 +508,44 @@ Sample Output
 ::1 - [Fri, 07 Dec 2018 17:04:38 JST] "GET /ping HTTP/1.1 200 122.767µs "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.80 Safari/537.36" "
 ```
 
+### Skip logging
+
+```go
+func main() {
+  router := gin.New()
+  
+  // skip logging for desired paths by setting SkipPaths in LoggerConfig
+  loggerConfig := gin.LoggerConfig{SkipPaths: []string{"/metrics"}}
+  
+  // skip logging based on your logic by setting Skip func in LoggerConfig
+  loggerConfig.Skip = func(c *gin.Context) bool {
+      // as an example skip non server side errors
+      return c.Writer.Status() < http.StatusInternalServerError
+  }
+  
+  engine.Use(gin.LoggerWithConfig(loggerConfig))
+  router.Use(gin.Recovery())
+  
+  // skipped
+  router.GET("/metrics", func(c *gin.Context) {
+      c.Status(http.StatusNotImplemented)
+  })
+
+  // skipped
+  router.GET("/ping", func(c *gin.Context) {
+      c.String(http.StatusOK, "pong")
+  })
+
+  // not skipped
+  router.GET("/data", func(c *gin.Context) {
+    c.Status(http.StatusNotImplemented)
+  })
+  
+  router.Run(":8080")
+}
+
+```
+
 ### Controlling Log output coloring
 
 By default, logs output on console should be colorized depending on the detected TTY.

logger.go

@@ -47,8 +47,15 @@ type LoggerConfig struct {
 	// SkipPaths is an url path array which logs are not written.
 	// Optional.
 	SkipPaths []string
+
+	// Skip is a Skipper that indicates which logs should not be written.
+	// Optional.
+	Skip Skipper
 }
 
+// Skipper is a function to skip logs based on provided Context
+type Skipper func(c *Context) bool
+
 // LogFormatter gives the signature of the formatter function passed to LoggerWithFormatter
 type LogFormatter func(params LogFormatterParams) string
 
@@ -241,8 +248,11 @@ func LoggerWithConfig(conf LoggerConfig) HandlerFunc {
 		// Process request
 		c.Next()
 
-		// Log only when path is not being skipped
-		if _, ok := skip[path]; !ok {
+		// Log only when it is not being skipped
+		if _, ok := skip[path]; ok || (conf.Skip != nil && conf.Skip(c)) {
+			return
+		}
+
 		param := LogFormatterParams{
 			Request: c.Request,
 			isTerm:  isTerm,
@@ -268,5 +278,4 @@ func LoggerWithConfig(conf LoggerConfig) HandlerFunc {
 
 		fmt.Fprint(out, formatter(param))
 	}
-	}
 }

logger_test.go

@@ -415,6 +415,26 @@ func TestLoggerWithConfigSkippingPaths(t *testing.T) {
 	assert.Contains(t, buffer.String(), "")
 }
 
+func TestLoggerWithConfigSkipper(t *testing.T) {
+	buffer := new(strings.Builder)
+	router := New()
+	router.Use(LoggerWithConfig(LoggerConfig{
+		Output: buffer,
+		Skip: func(c *Context) bool {
+			return c.Writer.Status() == http.StatusNoContent
+		},
+	}))
+	router.GET("/logged", func(c *Context) { c.Status(http.StatusOK) })
+	router.GET("/skipped", func(c *Context) { c.Status(http.StatusNoContent) })
+
+	PerformRequest(router, "GET", "/logged")
+	assert.Contains(t, buffer.String(), "200")
+
+	buffer.Reset()
+	PerformRequest(router, "GET", "/skipped")
+	assert.Contains(t, buffer.String(), "")
+}
+
 func TestDisableConsoleColor(t *testing.T) {
 	New()
 	assert.Equal(t, autoColor, consoleColorMode)