fix: concurrent-safe route registration with mutex synchronization

- Add sync.RWMutex to Engine struct for thread-safe access
- Protect addRoute() with Lock() for exclusive write access
- Protect Routes() with RLock() for concurrent read access
- Add concurrent test to verify no data race
- Add tests for serveError function coverage

Fixes #4457

gin.go

@@ -96,6 +96,9 @@ type Engine struct {
 	// (used for routing HTTP requests) happens only once, even if called multiple times concurrently.
 	routeTreesUpdated sync.Once
 
+	// mu protects the route trees during concurrent access.
+	mu sync.RWMutex
+
 	// RedirectTrailingSlash enables automatic redirection if the current route can't be matched but a
 	// handler for the path with (without) the trailing slash exists.
 	// For example if /foo/ is requested but a route only exists for /foo, the
@@ -368,6 +371,9 @@ func (engine *Engine) addRoute(method, path string, handlers HandlersChain) {
 
 	debugPrintRoute(method, path, handlers)
 
+	engine.mu.Lock()
+	defer engine.mu.Unlock()
+
 	root := engine.trees.get(method)
 	if root == nil {
 		root = new(node)
@@ -388,6 +394,8 @@ func (engine *Engine) addRoute(method, path string, handlers HandlersChain) {
 // Routes returns a slice of registered routes, including some useful information, such as:
 // the http method, path, and the handler name.
 func (engine *Engine) Routes() (routes RoutesInfo) {
+	engine.mu.RLock()
+	defer engine.mu.RUnlock()
 	for _, tree := range engine.trees {
 		routes = iterate("", tree.method, routes, tree.root)
 	}

gin_test.go

@@ -15,6 +15,7 @@ import (
 	"reflect"
 	"strconv"
 	"strings"
+	"sync"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -1084,3 +1085,56 @@ func TestUpdateRouteTreesCalledOnce(t *testing.T) {
 		assert.Equal(t, "ok", w.Body.String())
 	}
 }
+
+func TestConcurrentAddRouteAndRoutes(t *testing.T) {
+	router := New()
+	var wg sync.WaitGroup
+	for i := 0; i < 100; i++ {
+		wg.Add(2)
+		go func(i int) {
+			defer wg.Done()
+			router.GET(fmt.Sprintf("/route%d", i), func(c *Context) {
+				c.String(http.StatusOK, "ok")
+			})
+		}(i)
+		go func(i int) {
+			defer wg.Done()
+			_ = router.Routes()
+		}(i)
+	}
+	wg.Wait()
+	assert.Len(t, router.Routes(), 100)
+}
+
+func TestServeErrorWritten(t *testing.T) {
+	router := New()
+	router.GET("/", func(c *Context) {
+		c.Status(http.StatusInternalServerError)
+		_, _ = c.Writer.Write([]byte("custom error"))
+	})
+	router.NoRoute(func(c *Context) {
+		c.Status(http.StatusNotFound)
+	})
+
+	w := httptest.NewRecorder()
+	req, _ := http.NewRequest(http.MethodGet, "/notfound", nil)
+	router.ServeHTTP(w, req)
+	assert.Equal(t, http.StatusNotFound, w.Code)
+}
+
+func TestServeErrorStatusMismatch(t *testing.T) {
+	router := New()
+	router.HandleMethodNotAllowed = true
+	router.NoMethod(func(c *Context) {
+		c.Status(http.StatusForbidden)
+	})
+
+	router.GET("/exists", func(c *Context) {
+		c.String(http.StatusOK, "ok")
+	})
+
+	w := httptest.NewRecorder()
+	req, _ := http.NewRequest(http.MethodPost, "/exists", nil)
+	router.ServeHTTP(w, req)
+	assert.Equal(t, http.StatusForbidden, w.Code)
+}