test(gin): resolve race conditions in integration tests (#4453)

- Implement TestRebuild404Handlers to verify 404 handler chain rebuilding
  when global middleware is added via Use()
- Add waitForServerReady helper with exponential backoff to replace
  unreliable time.Sleep() calls in integration tests
- Fix race conditions in TestRunEmpty, TestRunEmptyWithEnv, and
  TestRunWithPort by using proper server readiness checks
- All tests now pass consistently with -race flag

This addresses the empty test function and eliminates flaky test failures
caused by insufficient wait times for server startup.

Co-authored-by: Bo-Yi Wu <appleboy.tw@gmail.com>

gin_integration_test.go

@@ -70,9 +70,10 @@ func TestRunEmpty(t *testing.T) {
 		router.GET("/example", func(c *Context) { c.String(http.StatusOK, "it worked") })
 		assert.NoError(t, router.Run())
 	}()
-	// have to wait for the goroutine to start and run the server
-	// otherwise the main thread will complete
-	time.Sleep(5 * time.Millisecond)
+
+	// Wait for server to be ready with exponential backoff
+	err := waitForServerReady("http://localhost:8080/example", 10)
+	require.NoError(t, err, "server should start successfully")
 
 	require.Error(t, router.Run(":8080"))
 	testRequest(t, "http://localhost:8080/example")
@@ -213,9 +214,10 @@ func TestRunEmptyWithEnv(t *testing.T) {
 		router.GET("/example", func(c *Context) { c.String(http.StatusOK, "it worked") })
 		assert.NoError(t, router.Run())
 	}()
-	// have to wait for the goroutine to start and run the server
-	// otherwise the main thread will complete
-	time.Sleep(5 * time.Millisecond)
+
+	// Wait for server to be ready with exponential backoff
+	err := waitForServerReady("http://localhost:3123/example", 10)
+	require.NoError(t, err, "server should start successfully")
 
 	require.Error(t, router.Run(":3123"))
 	testRequest(t, "http://localhost:3123/example")
@@ -234,9 +236,10 @@ func TestRunWithPort(t *testing.T) {
 		router.GET("/example", func(c *Context) { c.String(http.StatusOK, "it worked") })
 		assert.NoError(t, router.Run(":5150"))
 	}()
-	// have to wait for the goroutine to start and run the server
-	// otherwise the main thread will complete
-	time.Sleep(5 * time.Millisecond)
+
+	// Wait for server to be ready with exponential backoff
+	err := waitForServerReady("http://localhost:5150/example", 10)
+	require.NoError(t, err, "server should start successfully")
 
 	require.Error(t, router.Run(":5150"))
 	testRequest(t, "http://localhost:5150/example")

gin_test.go

@@ -545,6 +545,29 @@ func TestNoMethodWithoutGlobalHandlers(t *testing.T) {
 }
 
 func TestRebuild404Handlers(t *testing.T) {
+	var middleware0 HandlerFunc = func(c *Context) {}
+	var middleware1 HandlerFunc = func(c *Context) {}
+
+	router := New()
+
+	// Initially, allNoRoute should be nil
+	assert.Nil(t, router.allNoRoute)
+
+	// Set NoRoute handlers
+	router.NoRoute(middleware0)
+	assert.Len(t, router.allNoRoute, 1)
+	assert.Len(t, router.noRoute, 1)
+	compareFunc(t, router.allNoRoute[0], middleware0)
+
+	// Add Use middleware should trigger rebuild404Handlers
+	router.Use(middleware1)
+	assert.Len(t, router.allNoRoute, 2)
+	assert.Len(t, router.Handlers, 1)
+	assert.Len(t, router.noRoute, 1)
+
+	// Global middleware should come first
+	compareFunc(t, router.allNoRoute[0], middleware1)
+	compareFunc(t, router.allNoRoute[1], middleware0)
 }
 
 func TestNoMethodWithGlobalHandlers(t *testing.T) {

test_helpers.go

@@ -4,7 +4,11 @@
 
 package gin
 
-import "net/http"
+import (
+	"fmt"
+	"net/http"
+	"time"
+)
 
 // CreateTestContext returns a fresh Engine and a Context associated with it.
 // This is useful for tests that need to set up a new Gin engine instance
@@ -29,3 +33,28 @@ func CreateTestContextOnly(w http.ResponseWriter, r *Engine) (c *Context) {
 	c.writermem.reset(w)
 	return
 }
+
+// waitForServerReady waits for a server to be ready by making HTTP requests
+// with exponential backoff. This is more reliable than time.Sleep() for testing.
+func waitForServerReady(url string, maxAttempts int) error {
+	client := &http.Client{
+		Timeout: 100 * time.Millisecond,
+	}
+
+	for i := 0; i < maxAttempts; i++ {
+		resp, err := client.Get(url)
+		if err == nil {
+			resp.Body.Close()
+			return nil
+		}
+
+		// Exponential backoff: 10ms, 20ms, 40ms, 80ms, 160ms...
+		backoff := time.Duration(10*(1<<uint(i))) * time.Millisecond
+		if backoff > 500*time.Millisecond {
+			backoff = 500 * time.Millisecond
+		}
+		time.Sleep(backoff)
+	}
+
+	return fmt.Errorf("server at %s did not become ready after %d attempts", url, maxAttempts)
+}