test(engine): add regression tests for HandleContext with NoRoute (#4571)

Add two regression tests for GitHub issue #1848 to verify that
Context.handlers are properly isolated in HandleContext when used
from a NoRoute handler with group and engine middleware.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Bo-Yi Wu <appleboy.tw@gmail.com>

gin_test.go

@@ -743,6 +743,78 @@ func TestEngineHandleContextPreventsMiddlewareReEntry(t *testing.T) {
 	assert.Equal(t, int64(1), handlerCounterV2)
 }
 
+func TestEngineHandleContextNoRouteWithGroupMiddleware(t *testing.T) {
+	// Scenario from issue #1848:
+	// - Engine with no global middleware (gin.New())
+	// - A group with middleware
+	// - A route in that group
+	// - NoRoute handler that redirects via HandleContext
+	// The group middleware should run exactly once per HandleContext call,
+	// not accumulate across redirects.
+
+	var middlewareCount, handlerCount int64
+
+	r := New()
+	grp := r.Group("", func(c *Context) {
+		atomic.AddInt64(&middlewareCount, 1)
+		c.Next()
+	})
+	grp.GET("/target", func(c *Context) {
+		atomic.AddInt64(&handlerCount, 1)
+		c.String(http.StatusOK, "ok")
+	})
+
+	r.NoRoute(func(c *Context) {
+		c.Request.URL.Path = "/target"
+		r.HandleContext(c)
+	})
+
+	// Access a non-existent route to trigger NoRoute -> HandleContext
+	w := PerformRequest(r, "GET", "/nonexistent")
+	assert.Equal(t, http.StatusOK, w.Code)
+	assert.Equal(t, "ok", w.Body.String())
+	// Middleware and handler should each run exactly once
+	assert.Equal(t, int64(1), atomic.LoadInt64(&middlewareCount))
+	assert.Equal(t, int64(1), atomic.LoadInt64(&handlerCount))
+}
+
+func TestEngineHandleContextNoRouteWithEngineMiddleware(t *testing.T) {
+	// When engine middleware exists and NoRoute redirects via HandleContext,
+	// verify the handlers run the expected number of times.
+
+	var engineMwCount, groupMwCount, handlerCount int64
+
+	r := New()
+	r.Use(func(c *Context) {
+		atomic.AddInt64(&engineMwCount, 1)
+		c.Next()
+	})
+
+	grp := r.Group("", func(c *Context) {
+		atomic.AddInt64(&groupMwCount, 1)
+		c.Next()
+	})
+	grp.GET("/target", func(c *Context) {
+		atomic.AddInt64(&handlerCount, 1)
+		c.String(http.StatusOK, "ok")
+	})
+
+	r.NoRoute(func(c *Context) {
+		c.Request.URL.Path = "/target"
+		r.HandleContext(c)
+	})
+
+	w := PerformRequest(r, "GET", "/nonexistent")
+	assert.Equal(t, http.StatusOK, w.Code)
+	assert.Equal(t, "ok", w.Body.String())
+	// Handler and group middleware should each run once (from HandleContext)
+	assert.Equal(t, int64(1), atomic.LoadInt64(&handlerCount))
+	assert.Equal(t, int64(1), atomic.LoadInt64(&groupMwCount))
+	// Engine middleware runs twice: once for the NoRoute chain, once for the HandleContext chain
+	// This is expected behavior since HandleContext re-enters the full handler chain
+	assert.Equal(t, int64(2), atomic.LoadInt64(&engineMwCount))
+}
+
 func TestEngineHandleContextUseEscapedPathPercentEncoded(t *testing.T) {
 	r := New()
 	r.UseEscapedPath = true