fix(health): reset failed count after successful check (#5502)

Signed-off-by: racequite <quiterace@gmail.com>

client/health/health.go

@@ -119,6 +119,7 @@ func (monitor *Monitor) checkWorker() {
 
 		if err == nil {
 			xl.Tracef("do one health check success")
+			monitor.failedTimes = 0
 			if !monitor.statusOK && monitor.statusNormalFn != nil {
 				xl.Infof("health check status change to success")
 				monitor.statusOK = true

client/health/health_test.go

@@ -0,0 +1,65 @@
+// Copyright 2026 The frp Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package health
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+
+	v1 "github.com/fatedier/frp/pkg/config/v1"
+)
+
+func TestMonitorResetsFailedTimesAfterSuccess(t *testing.T) {
+	var checkCount atomic.Int32
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		count := checkCount.Add(1)
+		if count == 1 || count == 2 || count == 4 {
+			w.WriteHeader(http.StatusServiceUnavailable)
+			return
+		}
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer server.Close()
+
+	var failedCount atomic.Int32
+	monitor := NewMonitor(
+		context.Background(),
+		v1.HealthCheckConfig{
+			Type:            "http",
+			Path:            "/health",
+			TimeoutSeconds:  1,
+			IntervalSeconds: 1,
+			MaxFailed:       3,
+		},
+		strings.TrimPrefix(server.URL, "http://"),
+		func() {},
+		func() { failedCount.Add(1) },
+	)
+	monitor.interval = 10 * time.Millisecond
+	monitor.Start()
+	defer monitor.Stop()
+
+	require.Eventually(t, func() bool {
+		return checkCount.Load() >= 5
+	}, time.Second, 10*time.Millisecond)
+	require.Equal(t, int32(0), failedCount.Load())
+}