From 2c14970acbbd963c66c5b49ddf3c9c3bb02e18c6 Mon Sep 17 00:00:00 2001
From: Ecattea <147585044+Ecattea@users.noreply.github.com>
Date: Wed, 25 Feb 2026 22:03:27 +0800
Subject: [PATCH] fix(metric): avoid metrics mux conflict across instances

Use per-instance ServeMux in Run and add regression test for multi-instance panic.
---
 metric/metric.go          |  7 ++++---
 metric/metric_run_test.go | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+), 3 deletions(-)
 create mode 100644 metric/metric_run_test.go

diff --git a/metric/metric.go b/metric/metric.go
index cc7980495..d868f5524 100644
--- a/metric/metric.go
+++ b/metric/metric.go
@@ -103,10 +103,11 @@ func (m *metric) Run(ctx context.Context) (err error) {
 		return
 	}
 	if m.config.Addr != "" {
-		var errCh = make(chan error)
-		http.Handle("/metrics", promhttp.HandlerFor(m.registry, promhttp.HandlerOpts{}))
+		var errCh = make(chan error, 1)
+		mux := http.NewServeMux()
+		mux.Handle("/metrics", promhttp.HandlerFor(m.registry, promhttp.HandlerOpts{}))
 		go func() {
-			errCh <- http.ListenAndServe(m.config.Addr, nil)
+			errCh <- http.ListenAndServe(m.config.Addr, mux)
 		}()
 		select {
 		case err = <-errCh:
diff --git a/metric/metric_run_test.go b/metric/metric_run_test.go
new file mode 100644
index 000000000..5fe24e7f3
--- /dev/null
+++ b/metric/metric_run_test.go
@@ -0,0 +1,37 @@
+package metric
+
+import (
+	"context"
+	"testing"
+
+	"github.com/anyproto/any-sync/app"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+func TestRun_DoesNotPanicWithMultipleInstances(t *testing.T) {
+	defer func() {
+		if r := recover(); r != nil {
+			t.Fatalf("metric.Run should not panic with multiple metric instances: %v", r)
+		}
+	}()
+
+	first := newTestMetric()
+	if err := first.Run(context.Background()); err == nil {
+		t.Fatal("first metric run should return listen error with invalid test address")
+	}
+
+	second := newTestMetric()
+	if err := second.Run(context.Background()); err == nil {
+		t.Fatal("second metric run should return listen error with invalid test address")
+	}
+}
+
+func newTestMetric() *metric {
+	return &metric{
+		registry: prometheus.NewRegistry(),
+		// Intentionally invalid address to avoid creating long-lived listeners in unit tests.
+		config:      Config{Addr: "127.0.0.1"},
+		a:           &app.App{},
+		syncMetrics: map[string]SyncMetric{},
+	}
+}