From bd725fd6b8a27aef969ed97c28ced358f2f3bc99 Mon Sep 17 00:00:00 2001
From: machine424 <ayoubmrini424@gmail.com>
Date: Wed, 20 Aug 2025 11:53:24 +0200
Subject: [PATCH 1/2] test(notifier): add a test showing an alert mutation bug
 between alertmanager_config (alertmanagersets)

The alert_relabel_configs should only apply to the corresponding alertmanagerset

Signed-off-by: machine424 <ayoubmrini424@gmail.com>
---
 notifier/manager_test.go | 85 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 85 insertions(+)

diff --git a/notifier/manager_test.go b/notifier/manager_test.go
index 6f8f687f9a..09ca49e5c7 100644
--- a/notifier/manager_test.go
+++ b/notifier/manager_test.go
@@ -1132,3 +1132,88 @@ alerting:
 	require.NoError(t, n.ApplyConfig(cfg))
 	require.Empty(t, n.Alertmanagers())
 }
+
+// TestAlerstRelabelingIsIsolated ensures that a mutation alerts relabeling in an
+// alertmanagerSet doesn't affect others.
+// See https://github.com/prometheus/prometheus/pull/17063.
+func TestAlerstRelabelingIsIsolated(t *testing.T) {
+	var (
+		errc      = make(chan error, 1)
+		expected1 = make([]*Alert, 0)
+		expected2 = make([]*Alert, 0)
+
+		status1, status2 atomic.Int32
+	)
+	status1.Store(int32(http.StatusOK))
+	status2.Store(int32(http.StatusOK))
+
+	server1 := newTestHTTPServerBuilder(&expected1, errc, "", "", &status1)
+	server2 := newTestHTTPServerBuilder(&expected2, errc, "", "", &status2)
+
+	defer server1.Close()
+	defer server2.Close()
+
+	h := NewManager(&Options{}, model.UTF8Validation, nil)
+	h.alertmanagers = make(map[string]*alertmanagerSet)
+
+	am1Cfg := config.DefaultAlertmanagerConfig
+	am1Cfg.Timeout = model.Duration(time.Second)
+	am1Cfg.AlertRelabelConfigs = []*relabel.Config{
+		{
+			SourceLabels:         model.LabelNames{"alertname"},
+			Regex:                relabel.MustNewRegexp("(.*)"),
+			TargetLabel:          "parasite",
+			Action:               relabel.Replace,
+			Replacement:          "yes",
+			NameValidationScheme: model.UTF8Validation,
+		},
+	}
+
+	am2Cfg := config.DefaultAlertmanagerConfig
+	am2Cfg.Timeout = model.Duration(time.Second)
+
+	h.alertmanagers = map[string]*alertmanagerSet{
+		"am1": {
+			ams: []alertmanager{
+				alertmanagerMock{
+					urlf: func() string { return server1.URL },
+				},
+			},
+			cfg: &am1Cfg,
+		},
+		"am2": {
+			ams: []alertmanager{
+				alertmanagerMock{
+					urlf: func() string { return server2.URL },
+				},
+			},
+			cfg: &am2Cfg,
+		},
+	}
+
+	testAlert := &Alert{
+		Labels: labels.FromStrings("alertname", "test"),
+	}
+	h.queue = []*Alert{testAlert}
+
+	expected1 = append(expected1, &Alert{
+		Labels: labels.FromStrings("alertname", "test", "parasite", "yes"),
+	})
+
+	// am2 shouldn't get the parasite label.
+	expected2 = append(expected2, &Alert{
+		Labels: labels.FromStrings("alertname", "test"),
+	})
+
+	checkNoErr := func() {
+		t.Helper()
+		select {
+		case err := <-errc:
+			require.NoError(t, err)
+		default:
+		}
+	}
+
+	require.True(t, h.sendAll(h.queue...))
+	checkNoErr()
+}

From 8f79470ca91f49f1492e8a92527251d461cc65ae Mon Sep 17 00:00:00 2001
From: machine424 <ayoubmrini424@gmail.com>
Date: Mon, 25 Aug 2025 16:32:34 +0200
Subject: [PATCH 2/2] fix(notifier): create a new alert when relabeling alters
 labels

Signed-off-by: machine424 <ayoubmrini424@gmail.com>
---
 notifier/alert.go | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/notifier/alert.go b/notifier/alert.go
index 88245c9a7f..83e7a97fe0 100644
--- a/notifier/alert.go
+++ b/notifier/alert.go
@@ -84,7 +84,18 @@ func relabelAlerts(relabelConfigs []*relabel.Config, externalLabels labels.Label
 		if !keep {
 			continue
 		}
-		a.Labels = lb.Labels()
+
+		// If relabeling has altered the labels, create a new Alert to preserve immutability.
+		if !labels.Equal(a.Labels, lb.Labels()) {
+			a = &Alert{
+				Labels:       lb.Labels(),
+				Annotations:  a.Annotations,
+				StartsAt:     a.StartsAt,
+				EndsAt:       a.EndsAt,
+				GeneratorURL: a.GeneratorURL,
+			}
+		}
+
 		relabeledAlerts = append(relabeledAlerts, a)
 	}
 	return relabeledAlerts