fix(replication): handle absent pool in worker metrics

Signed-off-by: Feng Ruohang <rh@vonng.com>

CHANGELOG.md

@@ -17,6 +17,8 @@
   request validation and AWS FIPS endpoint signing fixes.
 - Bundle verified mcli 20261109 archives in source container builds and use
   that release by default in the integration-test client installer.
+- Prevent replication worker metrics from panicking when the replication
+  pool is absent, while retaining historical averages and peak counts.
 
 ## RELEASE.2026-10-26T00-00-00Z
 

cmd/bucket-replication-metrics.go

@@ -119,7 +119,10 @@ func (a *ActiveWorkerStat) update() {
 	if a == nil {
 		return
 	}
-	a.Curr = globalReplicationPool.Get().ActiveWorkers()
+	a.Curr = 0
+	if pool := globalReplicationPool.Get(); pool != nil {
+		a.Curr = pool.ActiveWorkers()
+	}
 	a.hist.Update(int64(a.Curr))
 	a.Avg = float32(a.hist.Mean())
 	a.Max = int(a.hist.Max())

cmd/bucket-replication-metrics_test.go

@@ -0,0 +1,32 @@
+// Copyright (c) 2026 PGSTY
+// SPDX-License-Identifier: AGPL-3.0-only
+
+package cmd
+
+import (
+	"testing"
+
+	"github.com/minio/minio/internal/once"
+	"github.com/rcrowley/go-metrics"
+)
+
+func TestReplicationWorkerMetricsWithoutPool(t *testing.T) {
+	previous := globalReplicationPool
+	t.Cleanup(func() { globalReplicationPool = previous })
+
+	globalReplicationPool = once.NewSingleton[ReplicationPool]()
+	globalReplicationPool.Set(&ReplicationPool{activeWorkers: 8})
+	stat := newActiveWorkerStat(metrics.NewRegistry())
+	stat.update()
+	if got := stat.get(); got.Curr != 8 || got.Avg != 8 || got.Max != 8 {
+		t.Fatalf("active pool metrics = %+v, want current/average/maximum 8", got)
+	}
+
+	// A collector may still run after replication has been disabled.
+	globalReplicationPool = once.NewSingleton[ReplicationPool]()
+	globalReplicationPool.Set(nil)
+	stat.update()
+	if got := stat.get(); got.Curr != 0 || got.Avg != 4 || got.Max != 8 {
+		t.Fatalf("disabled pool metrics = %+v, want current 0, average 4, maximum 8", got)
+	}
+}