fix: keep scanner response resets local and strengthen regression tests

Keep the legacy header resets and clear the unused NSScanner response
field at each construction site. Remove the generic stream Recycler call:
Recycler transfers resource ownership and cannot be treated as a reset
hook followed by another insertion into the handler's pool.

Exercise the actual NSScanner handler across repeated scans with dirty
response allocations, so the regression test does not depend on sync.Pool
reusing an object. Check both response variants after wire serialization.
Cover legacy DecodeMsg and reused indexed metadata headers as well.

Retain the original contributor's commits and DCO sign-offs.

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

cmd/storage-rest-server_test.go

@@ -1,6 +1,6 @@
-// Copyright (c) 2015-2023 MinIO, Inc.
+// Copyright (c) 2026 Julien Laurenceau and Feng Ruohang
 //
-// This file is part of MinIO Object Storage stack
+// This file is part of Silo Object Storage.
 //
 // This program is free software: you can redistribute it and/or modify
 // it under the terms of the GNU Affero General Public License as published by
@@ -17,20 +17,78 @@
 
 package cmd
 
-import "testing"
+import (
+	"context"
+	"testing"
 
-// TestNSScannerRespPooling verifies that recycled NSScanner stream
-// responses cannot leak stale fields from a previous scan into new
-// responses, which would otherwise produce incorrect bucket usage
-// data.
-func TestNSScannerRespPooling(t *testing.T) {
-	dirty := storageNSScannerRPC.NewResponse()
-	dirty.Update = &dataUsageEntry{}
-	dirty.Final = &dataUsageCache{}
-	storageNSScannerRPC.PutResponse(dirty)
+	"github.com/minio/madmin-go/v3"
+	"github.com/minio/minio/internal/grid"
+)
 
-	resp := storageNSScannerRPC.NewResponse()
-	if resp.Update != nil || resp.Final != nil {
-		t.Fatalf("pooled nsScannerResp leaked stale fields: Update %v, Final %v", resp.Update, resp.Final)
+type scannerResponseTestDisk struct {
+	StorageAPI
+}
+
+func (scannerResponseTestDisk) NSScanner(_ context.Context, cache dataUsageCache, updates chan<- dataUsageEntry, _ madmin.HealScanMode, _ func() bool) (dataUsageCache, error) {
+	defer close(updates)
+	updates <- dataUsageEntry{Size: int64(cache.Info.NextCycle)}
+	cache.Info.NextCycle++
+	return cache, nil
+}
+
+func TestNSScannerHandlerClearsStaleResponses(t *testing.T) {
+	endpoint, err := NewEndpoint(t.TempDir())
+	if err != nil {
+		t.Fatal(err)
+	}
+	globalLocalDrivesMu.Lock()
+	oldDrives, oldSets := globalLocalDrivesMap, globalLocalSetDrives
+	globalLocalDrivesMap = map[string]StorageAPI{endpoint.String(): scannerResponseTestDisk{}}
+	globalLocalSetDrives = nil
+	globalLocalDrivesMu.Unlock()
+	t.Cleanup(func() {
+		globalLocalDrivesMu.Lock()
+		globalLocalDrivesMap, globalLocalSetDrives = oldDrives, oldSets
+		globalLocalDrivesMu.Unlock()
+	})
+
+	oldRPC := storageNSScannerRPC
+	t.Cleanup(func() { storageNSScannerRPC = oldRPC })
+	// Every new allocation is dirty too: the check must not depend on
+	// sync.Pool returning a previously used object.
+	storageNSScannerRPC = grid.NewStream[*nsScannerOptions, grid.NoPayload, *nsScannerResp](grid.HandlerNSScanner,
+		func() *nsScannerOptions { return &nsScannerOptions{} }, nil,
+		func() *nsScannerResp {
+			return &nsScannerResp{Update: &dataUsageEntry{Size: -1}, Final: &dataUsageCache{}}
+		})
+	server := storageRESTServer{endpoint: endpoint}
+	for cycle := uint32(1); cycle <= 3; cycle++ {
+		out := make(chan *nsScannerResp, 2)
+		params := &nsScannerOptions{Cache: &dataUsageCache{Info: dataUsageCacheInfo{Name: "bucket", NextCycle: cycle}}}
+		if err := server.NSScannerHandler(t.Context(), params, out); err != nil {
+			t.Fatal(err)
+		}
+		if len(out) != 2 {
+			t.Fatalf("cycle %d: got %d responses, want an update and a final cache", cycle, len(out))
+		}
+		for i := 0; i < 2; i++ {
+			resp := <-out
+			wire, err := resp.MarshalMsg(nil)
+			storageNSScannerRPC.PutResponse(resp)
+			if err != nil {
+				t.Fatal(err)
+			}
+			var decoded nsScannerResp
+			if _, err := decoded.UnmarshalMsg(wire); err != nil {
+				t.Fatal(err)
+			}
+			if i == 0 {
+				if decoded.Update == nil || decoded.Final != nil || decoded.Update.Size != int64(cycle) {
+					t.Errorf("cycle %d: invalid update response: %+v", cycle, decoded)
+				}
+			} else if decoded.Update != nil || decoded.Final == nil || decoded.Final.Info.NextCycle != cycle+1 {
+				t.Errorf("cycle %d: invalid final response: %+v", cycle, decoded)
+			}
+		}
 	}
 }