Fix scanner deadlock on lost global lock (#16726)

cmd/erasure-server-pool.go

@@ -689,9 +689,11 @@ func (z *erasureServerPools) NSScanner(ctx context.Context, updates chan<- DataU
 	case updateCloser <- ch:
 		<-ch
 	case <-ctx.Done():
+		mu.Lock()
 		if firstErr == nil {
 			firstErr = ctx.Err()
 		}
+		defer mu.Unlock()
 	}
 	return firstErr
 }

cmd/erasure.go

@@ -412,9 +412,6 @@ func (er erasureObjects) nsScanner(ctx context.Context, buckets []BucketInfo, wa
 
 		for {
 			select {
-			case <-ctx.Done():
-				// Return without saving.
-				return
 			case <-t.C:
 				if cache.Info.LastUpdate.Equal(lastSave) {
 					continue
@@ -488,10 +485,13 @@ func (er erasureObjects) nsScanner(ctx context.Context, buckets []BucketInfo, wa
 				go func(name string) {
 					defer wg.Done()
 					for update := range updates {
-						bucketResults <- dataUsageEntryInfo{
+						select {
+						case <-ctx.Done():
+						case bucketResults <- dataUsageEntryInfo{
 							Name:   name,
 							Parent: dataUsageRoot,
 							Entry:  update,
+						}:
 						}
 					}
 				}(cache.Info.Name)
@@ -519,10 +519,14 @@ func (er erasureObjects) nsScanner(ctx context.Context, buckets []BucketInfo, wa
 					root = cache.flatten(*r)
 				}
 				t := time.Now()
-				bucketResults <- dataUsageEntryInfo{
+				select {
+				case <-ctx.Done():
+					return
+				case bucketResults <- dataUsageEntryInfo{
 					Name:   cache.Info.Name,
 					Parent: dataUsageRoot,
 					Entry:  root,
+				}:
 				}
 				// We want to avoid synchronizing up all writes in case
 				// the results are piled up.