storage: swap the volume data backend under the data lock

The tier-download swap closed v.DataBackend and assigned the new local DiskFile
without holding dataFileAccessLock, racing concurrent reads/writes (use of a
closed file / nil deref). Add an exported Volume.SwapDataBackend that performs
the close-and-replace under the lock, and call it from the tier download.

weed/server/volume_grpc_tier_download.go

@@ -115,10 +115,9 @@ func swapToLocalDatBackend(v *storage.Volume, datFileName string) error {
 	if err != nil {
 		return err
 	}
-	if v.DataBackend != nil {
-		v.DataBackend.Close()
-	}
-	v.DataBackend = backend.NewDiskFile(dataFile)
+	// Swap under the volume's data lock so concurrent reads never see a closed
+	// or half-swapped backend.
+	v.SwapDataBackend(backend.NewDiskFile(dataFile))
 	return nil
 }
 

weed/storage/volume.go

@@ -310,6 +310,19 @@ func (v *Volume) Close() {
 	v.doClose()
 }
 
+// SwapDataBackend atomically replaces the data backend (e.g. swapping a
+// remote-tier backend for a freshly downloaded local .dat), closing the old
+// one. Held under dataFileAccessLock so a concurrent read/write never observes
+// a half-swapped or closed backend.
+func (v *Volume) SwapDataBackend(newBackend backend.BackendStorageFile) {
+	v.dataFileAccessLock.Lock()
+	defer v.dataFileAccessLock.Unlock()
+	if v.DataBackend != nil {
+		v.DataBackend.Close()
+	}
+	v.DataBackend = newBackend
+}
+
 func (v *Volume) doClose() {
 	if v.nm != nil {
 		if err := v.nm.Sync(); err != nil {