fix: skip log files with deleted volumes in filer backup (#7692)

fix: skip log files with deleted volumes in filer backup (#3720)

When filer.backup or filer.meta.backup resumes after being stopped, it may
encounter persisted log files stored on volumes that have since been deleted
(via volume.deleteEmpty -force). Previously, this caused the backup to get
stuck in an infinite retry loop with 'volume X not found' errors.

This fix catches 'volume not found' errors when reading log files and skips
the problematic file instead of failing. The backup will now:
- Log a warning about the missing volume
- Skip the problematic log file
- Continue with the next log file, allowing progress

The VolumeNotFoundPattern regex was already defined but never used - this
change puts it to use.

Fixes #3720

weed/filer/filer_notify.go

@@ -152,9 +152,21 @@ func (f *Filer) logFlushFunc(logBuffer *log_buffer.LogBuffer, startTime, stopTim
 }
 
 var (
-	VolumeNotFoundPattern = regexp.MustCompile(`volume \d+? not found`)
+	volumeNotFoundPattern = regexp.MustCompile(`volume \d+? not found`)
+	chunkNotFoundPattern  = regexp.MustCompile(`(urls not found|File Not Found)`)
 )
 
+// isChunkNotFoundError checks if the error indicates that a volume or chunk
+// has been deleted and is no longer available. These errors can be skipped
+// when reading persisted log files since the data is unrecoverable.
+func isChunkNotFoundError(err error) bool {
+	if err == nil {
+		return false
+	}
+	errMsg := err.Error()
+	return volumeNotFoundPattern.MatchString(errMsg) || chunkNotFoundPattern.MatchString(errMsg)
+}
+
 func (f *Filer) ReadPersistedLogBuffer(startPosition log_buffer.MessagePosition, stopTsNs int64, eachLogEntryFn log_buffer.EachLogEntryFuncType) (lastTsNs int64, isDone bool, err error) {
 
 	visitor, visitErr := f.collectPersistedLogBuffer(startPosition, stopTsNs)