Function can_relocate 

pub(crate) fn can_relocate(config_dir: &str, cwd: &str, sid: &str) -> bool

Whether session sid for cwd under config_dir can be copied in by relocate: it exists (source_file) and holds a complete record. Its last record may still be being written, by a parent a fork starts from mid-turn: relocate cuts the copy back to its last complete one.