This is a slight optimization not to cause useless disk access. This
only applies to saving session automatically (see
--save-session-interval). aria2.saveSession and serialization at the
end of the session are always performed as before.
When serialization, we first check that whether there is any change
since the last serialization. To do this, we first calculate hash
value of serialized content without writing into file. Then compare
this value to the value of last serialization. If they do not match,
perform serialization.
IOFile provides the same interface of old BufferedFile. It provides
extension points as pure virtual functions. Those functions are
implemented by 2 concrete subclasses: BufferedFile and GZipFile.
Tatsuhiro Tsujikawa