As suggested by James in https://phabricator.services.mozilla.com/D131011#inline-760270 it would be good to have a helper that reads the sync header in the following way:

Not necessary for this change, but a helper like read_sync_header(stream: &mut TcpStream) -> IOResult<(SyncCommand, u32)> might be useful, since then you could just write

let (command, length) = read_sync_header(&mut stream)?;
match command {
  SyncCommand::Data => {
    stream.read_exact(&mut buf[0..length])?;
    […]
  },
  […]
}