Function tfrecord::io::async::try_read_len

pub async fn try_read_len<R>(
    reader: &mut R, 
    check_integrity: bool
) -> Result<Option<usize>> where
    R: AsyncRead + Unpin, 

Try to read the record length from a generic reader.

It is internally called by try_read_record. It returns Ok(None) if reaching the end of file.