Function write_record 

pub async fn write_record<W>(
    writer: &mut W,
    target: u32,
    generation: u32,
    record: RecordRef<'_>,
) -> Result<(), CommitError<<W as RecordWriter>::Error>>
where
    W: RecordWriter,

Write one committed record. Failure leaves any previously committed record untouched: the body lands first and the commit word last, so a mount never selects a partial write.