Function complete_record_count 

pub fn complete_record_count(contents: &str) -> usize

How many records in contents are known COMPLETE.

A record is complete only once the next record has begun, because nothing else marks its end. In a log still being written the final record may be half-flushed, so it is never counted and never compared: a partial write must not read as a difference. A buffer with one record start has zero complete records, and that is a real answer, not a failure.