...to bufio.Reader's internal buffer rollover.
BUG: #213
If we're dealing with multi-lined envelope (either by rows or by header/footer), readLine()
will be called several times, thus whatever ios.ByteReadLine, which uses bufio.Reader underneath,
returns in a previous call may be potentially be invalidated due to bufio.Reader's internal buf
rollover. If we read the previous line directly, it would cause corruption.
To fix the problem the easiest solution would be simply copying the return []byte from
ios.ByteReadLine every single time. But for files with single-line envelope, which are the vast
majority cases, this copy becomes unnecessary and burdensome on gc. So the trick is to has a flag
on reader.linesBuf's last element to tell if it contains a reference into the bufio.Reader's
internal buffer, or it's a copy. Every time before we call bufio.Reader read, we check
reader.liensBuf's last element flag, if it is not a copy, then we will turn it into a copy.
This way, we optimize for the vast majority cases without needing allocations, and avoid any potential
corruptions in the multi-lined envelope cases.