Re: BUG #17903: There is a bug in the KeepLogSeg()

At Thu, 20 Apr 2023 15:02:42 -0700, Nathan Bossart <nathandbossart@gmail.com> wrote in 
> I wonder if it'd be better to instead change
> 
>     if (currSegNo - segno > slot_keep_segs)
> to
>     if (currSegNo > segno + slot_keep_segs)
> 
> and
> 
>     if (currSegNo - segno < keep_segs)
> to
>     if (currSegNo < segNo + keep_segs)
> 
> If segno > currSegNo, the first conditional would be false as expected, and
> the second would be true as expected.  We could also use
> pg_sub_u64_overflow() to detect underflow, but that might be excessive in
> this case.

From what I understand, the XLogSegNo calculations are designed
without considering the actual value range. Basically, it assumes that
(XLogSegNo + <any positive int>) can overflow.  If we take the actual
value range into account, we can make that change.

The choice lies on whether we assume the actual value range or not.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center