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

On Thu, Apr 20, 2023 at 05:58:14PM +0900, Kyotaro Horiguchi wrote:
> + /*
> + * Slot minimum LSN could be greater than recptr. In such cases, no
> + * segments are protected by slots but we still need to keep segno in a
> + * reasonable range for subsequent calculations to avoid underflow.
> + */
> + if (segno > currSegNo)
> + segno = currSegNo;
> +

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.

--
Nathan Bossart
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