Date: Sun, 9 Jun 2024 21:37:03 -0500
From: Nathan Bossart <nathandbossart@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Joseph Koshakow <koshy44@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
	Andres Freund <andres@anarazel.de>
Subject: Re: Remove dependence on integer wrapping
Message-ID: <ZmZmz9mnTYNT4aCd@nathan>
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On Sun, Jun 09, 2024 at 09:57:54PM -0400, Tom Lane wrote:
> Nathan Bossart <nathandbossart@gmail.com> writes:
>> On Sun, Jun 09, 2024 at 04:59:22PM -0400, Joseph Koshakow wrote:
>>> The following comment was in the code for parsing timestamps:
>>> /* check for just-barely overflow (okay except time-of-day wraps) */
>>> /* caution: we want to allow 1999-12-31 24:00:00 */
>>> 
>>> I wasn't able to fully understand it even after staring at it for
>>> a while. Is the comment suggesting that it is ok for the months field,
>>> for example, to wrap around? That doesn't sound right to me I tested
>>> the supplied timestamp, 1999-12-31 24:00:00, and it behaves the same
>>> before and after the patch.
> 
>> I haven't stared at this for a while like you, but I am likewise confused
>> at first glance.  This dates back to commit 84df54b, and it looks like this
>> comment was present in the first version of the patch in the thread [0].  I
>> CTRL+F'd for any discussion about this but couldn't immediately find
>> anything.
> 
> I believe this is a copy-and-paste from 841b4a2d5, which added this:
> 
> +   *result = (date * INT64CONST(86400000000)) + time;
> +   /* check for major overflow */
> +   if ((*result - time) / INT64CONST(86400000000) != date)
> +       return -1;
> +   /* check for just-barely overflow (okay except time-of-day wraps) */
> +   if ((*result < 0) ? (date >= 0) : (date < 0))
> +       return -1;
> 
> I think you could replace the whole thing by using overflow-aware
> multiplication and addition primitives in the result calculation.
> Lines 2-4 basically check for mult overflow and 5-7 for addition
> overflow.

Ah, I see.  Joe's patch does that in one place.  It's probably worth doing
that in the other places this "just-barefly overflow" comment appears IMHO.

I was still confused by the comment about 1999, but I tracked it down to
commit 542eeba [0].  IIUC it literally means that we need special handling
for that date because POSTGRES_EPOCH_JDATE is 2000-01-01.

[0] https://postgr.es/m/CABUevEx5zUO%3DKRUg06a9qnQ_e9EvTKscL6HxAM_L3xj71R7AQw%40mail.gmail.com

-- 
nathan