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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Munro <thomas.munro@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: pg15b1: FailedAssertion("val > base", File: "...src/include/utils/relptr.h", Line: 67, PID: 30485)
Date: Tue, 31 May 2022 17:52:09 -0400
Message-ID: <347628.1654033929@sss.pgh.pa.us> (raw)
In-Reply-To: <CA+hUKGKWXaUHS7MZxxTYXGZ-04QeftwK+eeaycoQ3q7=+t9PTQ@mail.gmail.com>
References: <20220519193839.GT19626@telsasoft.com>
	<208993.1652994963@sss.pgh.pa.us>
	<20220520.120014.1649415417205853644.horikyota.ntt@gmail.com>
	<CA+Tgmob26sUtT8h2tnqttx1WRfi56FrRELs77ANCXvmVMv3w3g@mail.gmail.com>
	<321315.1654027805@sss.pgh.pa.us>
	<CA+hUKG+zSZ8ZkrS0GRhvaRQBMbfKcBJi_XdtA9nDKt4ODXxR1w@mail.gmail.com>
	<CA+TgmoaWZ-hs1vQ8C88ZE57oXpsWj5oxYZYR9y7c2H+uoikz_A@mail.gmail.com>
	<331489.1654031367@sss.pgh.pa.us>
	<CA+hUKGKWXaUHS7MZxxTYXGZ-04QeftwK+eeaycoQ3q7=+t9PTQ@mail.gmail.com>

Thomas Munro <thomas.munro@gmail.com> writes:
> Count we make the relptrs 1-based, so that 0 is reserved as a sentinel
> that has the nice memset(0) property?

Hm ... almost.  A +1 offset would mean that zero is ambiguous with a
pointer to the byte just before the relptr.  Maybe that case never
arises in practice, but now that we've seen this problem I'm not real
comfortable with such an assumption.  But how about a -1 offset?
Then zero would be ambiguous with a pointer to the second byte of the
relptr, and I think I *am* prepared to assume that that has no use-cases.

The other advantage of such a definition is that it'd help flush out
anybody breaking the relptr abstraction ;-)

			regards, tom lane





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