Re: BUG #16577: Segfault on altering a table located in a dropped tablespace

From: Michael Paquier <michael(at)paquier(dot)xyz>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, exclusion(at)gmail(dot)com, pgsql-bugs(at)lists(dot)postgresql(dot)org
Subject: Re: BUG #16577: Segfault on altering a table located in a dropped tablespace
Date: 2020-10-30 04:13:06
Message-ID: 20201030041306.GF1693@paquier.xyz
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On Wed, Oct 28, 2020 at 09:59:24AM -0400, Tom Lane wrote:
> Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> writes:
>> I haven't started on this one yet, but I intend to do so shortly.

Cool. Glad to hear that.

>> Strictly speaking, we can still introduce a new category of pg_shdepend
>> entries in back branches; it won't break anything that works today.
>
> Yeah, as long as the patched version won't actively fail when those
> pg_shdepend entries are missing, I don't think a backpatch is too
> hazardous. It might be worth checking that the extra entries don't
> create huge problems if one does downgrade after some of them exist
> --- but my feeling for how that mechanism works is that it'd Just
> Work, and indeed provide the missing DROP protection even without
> explicit action by the backend.

Good point. Even with that, we have never considered the case of a
downgrade as something actually supported, right? We had in the past
for example bug fixes that involve slight tweaks in the WAL records
that are upward-compatible, changing the way these get interpreted
when replayed, but it could be possible to finish with logical
corruptions or replay failures if a downgraded version replays a
slightly-modified record generated by a newer version, no?
--
Michael

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