Date: Thu, 12 Sep 2019 10:16:12 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Andrey Borodin <x4mmm@yandex-team.ru>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: Amcheck: do rightlink verification with lock coupling
Message-ID: <20190912131612.GA24926@alvherre.pgsql>
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On 2019-Sep-12, Andrey Borodin wrote:

> This patch violates one of amcheck design principles: current code
> does not ever take more than one page lock. I do not know: should we
> hold this rule or should we use more deep check?

The check does seem worthwhile to me.

As far as I know, in btree you can lock the right sibling of a page
while keeping lock on the page itself, without risk of deadlock.  So I'm
not sure what's the issue with that.  This is in the README:

  In most cases we release our lock and pin on a page before attempting
  to acquire pin and lock on the page we are moving to.  In a few places
  it is necessary to lock the next page before releasing the current one.
  This is safe when moving right or up, but not when moving left or down
  (else we'd create the possibility of deadlocks).

I suppose Peter was concerned about being able to run amcheck without
causing any trouble at all for concurrent operation; maybe we can retain
that property by making this check optional.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services