Re: [PATCH] Discard aborted updaters when expanding a multixact

Hi,

I ran into "new multixact has more than one updating member" while load
testing a trigger-based extension, and found this thread. In case it
helps with committing and back-patching, here is a plain-SQL reproducer
that needs no injection points and no crash, at READ COMMITTED with
default settings.

Schema:

CREATE TABLE tb_user (pk_user bigint PRIMARY KEY, name text);
CREATE TABLE tb_post (pk_post bigserial PRIMARY KEY,
fk_user bigint NOT NULL REFERENCES tb_user,
title text);
INSERT INTO tb_user SELECT g, 'u' FROM generate_series(1, 10) g;

ins.pgb (the FK check takes FOR KEY SHARE on the parent):

\set u random(1, 10)
INSERT INTO tb_post (fk_user, title) VALUES (:u, 't');

ren.pgb (a non-key update of the parent that always rolls back):

\set u random(1, 10)
BEGIN;
UPDATE tb_user SET name = md5(random()::text) WHERE pk_user = :u;
ROLLBACK;

pgbench -n -c 48 -j 8 -T 30 -f ins(dot)pgb(at)8 -f ren(dot)pgb(at)2

On REL_18_STABLE (a85b552), 24 cores, 43 of the 48 clients failed with
"ERROR: new multixact has more than one updating member" within 30
seconds, in each of three runs. With the heapam.c and xact.c changes of
v5 applied to the same tree: no errors over about 16 million
transactions in three runs, and no other errors. I also hit it on 18.1
and 18.4 under REPEATABLE READ.

Tested-by: Lionel Hamayon <lionel(dot)hamayon(at)evolution-digitale(dot)fr>

Regards,

Lionel