Date: Sun, 24 Jan 2021 14:17:58 +0800
From: Julien Rouhaud <rjuju123@gmail.com>
To: pgsql-hackers@lists.postgresql.org
Subject: Faulty HEAP_XMAX_LOCK_ONLY & HEAP_KEYS_UPDATED hintbit combination
Message-ID: <20210124061758.GA11756@nol>
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Hi,

While working on pg14 compatibility for an extension relying on an apparently
uncommon combination of FOR UPDATE and stored function calls, I hit some new
Asserts introduced in 866e24d47db (Extend amcheck to check heap pages):

+	/*
+	 * Do not allow tuples with invalid combinations of hint bits to be placed
+	 * on a page.  These combinations are detected as corruption by the
+	 * contrib/amcheck logic, so if you disable one or both of these
+	 * assertions, make corresponding changes there.
+	 */
+	Assert(!((tuple->t_data->t_infomask & HEAP_XMAX_LOCK_ONLY) &&
+			 (tuple->t_data->t_infomask2 & HEAP_KEYS_UPDATED)));


I attach a simple self contained script to reproduce the problem, the last
UPDATE triggering the Assert.

I'm not really familiar with this part of the code, so it's not exactly clear
to me if some logic is missing in compute_new_xmax_infomask() /
heap_prepare_insert(), or if this should actually be an allowed combination of
hint bit.

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DROP TABLE IF EXISTS t1;
CREATE TABLE t1(id integer, val text);
INSERT INTO t1 SELECT i, 'val' FROM generate_series(1, 500) i;

BEGIN;
SAVEPOINT s1;
SELECT 1 FROM t1 WHERE id = 123 FOR UPDATE;
UPDATE t1 SET val = 'hoho' WHERE id = 123;
release s1;
savepoint s1;
SELECT 1 FROM t1 WHERE id = 123 FOR UPDATE;
UPDATE t1 SET val = 'hoho' WHERE id = 123;
COMMIT;

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