From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
cc: Andres Freund <andres@anarazel.de>, Daniel Gustafsson <daniel@yesql.se>,
        Kyotaro Horiguchi <horikyota.ntt@gmail.com>, deniel1495@mail.ru,
        Ibrar Ahmed <ibrar.ahmad@gmail.com>, tejeswarm@hotmail.com,
        hlinnaka <hlinnaka@iki.fi>,
        Masahiko Sawada <masahiko.sawada@2ndquadrant.com>,
        "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>,
        Daniel Wood <hexexpert@comcast.net>
Subject: Re: Corruption during WAL replay
In-reply-to: 
 <CA+TgmobXjkeptO-rNiOrq8zjUK2QusXKGgsRL-fqPCL0umc+3Q@mail.gmail.com>
References: 
 <CA+Tgmoa_TAJsUnFWzTevg=FzgXFyiHLJAY_L-jMDA=mH=ytysg@mail.gmail.com>
 <3170060.1648171358@sss.pgh.pa.us> <3173721.1648173548@sss.pgh.pa.us>
 <20220325022010.iqdj37tjqrqwxfrh@alap3.anarazel.de>
 <20220325024302.lhhi7jaapyetqviv@alap3.anarazel.de>
 <20220325034301.htu27xf54xjgyoca@alap3.anarazel.de>
 <3185871.1648181300@sss.pgh.pa.us>
 <20220325045438.enwakjqhrafzq5f2@alap3.anarazel.de>
 <20220325052654.3xpbmntatyofau2w@alap3.anarazel.de>
 <3193652.1648186725@sss.pgh.pa.us>
 <20220325060737.iayq5cs36jktqlag@alap3.anarazel.de>
 <3237467.1648216174@sss.pgh.pa.us>
 <CA+Tgmoa=1a+A_gK4gqBf7ypK9NyY5bo--Wqc7Wp3xm8fjsrUOQ@mail.gmail.com>
 <3238876.1648216924@sss.pgh.pa.us>
 <CA+TgmoaH=6MJtcqN44geA6uQqt1PCHg-2UvehScAxSmsUm4KVA@mail.gmail.com>
 <3241547.1648218889@sss.pgh.pa.us>
 <CA+TgmobXjkeptO-rNiOrq8zjUK2QusXKGgsRL-fqPCL0umc+3Q@mail.gmail.com>
Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Fri, 25 Mar 2022 10:49:22 -0400"
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Date: Fri, 25 Mar 2022 11:50:48 -0400
Message-ID: <3261706.1648223448@sss.pgh.pa.us>
Archived-At: 
 <https://www.postgresql.org/message-id/3261706.1648223448%40sss.pgh.pa.us>
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Robert Haas <robertmhaas@gmail.com> writes:
> ... It's not
> like a 16-bit checksum was state-of-the-art even when we introduced
> it. We just did it because we had 2 bytes that we could repurpose
> relatively painlessly, and not any larger number. And that's still the
> case today, so at least in the short term we will have to choose some
> other solution to this problem.

Indeed.  I propose the attached, which also fixes the unsafe use
of seek() alongside syswrite(), directly contrary to what "man perlfunc"
says to do.

			regards, tom lane


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diff --git a/src/bin/pg_checksums/t/002_actions.pl b/src/bin/pg_checksums/=
t/002_actions.pl
index 62c608eaf6..8c70453a45 100644
--- a/src/bin/pg_checksums/t/002_actions.pl
+++ b/src/bin/pg_checksums/t/002_actions.pl
@@ -24,6 +24,7 @@ sub check_relation_corruption
 	my $tablespace =3D shift;
 	my $pgdata     =3D $node->data_dir;
 =

+	# Create table and discover its filesystem location.
 	$node->safe_psql(
 		'postgres',
 		"CREATE TABLE $table AS SELECT a FROM generate_series(1,10000) AS a;
@@ -37,9 +38,6 @@ sub check_relation_corruption
 	my $relfilenode_corrupted =3D $node->safe_psql('postgres',
 		"SELECT relfilenode FROM pg_class WHERE relname =3D '$table';");
 =

-	# Set page header and block size
-	my $pageheader_size =3D 24;
-	my $block_size =3D $node->safe_psql('postgres', 'SHOW block_size;');
 	$node->stop;
 =

 	# Checksums are correct for single relfilenode as the table is not
@@ -55,8 +53,12 @@ sub check_relation_corruption
 =

 	# Time to create some corruption
 	open my $file, '+<', "$pgdata/$file_corrupted";
-	seek($file, $pageheader_size, SEEK_SET);
-	syswrite($file, "\0\0\0\0\0\0\0\0\0");
+	my $pageheader;
+	sysread($file, $pageheader, 24) or die "sysread failed";
+	# This inverts the pd_checksum field (only); see struct PageHeaderData
+	$pageheader ^=3D "\0\0\0\0\0\0\0\0\xff\xff";
+	sysseek($file, 0, 0) or die "sysseek failed";
+	syswrite($file, $pageheader) or die "syswrite failed";
 	close $file;
 =

 	# Checksum checks on single relfilenode fail

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