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From: Tom Lane <[email protected]>
To: Robert Haas <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: Daniel Gustafsson <[email protected]>
Cc: Kyotaro Horiguchi <[email protected]>
Cc: [email protected]
Cc: Ibrar Ahmed <[email protected]>
Cc: [email protected]
Cc: hlinnaka <[email protected]>
Cc: Masahiko Sawada <[email protected]>
Cc: [email protected] <[email protected]>
Cc: Daniel Wood <[email protected]>
Subject: Re: Corruption during WAL replay
Date: Fri, 25 Mar 2022 11:50:48 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <CA+TgmobXjkeptO-rNiOrq8zjUK2QusXKGgsRL-fqPCL0umc+3Q@mail.gmail.com>
References: <CA+Tgmoa_TAJsUnFWzTevg=FzgXFyiHLJAY_L-jMDA=mH=ytysg@mail.gmail.com>
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Robert Haas <[email protected]> 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



Attachments:

  [text/x-diff] make-checksum-corruption-more-reliable.patch (1.4K, ../[email protected]/2-make-checksum-corruption-more-reliable.patch)
  download | inline diff:
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 = shift;
 	my $pgdata     = $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 = $node->safe_psql('postgres',
 		"SELECT relfilenode FROM pg_class WHERE relname = '$table';");
 
-	# Set page header and block size
-	my $pageheader_size = 24;
-	my $block_size = $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
+	$pageheader ^= "\0\0\0\0\0\0\0\0\xff\xff";
+	sysseek($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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