Add test for multixid wraparound

Author: Andrey Borodin <amborodin@acm.org>
Discussion: https://www.postgresql.org/message-id/7de697df-d74d-47db-9f73-e069b7349c4b@iki.fi

src/test/modules/test_slru/meson.build

@@ -38,7 +38,8 @@ tests += {
        'enable_injection_points': get_option('injection_points') ? 'yes' : 'no',
     },
     'tests': [
-      't/001_multixact.pl'
+      't/001_multixact.pl',
+      't/002_multixact_wraparound.pl'
     ],
   },
 }

src/test/modules/test_slru/t/002_multixact_wraparound.pl

@@ -0,0 +1,74 @@
+# Copyright (c) 2024-2025, PostgreSQL Global Development Group
+
+# Test multixact wraparound
+
+use strict;
+use warnings FATAL => 'all';
+
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+
+use Test::More;
+
+my $node = PostgreSQL::Test::Cluster->new('main');
+$node->init;
+$node->append_conf('postgresql.conf',
+	"shared_preload_libraries = 'test_slru'");
+
+# Set the cluster's next multitransaction close to wraparound
+my $node_pgdata = $node->data_dir;
+command_ok(
+	[
+		'pg_resetwal',
+		'--multixact-ids' => '0xFFFFFFF8,0xFFFFFFF8',
+		$node_pgdata
+	],
+	"set the cluster's next multitransaction to 0xFFFFFFF8");
+
+# Fixup the SLRU files to match the state we reset to.
+
+# initialize SLRU file with zeros (65536 entries * 4 bytes = 262144 bytes)
+my $slru_file = "$node_pgdata/pg_multixact/offsets/FFFF";
+open my $fh, ">", $slru_file
+  or die "could not open \"$slru_file\": $!";
+binmode $fh;
+# Write 65536 entries of 4 bytes each (all zeros)
+syswrite($fh, "\0" x 262144) == 262144
+  or die "could not write to \"$slru_file\": $!";
+close $fh;
+
+# remove old SLRU file
+unlink("$node_pgdata/pg_multixact/offsets/0000")
+  or die "could not unlink \"$node_pgdata/pg_multixact/offsets/0000\": $!";
+
+# Consume multixids to wrap around.  We start at 0xFFFFFFF8, so after
+# creating 16 multixacts we should definitely have wrapped around.
+$node->start;
+$node->safe_psql('postgres', q(CREATE EXTENSION test_slru));
+
+my @multixact_ids;
+foreach my $i (1 .. 16)
+{
+	my $multi =
+	  $node->safe_psql('postgres', q{SELECT test_create_multixact();});
+	push @multixact_ids, $multi;
+}
+
+# Verify that wraparound occurred (last_multi should be numerically
+# smaller than first_multi)
+my $first_multi = $multixact_ids[0];
+my $last_multi = $multixact_ids[-1];
+ok( $last_multi < $first_multi,
+	"multixact wraparound occurred (first: $first_multi, last: $last_multi)");
+
+# Verify that all the multixacts we created are readable
+foreach my $i (0 .. $#multixact_ids)
+{
+	my $multi = $multixact_ids[$i];
+	is( $node->safe_psql(
+			'postgres', qq{SELECT test_read_multixact('$multi');}),
+		'',
+		"multixact $i (ID: $multi) is readable after wraparound");
+}
+
+done_testing();