From 820e5bdc8c9e80777dc3d27307f4152dd5f9cc54 Mon Sep 17 00:00:00 2001
From: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Date: Fri, 23 Oct 2026 20:12:35 +0900
Subject: [PATCH v7 2/2] Add a test for the end LSN of an xlog-switch record

An xlog-switch record that starts SizeOfXLogRecord bytes before a
segment boundary ends exactly on it, and pg_switch_wal() must report
the boundary.  Without the fix, it reports the boundary plus
SizeOfXLogLongPHD.

The test goes into 039_end_of_wal.pl, whose node is set up so that
nothing else writes WAL.  It pads the segment with logical messages,
measuring the record overhead and page header size at run time
instead of hard-coding them.
---
 src/test/recovery/t/039_end_of_wal.pl | 69 +++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)

diff --git a/src/test/recovery/t/039_end_of_wal.pl b/src/test/recovery/t/039_end_of_wal.pl
index 5fe8835e094..fa4ca73fd73 100644
--- a/src/test/recovery/t/039_end_of_wal.pl
+++ b/src/test/recovery/t/039_end_of_wal.pl
@@ -376,4 +376,73 @@ ok( $node->log_contains(
 		"invalid contrecord length 123456 .* at .*", $log_size),
 	"xlp_rem_len bad (split record header)");
 
+
+###########################################################################
+note "xlog-switch record ending on a segment boundary";
+###########################################################################
+
+# A switch record that starts SizeOfXLogRecord bytes before the end of a
+# segment ends exactly on the boundary, and pg_switch_wal() has to return
+# the boundary, not a position past the next page header.
+
+# Emit a non-transactional logical message (no XID, no commit record)
+# with a payload of $size bytes, returning its end LSN in bytes.
+sub emit_message
+{
+	my ($node, $size) = @_;
+	return int(
+		$node->safe_psql(
+			'postgres',
+			"SELECT pg_logical_emit_message(false, '', repeat('a', $size)) - '0/0'"
+		));
+}
+
+my $SizeOfXLogRecord = 24;
+
+# Start on a fresh segment.  Nothing else writes WAL on this node, so the
+# positions below are exact.
+$node->safe_psql('postgres', "SELECT pg_switch_wal()");
+my $lsn = int(
+	$node->safe_psql(
+		'postgres', "SELECT pg_current_wal_insert_lsn() - '0/0'"));
+my $seg_end = ($lsn / $WAL_SEGMENT_SIZE + 1) * $WAL_SEGMENT_SIZE;
+
+# Measure the space a message takes beyond its payload, padding included.
+# 256 is a multiple of MAXALIGN, over 255 so the record uses the long
+# main-data header like the fill, and small enough to stay on the first page.
+my $probe_size = 256;
+my $end = emit_message($node, $probe_size);
+my $overhead = $end - $lsn - $probe_size;
+$lsn = $end;
+
+# Measure the page header size with a message crossing one page boundary.
+# The first page of a segment has a longer header, but nothing below
+# crosses a segment boundary.
+my $size = start_of_next_page($lsn) - $lsn;
+$end = emit_message($node, $size);
+my $SizeOfXLogShortPHD = $end - $lsn - $size - $overhead;
+$lsn = $end;
+
+# Fill the segment in steps, so that no record gets too big for any
+# segment size, until the rest fits in one message.
+my $step = $WAL_SEGMENT_SIZE / 16;
+$lsn = emit_message($node, $step) while $seg_end - $lsn > 2 * $step;
+
+# Size the last message to end SizeOfXLogRecord bytes before the
+# boundary, less the page headers it crosses.  A record ending exactly on
+# a page boundary reports the boundary while the next one starts after
+# the page header, hence the - 1.
+my $headers =
+  ($seg_end - start_of_next_page($lsn - 1)) / $WAL_BLOCK_SIZE *
+  $SizeOfXLogShortPHD;
+$lsn = emit_message($node,
+	$seg_end - $SizeOfXLogRecord - $lsn - $headers - $overhead);
+is($lsn, $seg_end - $SizeOfXLogRecord,
+	"message ends SizeOfXLogRecord bytes before the segment end");
+
+# The switch record is only a header, so it ends on the boundary.
+is( $node->safe_psql('postgres', "SELECT pg_switch_wal() - '0/0'"),
+	$seg_end,
+	"pg_switch_wal() returns the segment boundary");
+
 done_testing();
-- 
2.50.1 (Apple Git-155)