From 2f2a37c62f74779bbe5d988040922cf7a21eabc9 Mon Sep 17 00:00:00 2001
From: Sergey Fukanchik <s.fukanchik@postgrespro.ru>
Date: Sun, 7 Sep 2025 12:06:03 +0300
Subject: [PATCH 1/2] Perform check for oversized WAL record before calculating
 record CRC

---
 src/backend/access/transam/xloginsert.c | 26 ++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/src/backend/access/transam/xloginsert.c b/src/backend/access/transam/xloginsert.c
index c7571429e8e9..e8c55924c125 100644
--- a/src/backend/access/transam/xloginsert.c
+++ b/src/backend/access/transam/xloginsert.c
@@ -904,19 +904,6 @@ XLogRecordAssemble(RmgrId rmid, uint8 info,
 	hdr_rdt.len = (scratch - hdr_scratch);
 	total_len += hdr_rdt.len;
 
-	/*
-	 * Calculate CRC of the data
-	 *
-	 * Note that the record header isn't added into the CRC initially since we
-	 * don't know the prev-link yet.  Thus, the CRC will represent the CRC of
-	 * the whole record in the order: rdata, then backup blocks, then record
-	 * header.
-	 */
-	INIT_CRC32C(rdata_crc);
-	COMP_CRC32C(rdata_crc, hdr_scratch + SizeOfXLogRecord, hdr_rdt.len - SizeOfXLogRecord);
-	for (rdt = hdr_rdt.next; rdt != NULL; rdt = rdt->next)
-		COMP_CRC32C(rdata_crc, rdt->data, rdt->len);
-
 	/*
 	 * Ensure that the XLogRecord is not too large.
 	 *
@@ -930,6 +917,19 @@ XLogRecordAssemble(RmgrId rmid, uint8 info,
 				 errdetail_internal("WAL record would be %" PRIu64 " bytes (of maximum %u bytes); rmid %u flags %u.",
 									total_len, XLogRecordMaxSize, rmid, info)));
 
+	/*
+	 * Calculate CRC of the data
+	 *
+	 * Note that the record header isn't added into the CRC initially since we
+	 * don't know the prev-link yet.  Thus, the CRC will represent the CRC of
+	 * the whole record in the order: rdata, then backup blocks, then record
+	 * header.
+	 */
+	INIT_CRC32C(rdata_crc);
+	COMP_CRC32C(rdata_crc, hdr_scratch + SizeOfXLogRecord, hdr_rdt.len - SizeOfXLogRecord);
+	for (rdt = hdr_rdt.next; rdt != NULL; rdt = rdt->next)
+		COMP_CRC32C(rdata_crc, rdt->data, rdt->len);
+
 	/*
 	 * Fill in the fields in the record header. Prev-link is filled in later,
 	 * once we know where in the WAL the record will be inserted. The CRC does

From 9dbcd1c281c2f623b7033ba0330bbdeae5144f74 Mon Sep 17 00:00:00 2001
From: Sergey Fukanchik <s.fukanchik@postgrespro.ru>
Date: Sun, 7 Sep 2025 12:06:27 +0300
Subject: [PATCH 2/2] Add a TAP test for oversized WAL records

---
 .../recovery/t/049_oversized_wal_record.pl    | 50 +++++++++++++++++++
 1 file changed, 50 insertions(+)
 create mode 100644 src/test/recovery/t/049_oversized_wal_record.pl

diff --git a/src/test/recovery/t/049_oversized_wal_record.pl b/src/test/recovery/t/049_oversized_wal_record.pl
new file mode 100644
index 000000000000..8604ef53d48b
--- /dev/null
+++ b/src/test/recovery/t/049_oversized_wal_record.pl
@@ -0,0 +1,50 @@
+
+# Copyright (c) 2025, PostgreSQL Global Development Group
+
+# Try to create a WAL record which is larger than the limit XLogRecordMaxSize.
+# That should raise an error.
+
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Cluster;
+use Test::More;
+
+# Disable the test unless requested explicitly because it requires 1Gb of memory
+if (exists $ENV{PG_TEST_EXTRA}
+	&& $ENV{PG_TEST_EXTRA} =~ m/\boversized_wal_record\b/)
+{
+	plan tests => 4;
+}
+else
+{
+	plan skip_all =>
+      'Skipping oversized_wal_record test as it requires a lot of memory';
+}
+
+note "Check handing of oversized WAL records";
+
+# Initialize and start basic node
+my $node = PostgreSQL::Test::Cluster->new('node');
+
+$node->init;
+
+$node->start;
+
+# Insert oversized record
+my ($ret, $stdout, $stderr) = $node->psql(
+	'postgres',
+	"select pg_logical_emit_message(false, 'a', repeat('00000000', (1020 * 1024 * 1024)/8+1), true)",
+	on_error_die => 0
+  );
+
+# Verify we have error
+ok($ret != 0, 'exit code is non-zero');
+
+ok($stderr =~ qr/ERROR:  oversized WAL record/, 'expect error');
+
+ok($stderr =~ qr/DETAIL:  WAL record would be [0-9]+ bytes \(of maximum [0-9]+ bytes\);/,
+   'error details');
+
+$node->stop;
+
+done_testing();