From 6ded021b3b22a8ad205a81d3af7d987470fa95c8 Mon Sep 17 00:00:00 2001
From: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Date: Sun, 20 Sep 2026 00:29:44 +0530
Subject: [PATCH v1 1/2] Fix legacy shmem reattachment in single-user mode

ShmemInitStruct() only looks for an existing allocation when
IsUnderPostmaster is true.  In a standalone backend, a second
call for the same area instead attempts to initialize it again
and errors out.  This differs from the legacy API's behavior
before the shared-memory registration refactor.

Look up existing allocations regardless of process type, as the
old implementation did.  Add a test that checks reattachment
returns the same pointer in normal and single-user backends.

Backpatch-through: 19
---
 src/backend/storage/ipc/shmem.c               |  7 +-----
 .../test_shmem/t/001_late_shmem_alloc.pl      | 23 +++++++++++--------
 .../modules/test_shmem/test_shmem--1.0.sql    |  4 ++++
 src/test/modules/test_shmem/test_shmem.c      | 16 +++++++++++++
 4 files changed, 35 insertions(+), 15 deletions(-)

diff --git a/src/backend/storage/ipc/shmem.c b/src/backend/storage/ipc/shmem.c
index 3345735dff0c5923b491d72c9440cafc1b265998..c0a8fb39e47a7ff773d0ac3740d34cdec2c03339 100644
--- a/src/backend/storage/ipc/shmem.c
+++ b/src/backend/storage/ipc/shmem.c
@@ -1131,12 +1131,7 @@ ShmemInitStruct(const char *name, Size size, bool *foundPtr)
 
 	LWLockAcquire(ShmemIndexLock, LW_EXCLUSIVE);
 
-	/*
-	 * During postmaster startup, look up the existing entry if any.
-	 */
-	*foundPtr = false;
-	if (IsUnderPostmaster)
-		*foundPtr = AttachShmemIndexEntry(&request, true);
+	*foundPtr = AttachShmemIndexEntry(&request, true);
 
 	/* Initialize it if not found */
 	if (!*foundPtr)
diff --git a/src/test/modules/test_shmem/t/001_late_shmem_alloc.pl b/src/test/modules/test_shmem/t/001_late_shmem_alloc.pl
index 7d961fe279085581b4ebdc2c3a95a8e21b02383c..cd0077229484ee3a400da5d8d389ca9416b1616f 100644
--- a/src/test/modules/test_shmem/t/001_late_shmem_alloc.pl
+++ b/src/test/modules/test_shmem/t/001_late_shmem_alloc.pl
@@ -29,6 +29,9 @@ my $attach_count2 =
 cmp_ok($attach_count2, '>', $attach_count1,
 	"attach callback is called in each backend");
 
+is($node->safe_psql("postgres", "SELECT test_shmem_legacy();"), 't',
+	"legacy shared memory allocation can attach to an existing area");
+
 $node->stop;
 
 ###
@@ -55,19 +58,21 @@ SKIP:
 {
 	# Skip the test on Windows, as single-user mode would fail on permission
 	# failure with privileged accounts.
-	skip 'single-user test is not supported by this platform', 1
+	skip 'single-user test is not supported by this platform', 2
 	  if $windows_os;
+	my @command = (
+		'postgres', '--single', '-F',
+		'-c' => 'exit_on_error=true',
+		'-D' => $node->data_dir,
+		'postgres');
 	my $query = "SELECT get_test_shmem_attach_count();\n";
-	my $result = run_log(
-		[
-			'postgres', '--single', '-F',
-			'-c' => 'exit_on_error=true',
-			'-D' => $node->data_dir,
-			'postgres'
-		],
-		'<' => \$query);
+	my $result = run_log(\@command, '<' => \$query);
 
 	ok($result, "shmem area is initialized in single-user mode");
+
+	$query = "SELECT test_shmem_legacy();\n";
+	$result = run_log(\@command, '<' => \$query);
+	ok($result, "legacy shared memory reattachment works in single-user mode");
 }
 
 ###
diff --git a/src/test/modules/test_shmem/test_shmem--1.0.sql b/src/test/modules/test_shmem/test_shmem--1.0.sql
index 2d01fd9256c1bd27b84bbd2d4c987ba20e8636ab..04ead79e3482465ad4c71e441f3b1043bca5f5a5 100644
--- a/src/test/modules/test_shmem/test_shmem--1.0.sql
+++ b/src/test/modules/test_shmem/test_shmem--1.0.sql
@@ -4,6 +4,10 @@
 \echo Use "CREATE EXTENSION test_shmem" to load this file. \quit
 
 
+CREATE FUNCTION test_shmem_legacy()
+RETURNS pg_catalog.bool STRICT
+AS 'MODULE_PATHNAME' LANGUAGE C;
+
 CREATE FUNCTION get_test_shmem_attach_count()
 RETURNS pg_catalog.int4 STRICT
 AS 'MODULE_PATHNAME' LANGUAGE C;
diff --git a/src/test/modules/test_shmem/test_shmem.c b/src/test/modules/test_shmem/test_shmem.c
index 2a7e13512bfa21ddfb5d6047423d4e1bc4390d9d..d2f1e004886e2a0560450381a270fbcd579616b5 100644
--- a/src/test/modules/test_shmem/test_shmem.c
+++ b/src/test/modules/test_shmem/test_shmem.c
@@ -129,3 +129,19 @@ get_test_shmem_attach_count(PG_FUNCTION_ARGS)
 		elog(ERROR, "shmem area not yet initialized");
 	PG_RETURN_INT32(TestShmem->attach_count);
 }
+
+PG_FUNCTION_INFO_V1(test_shmem_legacy);
+Datum
+test_shmem_legacy(PG_FUNCTION_ARGS)
+{
+	void	   *first;
+	void	   *second;
+	bool		found;
+
+	first = ShmemInitStruct("test_shmem legacy", 64, &found);
+	second = ShmemInitStruct("test_shmem legacy", 64, &found);
+	if (!found || first != second)
+		elog(ERROR, "could not reattach to legacy shared memory area");
+
+	PG_RETURN_BOOL(found);
+}

base-commit: e73841ffbceea314cf9fa3f64a5eae9f87a46449
-- 
2.34.1