On 24/09/2026 16:07, Ashutosh Bapat wrote:
On Wed, Sep 23, 2026 at 6:50 PM Ayush Tiwari
<[email protected]> wrote:
On Wed, 23 Sept 2026 at 17:53, Heikki Linnakangas <[email protected]> wrote:
On 19/09/2026 22:12, Ayush Tiwari wrote:
The other one is a bit odd: ask for an existing area with
SHMEM_ATTACH_UNKNOWN_SIZE after startup in single-user mode, and we
tell you it "cannot be used during startup".
IIUC, the distinction we need here is whether we're still working out
the initial shmem requirements, not whether we have a postmaster.
0002 adds SRS_REQUESTING_AFTER_STARTUP for that. I couldn't see a clean
way to reuse SRS_REQUESTING without mixing those cases up.
I'm a bit on the fence about adding another state just for this.
With the PG19 release getting close, I thought I'd send this out for
feedback before spending more time iterating on it. Does the extra
state seem like the right approach?
Extra state sounds reasonable. Thanks, I'll take a closer look, and I'll
double-check all the other places in shmem.c where we use
IsUnderPostmaster, too.
The IsUnderPostmaster variable is deceptive. It's easy to forget about
single-user code, and incorrectly assume that IsUnderPostmaster == true
means you're a backend and IsUnderPostmaster == false means you're
postmaster. I think that's what happened to me here and with the
previous single-user mode bugs. I remember I've struggled to keep that
in mind in the past too. We should perhaps replace IsUnderPostmaster
with a three-valued enum or something (postmaster, backend, single-user
backend).
If we do this, do we still need extra state?
I came up with a simpler idea: we can check "ShmemIndex == NULL" to know
if shared memory has already been initialized and we're in the "after
startup" case, or not. That feels like a pretty direct way of checking
for exactly the property we care about, without needing another state.
I also reworked the tests. I added a very generic test_shmem_register()
function that [registers a callback that] calls ShmemRequestStruct()
with given name and size. And then the perl script can call it with
different sizes, to test the "unknown-size" case, as well as trying to
attach with incorrect size etc. So most of the logic is now in the perl
script.
What do you think?
- Heikki
From 904e010acc467ba291f1d29248374c7f917fe090 Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <[email protected]>
Date: Thu, 24 Sep 2026 16:20:51 +0300
Subject: [PATCH v2 1/1] Allow unknown-size shmem attachments in single-user
mode
ShmemRequestInternal() rejected SHMEM_ATTACH_UNKNOWN_SIZE whenever
IsUnderPostmaster was false. That also rejected an after-startup
attachment in a standalone backend, even if the area exists.
To fix, check whether the shared memory has been initialized
(ShmemIndex == NULL) rather than IsUnderPostmaster.
Refactor the tests, and add more test cases for after-startup
attachment, with SHMEM_ATTACH_UNKNOWN_SIZE and a mismatching size, and
for this single-user case.
Author: Ayush Tiwari <[email protected]>
Discusssion: https://www.postgresql.org/message-id/cajtyswxregxcnwdzjsxt8pt2uy_y2gsx+houu2x3s8ydclj...@mail.gmail.com
Backpatch-through: 19
---
src/backend/storage/ipc/shmem.c | 24 ++-
.../test_shmem/t/001_late_shmem_alloc.pl | 144 ++++++++++--------
.../modules/test_shmem/test_shmem--1.0.sql | 4 +
src/test/modules/test_shmem/test_shmem.c | 55 +++++++
4 files changed, 152 insertions(+), 75 deletions(-)
diff --git a/src/backend/storage/ipc/shmem.c b/src/backend/storage/ipc/shmem.c
index 0e827344462..9ceff1723ed 100644
--- a/src/backend/storage/ipc/shmem.c
+++ b/src/backend/storage/ipc/shmem.c
@@ -348,33 +348,27 @@ ShmemRequestInternal(ShmemStructOpts *options, ShmemRequestKind kind)
MemoryContext oldcontext;
ShmemRequest *request;
+ /* Check that we're in the right state */
+ if (shmem_request_state != SRS_REQUESTING)
+ elog(ERROR, "ShmemRequestStruct can only be called from a shmem_request callback");
+
/* Check the options */
if (options->name == NULL)
elog(ERROR, "shared memory request is missing 'name' option");
- if (IsUnderPostmaster)
- {
- if (options->size <= 0 && options->size != SHMEM_ATTACH_UNKNOWN_SIZE)
- elog(ERROR, "invalid size %zd for shared memory request for \"%s\"",
- options->size, options->name);
- }
- else
+ if (options->size == SHMEM_ATTACH_UNKNOWN_SIZE)
{
- if (options->size == SHMEM_ATTACH_UNKNOWN_SIZE)
+ if (ShmemIndex == NULL)
elog(ERROR, "SHMEM_ATTACH_UNKNOWN_SIZE cannot be used during startup");
- if (options->size <= 0)
- elog(ERROR, "invalid size %zd for shared memory request for \"%s\"",
- options->size, options->name);
}
+ else if (options->size <= 0)
+ elog(ERROR, "invalid size %zd for shared memory request for \"%s\"",
+ options->size, options->name);
if (options->alignment != 0 && pg_nextpower2_size_t(options->alignment) != options->alignment)
elog(ERROR, "invalid alignment %zu for shared memory request for \"%s\"",
options->alignment, options->name);
- /* Check that we're in the right state */
- if (shmem_request_state != SRS_REQUESTING)
- elog(ERROR, "ShmemRequestStruct can only be called from a shmem_request callback");
-
/* Check that it's not already registered in this process */
foreach_ptr(ShmemRequest, existing, pending_shmem_requests)
{
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 7d961fe2790..c52bbed7bf5 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
@@ -16,62 +16,7 @@ $node->safe_psql("postgres", "CREATE EXTENSION test_shmem");
$node->stop;
###
-# Test allocating memory after startup, i.e. when the library is not
-# in shared_preload_libraries
-###
-$node->start;
-
-# Check that the attach counter is incremented on a new connection
-my $attach_count1 =
- $node->safe_psql("postgres", "SELECT get_test_shmem_attach_count();");
-my $attach_count2 =
- $node->safe_psql("postgres", "SELECT get_test_shmem_attach_count();");
-cmp_ok($attach_count2, '>', $attach_count1,
- "attach callback is called in each backend");
-
-$node->stop;
-
-###
-# Test that trying to allocate a new shmem area with size =
-# SHMEM_ATTACH_UNKNOWN_SIZE (-1) fails.
-###
-$node->append_conf('postgresql.conf', "test_shmem.area_size = -1");
-$node->start;
-
-my (undef, undef, $stderr) =
- $node->psql("postgres", "SELECT get_test_shmem_attach_count();");
-like(
- $stderr,
- qr/cannot attach to shared memory struct "test_shmem area" because it does not exist/,
- "unknown size request for a nonexistent area fails");
-
-$node->stop;
-$node->adjust_conf('postgresql.conf', 'test_shmem.area_size', undef);
-
-###
-# Test allocating memory after startup in single-user mode
-###
-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
- if $windows_os;
- my $query = "SELECT get_test_shmem_attach_count();\n";
- my $result = run_log(
- [
- 'postgres', '--single', '-F',
- '-c' => 'exit_on_error=true',
- '-D' => $node->data_dir,
- 'postgres'
- ],
- '<' => \$query);
-
- ok($result, "shmem area is initialized in single-user mode");
-}
-
-###
-# Test that loading via shared_preload_libraries also works
+# Test that loading via shared_preload_libraries works
###
$node->append_conf('postgresql.conf',
"shared_preload_libraries = 'test_shmem'");
@@ -81,9 +26,9 @@ $node->start;
# called or not, depending on whether this is an EXEC_BACKEND build.
my $exec_backend =
$node->safe_psql("postgres", "SHOW debug_exec_backend;") eq 'on';
-$attach_count1 =
+my $attach_count1 =
$node->safe_psql("postgres", "SELECT get_test_shmem_attach_count();");
-$attach_count2 =
+my $attach_count2 =
$node->safe_psql("postgres", "SELECT get_test_shmem_attach_count();");
if ($exec_backend)
@@ -99,9 +44,8 @@ else
);
}
-# clean up
$node->stop;
-$node->adjust_conf('postgresql.conf', "shared_preload_libraries", undef);
+$node->adjust_conf('postgresql.conf', 'shared_preload_libraries', undef);
###
# Test a failure in initializing the shared memory area
@@ -139,6 +83,49 @@ SKIP:
$node->stop;
}
+###
+# Test allocating memory after startup, i.e. when the library is not
+# in shared_preload_libraries
+###
+$node->start;
+
+# This first call to the function after startup loads the library
+# and initializes the shmem area.
+$attach_count1 =
+ $node->safe_psql("postgres", "SELECT get_test_shmem_attach_count();");
+
+# Check that the attach counter is incremented on a new connection
+$attach_count2 =
+ $node->safe_psql("postgres", "SELECT get_test_shmem_attach_count();");
+cmp_ok($attach_count2, '>', $attach_count1,
+ "attach callback is called in each backend");
+
+# Allocate another shmem area, after the library is loaded.
+my $stderr;
+my $res = $node->safe_psql("postgres",
+ "SELECT test_shmem_register('test_shmem after startup', 20, 1);");
+is($res, 0, 'allocate after startup');
+
+# Test attaching to it again
+$res = $node->safe_psql("postgres",
+ "SELECT test_shmem_register('test_shmem after startup', 20, 2);");
+is($res, 1, 'attach after startup');
+
+# If the size doesn't match when attaching, you get an error
+(undef, undef, $stderr) =
+ $node->psql("postgres", "SELECT test_shmem_register('test_shmem after startup', 25, 3);");
+like(
+ $stderr,
+ qr/ERROR: shared memory struct "test_shmem after startup" was created with different size: existing 20, requested 25/,
+ "attaching with different size fails");
+
+# Test attaching with SHMEM_ATTACH_UNKNOWN_SIZE
+$res =
+ $node->safe_psql("postgres", "SELECT test_shmem_register('test_shmem after startup', -1, 4);");
+is($res, 2, 'attach with SHMEM_ATTACH_UNKNOWN_SIZE');
+
+$node->stop;
+
###
# Test "out of shared memory" in an after-startup request
###
@@ -167,4 +154,41 @@ $session->query_safe("SELECT get_test_shmem_attach_count();");
$session->quit;
$node->stop;
+
+###
+# Test allocating memory after startup in single-user mode
+###
+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
+ if $windows_os;
+
+ my @command = (
+ 'postgres', '--single', '-F',
+ '-c' => 'exit_on_error=true',
+ '-D' => $node->data_dir,
+ 'postgres');
+
+ my $queries = "SELECT get_test_shmem_attach_count();\n";
+ my $result = run_log([@command], '<' => \$queries);
+ ok($result, "shmem area is initialized in single-user mode");
+
+ $queries = qq{
+-- allocate
+SELECT test_shmem_register('test_shmem after startup', 25, 1);
+-- attach
+SELECT test_shmem_register('test_shmem after startup', 25, 2);
+-- attach with SHMEM_ATTACH_UNKNOWN_SIZE
+SELECT test_shmem_register('test_shmem after startup', -1, 3);
+};
+ $result = run_log([@command], '<' => \$queries);
+ ok($result, "shmem area is initialized in single-user mode");
+}
+
+# clean up
+$node->stop;
+$node->adjust_conf('postgresql.conf', "shared_preload_libraries", undef);
+
done_testing();
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 2d01fd9256c..1dec3ce0d68 100644
--- a/src/test/modules/test_shmem/test_shmem--1.0.sql
+++ b/src/test/modules/test_shmem/test_shmem--1.0.sql
@@ -7,3 +7,7 @@
CREATE FUNCTION get_test_shmem_attach_count()
RETURNS pg_catalog.int4 STRICT
AS 'MODULE_PATHNAME' LANGUAGE C;
+
+CREATE FUNCTION test_shmem_register(pg_catalog.text, pg_catalog.int8, pg_catalog.int4)
+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 2a7e13512bf..772661abce7 100644
--- a/src/test/modules/test_shmem/test_shmem.c
+++ b/src/test/modules/test_shmem/test_shmem.c
@@ -20,6 +20,7 @@
#include "fmgr.h"
#include "miscadmin.h"
#include "storage/shmem.h"
+#include "utils/builtins.h"
#include "utils/guc.h"
#include "utils/injection_point.h"
@@ -129,3 +130,57 @@ get_test_shmem_attach_count(PG_FUNCTION_ARGS)
elog(ERROR, "shmem area not yet initialized");
PG_RETURN_INT32(TestShmem->attach_count);
}
+
+
+/*
+ * Callback for test_shmem_register(). test_shmem_register() provides the
+ * options, we just pass them through to ShmemRequestStruct.
+ */
+static void
+test_shmem_after_startup_request(void *arg)
+{
+ ShmemStructOpts *opts = (ShmemStructOpts *) arg;
+
+ elog(LOG, "test_shmem_after_startup_request callback called");
+
+ ShmemRequestStructWithOpts(opts);
+}
+
+/*
+ * Allocate or attach to a shmem segment, with the caller-supplied name and
+ * size.
+ *
+ * The given integer 'new_value' is stored in the segment, and the old value
+ * is returned.
+ */
+PG_FUNCTION_INFO_V1(test_shmem_register);
+Datum
+test_shmem_register(PG_FUNCTION_ARGS)
+{
+ char *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
+ int64 size = PG_GETARG_INT64(1);
+ int new_value = PG_GETARG_INT32(2);
+ int old_value;
+ int *attached = NULL;
+
+ ShmemStructOpts opts = {
+ .name = name,
+ .size = size,
+ .ptr = (void **) &attached,
+ };
+
+ ShmemCallbacks callbacks = {
+ .flags = SHMEM_CALLBACKS_ALLOW_AFTER_STARTUP,
+ .request_fn = test_shmem_after_startup_request,
+ .opaque_arg = &opts,
+ };
+
+ RegisterShmemCallbacks(&callbacks);
+ if (attached == NULL)
+ elog(ERROR, "could not attach to shared memory");
+
+ old_value = *attached;
+ *attached = new_value;
+
+ PG_RETURN_INT32(old_value);
+}
--
2.47.3
