I quickly put together a patch for the stuff we've discussed in this
thread. WDYT?
--
nathan
>From 11289e1e69fc7c6fdbe5a73483efc2daf1197ec9 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nat...@postgresql.org>
Date: Mon, 8 Sep 2025 16:08:00 -0500
Subject: [PATCH v1 1/1] Fix shmem_startup_hook documentation.
---
doc/src/sgml/xfunc.sgml | 10 ++++++----
src/test/modules/test_slru/test_slru.c | 21 +++++++++++++++------
2 files changed, 21 insertions(+), 10 deletions(-)
diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml
index da21ef56891..80d862ff9c2 100644
--- a/doc/src/sgml/xfunc.sgml
+++ b/doc/src/sgml/xfunc.sgml
@@ -3668,11 +3668,13 @@ LWLockRelease(AddinShmemInitLock);
</programlisting>
<literal>shmem_startup_hook</literal> provides a convenient place for the
initialization code, but it is not strictly required that all such code
- be placed in this hook. Each backend will execute the registered
- <literal>shmem_startup_hook</literal> shortly after it attaches to shared
- memory. Note that add-ins should still acquire
+ be placed in this hook. In some builds, each backend will execute the
+ registered <literal>shmem_startup_hook</literal> shortly after it
+ attaches to shared memory, so add-ins should still acquire
<function>AddinShmemInitLock</function> within this hook, as shown in the
- example above.
+ example above. In other builds, the postmaster process will execute the
+ <literal>shmem_startup_hook</literal> once, and each backend will
+ automatically inherit the pointers to shared memory.
</para>
<para>
diff --git a/src/test/modules/test_slru/test_slru.c
b/src/test/modules/test_slru/test_slru.c
index 8c0367eeee4..f41422cca7d 100644
--- a/src/test/modules/test_slru/test_slru.c
+++ b/src/test/modules/test_slru/test_slru.c
@@ -219,8 +219,8 @@ test_slru_shmem_startup(void)
*/
const bool long_segment_names = true;
const char slru_dir_name[] = "pg_test_slru";
- int test_tranche_id;
- int test_buffer_tranche_id;
+ int test_tranche_id = -1;
+ int test_buffer_tranche_id = -1;
if (prev_shmem_startup_hook)
prev_shmem_startup_hook();
@@ -231,10 +231,19 @@ test_slru_shmem_startup(void)
*/
(void) MakePGDirectory(slru_dir_name);
- /* initialize the SLRU facility */
- test_tranche_id = LWLockNewTrancheId("test_slru_tranche");
-
- test_buffer_tranche_id = LWLockNewTrancheId("test_buffer_tranche");
+ /*
+ * Initialize the SLRU facility.
+ *
+ * In EXEC_BACKEND builds, the shmem_startup_hook is called in the
+ * postmaster and in each backend, but we only need to generate the
LWLock
+ * tranches once. Note that these IDs are not used by SimpleLruInit()
in
+ * the IsUnderPostmaster (i.e., backend) case.
+ */
+ if (!IsUnderPostmaster)
+ {
+ test_tranche_id = LWLockNewTrancheId("test_slru_tranche");
+ test_buffer_tranche_id =
LWLockNewTrancheId("test_buffer_tranche");
+ }
TestSlruCtl->PagePrecedes = test_slru_page_precedes_logically;
SimpleLruInit(TestSlruCtl, "TestSLRU",
--
2.39.5 (Apple Git-154)
