Fix shmem_startup_hook documentation.

Author: nathan-bossart

doc/src/sgml/xfunc.sgml

@@ -3668,11 +3668,14 @@ 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.  On Windows (and anywhere else where
+      <literal>EXEC_BACKEND</literal> is defined), each backend executes 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.  On other platforms, the postmaster process executes the
+      <literal>shmem_startup_hook</literal> once, and each backend
+      automatically inherits the pointers to shared memory.
      </para>
 
      <para>

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",