From bd1788051b02cfddcd9ef0e2fd094972f372b8fd Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Wed, 8 Sep 2021 12:02:30 +0900
Subject: [PATCH] Introduce GUC shared_memory_size

This runtime-computed GUC shows the size of the server's main shared
memory area, taking into account the amount of shared memory allocated
by extensions as this is calculated after processing
shared_preload_libraries.

Author: Nathan Bossart
Discussion: https://postgr.es/m/F2772387-CE0F-46BF-B5F1-CC55516EB885@amazon.com
---
 doc/src/sgml/config.sgml            | 14 ++++++++++++++
 src/backend/postmaster/postmaster.c |  7 +++++++
 src/backend/storage/ipc/ipci.c      | 22 ++++++++++++++++++++++
 src/backend/utils/misc/guc.c        | 12 ++++++++++++
 src/include/storage/ipc.h           |  1 +
 5 files changed, 56 insertions(+)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 2c31c35a6b1..ef0e2a77462 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -10275,6 +10275,20 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-shared-memory-size" xreflabel="shared_memory_size">
+      <term><varname>shared_memory_size</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>shared_memory_size</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the size of the main shared memory area, rounded up to the
+        nearest megabyte.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-ssl-library" xreflabel="ssl_library">
       <term><varname>ssl_library</varname> (<type>string</type>)
       <indexterm>
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 63043ed8d1f..b2fe420c3cf 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -1026,6 +1026,13 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeMaxBackends();
 
+	/*
+	 * Now that loadable modules have had their chance to request additional
+	 * shared memory, determine the value of any runtime-computed GUCs that
+	 * depend on the amount of shared memory required.
+	 */
+	InitializeShmemGUCs();
+
 	/*
 	 * Set up shared memory and semaphores.
 	 */
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 64bc16fa848..1a408ad77e5 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -313,3 +313,25 @@ CreateSharedMemoryAndSemaphores(void)
 	if (shmem_startup_hook)
 		shmem_startup_hook();
 }
+
+/*
+ * InitializeShmemGUCs
+ *
+ * This function initializes runtime-computed GUCs related to the amount of
+ * shared memory required for the current configuration.
+ */
+void
+InitializeShmemGUCs(void)
+{
+	char		buf[64];
+	Size		size_b;
+	Size		size_mb;
+
+	/*
+	 * Calculate the shared memory size and round up to the nearest megabyte.
+	 */
+	size_b = CalculateShmemSize(NULL);
+	size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
+	sprintf(buf, "%lu", size_mb);
+	SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+}
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 467b0fd6fe7..c339acf0670 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -664,6 +664,7 @@ static int	max_index_keys;
 static int	max_identifier_length;
 static int	block_size;
 static int	segment_size;
+static int	shared_memory_size_mb;
 static int	wal_block_size;
 static bool data_checksums;
 static bool integer_datetimes;
@@ -2337,6 +2338,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"shared_memory_size", PGC_INTERNAL, RESOURCES_MEM,
+			gettext_noop("Shows the size of the server's main shared memory area (rounded up to the nearest MB)."),
+			NULL,
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB
+		},
+		&shared_memory_size_mb,
+		0, 0, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"temp_buffers", PGC_USERSET, RESOURCES_MEM,
 			gettext_noop("Sets the maximum number of temporary buffers used by each session."),
diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index 80e191d407d..7a1ebc8559c 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -79,5 +79,6 @@ extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
 extern Size CalculateShmemSize(int *num_semaphores);
 extern void CreateSharedMemoryAndSemaphores(void);
+extern void InitializeShmemGUCs(void);
 
 #endif							/* IPC_H */