From 2c6ed729d951621e34214abd1c532539530479ea Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Sat, 28 Aug 2021 05:02:55 +0000
Subject: [PATCH 1/1] add shared_memory_size GUC

---
 src/backend/postmaster/postmaster.c | 25 +++++++++++++++++++++++--
 src/backend/utils/misc/guc.c        | 13 +++++++++++++
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 9c2c98614a..38e28b3f8f 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -893,7 +893,8 @@ PostmasterMain(int argc, char *argv[])
 	if (!SelectConfigFiles(userDoption, progname))
 		ExitPostmaster(2);
 
-	if (output_config_variable != NULL)
+	if (output_config_variable != NULL &&
+		strcmp(output_config_variable, "shared_memory_size") != 0)
 	{
 		/*
 		 * "-C guc" was specified, so print GUC's value and exit.  No extra
@@ -983,7 +984,8 @@ PostmasterMain(int argc, char *argv[])
 	 * so it must happen before opening sockets so that at exit, the socket
 	 * lockfiles go away after CloseServerPorts runs.
 	 */
-	CreateDataDirLockFile(true);
+	if (output_config_variable == NULL)
+		CreateDataDirLockFile(true);
 
 	/*
 	 * Read the control file (for error checking and config info).
@@ -1026,6 +1028,25 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeMaxBackends();
 
+	{
+		char buf[64];
+		Size size;
+
+		size = CalculateShmemSize(NULL);
+		size += (1024 * 1024) - 1;
+		size /= (1024 * 1024);
+
+		sprintf(buf, "%lu MB", size);
+		SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+	}
+
+	if (output_config_variable != NULL)
+	{
+		const char *val = GetConfigOption(output_config_variable, false, false);
+		puts(val);
+		ExitPostmaster(0);
+	}
+
 	/*
 	 * Set up shared memory and semaphores.
 	 */
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 467b0fd6fe..7c88dbeae9 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -620,6 +620,8 @@ char	   *pgstat_temp_directory;
 
 char	   *application_name;
 
+int			shmem_size_mb;
+
 int			tcp_keepalives_idle;
 int			tcp_keepalives_interval;
 int			tcp_keepalives_count;
@@ -2337,6 +2339,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"shared_memory_size", PGC_INTERNAL, RESOURCES_MEM,
+			gettext_noop("Shows the amount of shared memory allocated by the server (rounded up to the nearest MB)."),
+			NULL,
+			GUC_UNIT_MB
+		},
+		&shmem_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."),
-- 
2.16.6