From 55af4473761a8bcfe8c2ff940a5df4625a40653a Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathandbossart@gmail.com>
Date: Tue, 12 Apr 2022 14:57:00 -0700
Subject: [PATCH v4 1/2] Fix comments about bgworker registration before
 MaxBackends initialization

Since 6bc8ef0b, InitializeMaxBackends() has used max_worker_processes
instead of adapting MaxBackends to the number of background workers
registered by modules loaded in shared_preload_libraries (at this time,
bgworkers were only static, but gained dynamic capabilities as a matter
of supporting parallel queries meaning that a control cap was
necessary).

Some comments referred to the past registration logic, making them
confusing and incorrect, so fix these.

Some of the out-of-core modules that could be loaded in this path
sometimes like to manipulate dynamically some of the resource-related
GUCs for their own needs, this commit adds a note about that.

Author: Nathan Bossart
Discussion: https://postgr.es/m/20220127181815.GA551692@nathanxps13
---
 src/backend/postmaster/postmaster.c | 10 ++++------
 src/backend/utils/init/postinit.c   |  5 ++---
 2 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 964a56dec4..ce4007bb2c 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -1005,10 +1005,8 @@ PostmasterMain(int argc, char *argv[])
 	LocalProcessControlFile(false);
 
 	/*
-	 * Register the apply launcher.  Since it registers a background worker,
-	 * it needs to be called before InitializeMaxBackends(), and it's probably
-	 * a good idea to call it before any modules had chance to take the
-	 * background worker slots.
+	 * Register the apply launcher.  It's probably a good idea to call this
+	 * before any modules had a chance to take the background worker slots.
 	 */
 	ApplyLauncherRegister();
 
@@ -1029,8 +1027,8 @@ PostmasterMain(int argc, char *argv[])
 #endif
 
 	/*
-	 * Now that loadable modules have had their chance to register background
-	 * workers, calculate MaxBackends.
+	 * Now that loadable modules have had their chance to alter any GUCs,
+	 * calculate MaxBackends.
 	 */
 	InitializeMaxBackends();
 
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 9139fe895c..a28612b375 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -538,9 +538,8 @@ pg_split_opts(char **argv, int *argcp, const char *optstr)
 /*
  * Initialize MaxBackends value from config options.
  *
- * This must be called after modules have had the chance to register background
- * workers in shared_preload_libraries, and before shared memory size is
- * determined.
+ * This must be called after modules have had the chance to alter GUCs in
+ * shared_preload_libraries and before shared memory size is determined.
  *
  * Note that in EXEC_BACKEND environment, the value is passed down from
  * postmaster to subprocesses via BackendParameters in SubPostmasterMain; only
-- 
2.25.1

