Subject: [PATCH v1 2/3] Calculate MaxBackends earlier in PostmasterMain().

From: Nathan Bossart <[email-masked]>
Date: Mon, 2 Aug 2021 17:42:25 +0000

Presently, InitializeMaxBackends() is called after processing
shared_preload_libraries because it used to tally up the number of
registered background workers requested by the libraries.  Since
6bc8ef0b, InitializeMaxBackends() has simply used the
max_worker_processes GUC instead, so all the comments about needing
to register background workers before initializing MaxBackends are
no longer correct.

In addition to revising the comments, this patch reorders
InitializeMaxBackends() to before shared_preload_libraries is
processed so that modules can make use of MaxBackends in their
_PG_init() functions.
---
 src/backend/postmaster/postmaster.c | 19 +++++++++----------
 src/backend/utils/init/postinit.c   |  4 +---
 2 files changed, 10 insertions(+), 13 deletions(-)

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 3dcaf8a4a5..3d3b2e376c 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -1005,10 +1005,15 @@ 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.
+	 * Calculate MaxBackends.  This is done before processing
+	 * shared_preload_libraries so that such libraries can make use of it in
+	 * _PG_init().
+	 */
+	InitializeMaxBackends();
+
+	/*
+	 * Register the apply launcher.  It's probably a good idea to call it before
+	 * any modules had chance to take the background worker slots.
 	 */
 	ApplyLauncherRegister();
 
@@ -1028,12 +1033,6 @@ PostmasterMain(int argc, char *argv[])
 	}
 #endif
 
-	/*
-	 * Now that loadable modules have had their chance to register background
-	 * workers, calculate MaxBackends.
-	 */
-	InitializeMaxBackends();
-
 	/*
 	 * Now that loadable modules have had their chance to request additional
 	 * shared memory, determine the value of any runtime-computed GUCs that
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 9139fe895c..0be1eb0f44 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -538,9 +538,7 @@ 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 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

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