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From: Wang, Shenhao <[email protected]>
To: [email protected] <[email protected]>
Subject: make MaxBackends available in _PG_init
Date: Mon, 21 Sep 2020 03:43:41 +0000
Message-ID: <4f20d57b2aeb447b8eb1495319940c5f@G08CNEXMBPEKD06.g08.fujitsu.local> (raw)

Hi Hackers:

I find the value in function _PG_init, the value of MaxBackends is 0.
In source, I find that the postmaster will first load library, and then calculate the value of MaxBackends.

In the old version, the MaxBackends was calculated by:
	 MaxBackends = MaxConnections + autovacuum_max_workers + 1 +
		GetNumShmemAttachedBgworkers();
Because any extension can register workers which will affect the return value of GetNumShmemAttachedBgworkers.
InitializeMaxBackends must be called after shared_preload_libraries. This is also mentioned in comments.

Now, function GetNumShmemAttachedBgworkers was deleted and replaced by guc max_worker_processes, 
so if we changed the calling order like:
	Step1: calling InitializeMaxBackends.
	Step2: calling process_shared_preload_libraries

In this order extension can get the correct value of MaxBackends in _PG_init.

Any thoughts?

Regards




Attachments:

  [application/octet-stream] 0001-Make-MaxBackends-available-in-_PG_init.patch (2.2K, ../4f20d57b2aeb447b8eb1495319940c5f@G08CNEXMBPEKD06.g08.fujitsu.local/2-0001-Make-MaxBackends-available-in-_PG_init.patch)
  download | inline diff:
From 1bc1f06337bfffc57d4ac25147a7b981ef89bcbf Mon Sep 17 00:00:00 2001
From: Shenhao Wang <[email protected]>
Date: Fri, 18 Sep 2020 18:35:54 +0800
Subject: [PATCH] Make MaxBackends available in _PG_init

---
 src/backend/postmaster/postmaster.c | 17 +++++++----------
 src/backend/utils/init/postinit.c   |  4 +---
 2 files changed, 8 insertions(+), 13 deletions(-)

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 959e3b8873..8da50cb2a1 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -989,13 +989,16 @@ 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 it
+	 * before any modules had chance to take the background worker slots.
 	 */
 	ApplyLauncherRegister();
 
+	/*
+	 * Calculate MaxBackends.
+	 */
+	InitializeMaxBackends();
+
 	/*
 	 * process any libraries that should be preloaded at postmaster start
 	 */
@@ -1012,12 +1015,6 @@ PostmasterMain(int argc, char *argv[])
 	}
 #endif
 
-	/*
-	 * Now that loadable modules have had their chance to register background
-	 * workers, calculate MaxBackends.
-	 */
-	InitializeMaxBackends();
-
 	/*
 	 * Set up shared memory and semaphores.
 	 */
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index d4ab4c7e23..06d813f76b 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -513,9 +513,7 @@ 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
-- 
2.21.0

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