Add a new shmem_request_hook hook.

Currently, preloaded libraries are expected to request additional
shared memory and LWLocks in _PG_init().  However, it is not unusal
for such requests to depend on MaxBackends, which won't be
initialized at that time.  Such requests could also depend on GUCs
that other modules might change.  This introduces a new hook where
modules can safely use MaxBackends and GUCs to request additional
shared memory and LWLocks.

Furthermore, this change restricts requests for shared memory and
LWLocks to this hook.  Previously, libraries could make requests
until the size of the main shared memory segment was calculated.
Unlike before, we no longer silently ignore requests received at
invalid times.  Instead, we FATAL if someone tries to request
additional shared memory or LWLocks outside of the hook.

Nathan Bossart and Julien Rouhaud

Discussion: https://postgr.es/m/20220412210112.GA2065815%40nathanxps13
Discussion: https://postgr.es/m/Yn2jE/lmDhKtkUdr@paquier.xyz

1 files changed, 15 insertions, 0 deletions

diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 6ed584e1143..ec6a61594a4 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -1618,6 +1618,9 @@ char	   *local_preload_libraries_string = NULL;
 bool		process_shared_preload_libraries_in_progress = false;
 bool		process_shared_preload_libraries_done = false;
 
+shmem_request_hook_type shmem_request_hook = NULL;
+bool		process_shmem_requests_in_progress = false;
+
 /*
  * load the shared libraries listed in 'libraries'
  *
@@ -1701,6 +1704,18 @@ process_session_preload_libraries(void)
 				   true);
 }
 
+/*
+ * process any shared memory requests from preloaded libraries
+ */
+void
+process_shmem_requests(void)
+{
+	process_shmem_requests_in_progress = true;
+	if (shmem_request_hook)
+		shmem_request_hook();
+	process_shmem_requests_in_progress = false;
+}
+
 void
 pg_bindtextdomain(const char *domain)
 {