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, 6 insertions, 14 deletions

diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 75e456360be..26372d95b38 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -55,25 +55,21 @@ int			shared_memory_type = DEFAULT_SHARED_MEMORY_TYPE;
 shmem_startup_hook_type shmem_startup_hook = NULL;
 
 static Size total_addin_request = 0;
-static bool addin_request_allowed = true;
-
 
 /*
  * RequestAddinShmemSpace
  *		Request that extra shmem space be allocated for use by
  *		a loadable module.
  *
- * This is only useful if called from the _PG_init hook of a library that
- * is loaded into the postmaster via shared_preload_libraries.  Once
- * shared memory has been allocated, calls will be ignored.  (We could
- * raise an error, but it seems better to make it a no-op, so that
- * libraries containing such calls can be reloaded if needed.)
+ * This may only be called via the shmem_request_hook of a library that is
+ * loaded into the postmaster via shared_preload_libraries.  Calls from
+ * elsewhere will fail.
  */
 void
 RequestAddinShmemSpace(Size size)
 {
-	if (IsUnderPostmaster || !addin_request_allowed)
-		return;					/* too late */
+	if (!process_shmem_requests_in_progress)
+		elog(FATAL, "cannot request additional shared memory outside shmem_request_hook");
 	total_addin_request = add_size(total_addin_request, size);
 }
 
@@ -83,9 +79,6 @@ RequestAddinShmemSpace(Size size)
  *
  * If num_semaphores is not NULL, it will be set to the number of semaphores
  * required.
- *
- * Note that this function freezes the additional shared memory request size
- * from loadable modules.
  */
 Size
 CalculateShmemSize(int *num_semaphores)
@@ -152,8 +145,7 @@ CalculateShmemSize(int *num_semaphores)
 	size = add_size(size, ShmemBackendArraySize());
 #endif
 
-	/* freeze the addin request size and include it */
-	addin_request_allowed = false;
+	/* include additional requested shmem from preload libraries */
 	size = add_size(size, total_addin_request);
 
 	/* might as well round it off to a multiple of a typical page size */