Register built-in dynamic managers in EXEC_BACKEND child

Under EXEC_BACKEND the exec'd child starts with an empty (palloc'd) smgr
table; register_builtin_dynamic_managers() is only called from the
postmaster (inherited via fork) and InitStandaloneProcess. Re-run it in
SubPostmasterMain, before process_shared_preload_libraries(), mirroring
the postmaster so extensions can still register their own smgrs.

Author: dutow

src/backend/postmaster/launch_backend.c

@@ -669,6 +669,15 @@ SubPostmasterMain(int argc, char *argv[])
 
 	RegisterBuiltinShmemCallbacks();
 
+	/*
+	 * Re-register built-in dynamic managers (e.g. md smgr). On fork-based
+	 * platforms the child inherits the postmaster's already-populated smgr
+	 * table; under EXEC_BACKEND the child starts with an empty table, so we
+	 * must re-run registration. Must happen before shared_preload_libraries
+	 * so extensions can still register their own smgrs from _PG_init().
+	 */
+	register_builtin_dynamic_managers();
+
 	/*
 	 * Reload any libraries that were preloaded by the postmaster.  Since we
 	 * exec'd this process, those libraries didn't come along with us; but we