Defer restoration of libraries in parallel workers.

Several users of extensions complained of crashes in parallel workers
that turned out to be due to syscache access from their _PG_init()
functions.  Reorder the initialization of parallel workers so that
libraries are restored after the caches are initialized, and inside a
transaction.

This was reported in bug #15350 and elsewhere.  We don't consider it
to be a bug: extensions shouldn't do that, because then they can't be
used in shared_preload_libraries.  However, it's a fairly obscure
hazard and these extensions worked in practice before parallel query
came along.  So let's make it work.  Later commits might add a warning
message and eventually an error.

Back-patch to 9.6, where parallel query landed.

Author: Thomas Munro
Reviewed-by: Amit Kapila
Reported-by: Kieran McCusker, Jimmy
Discussion: https://postgr.es/m/153512195228.1489.8545997741965926448%40wrigleys.postgresql.org

src/backend/access/transam/parallel.c

@@ -1118,14 +1118,6 @@ ParallelWorkerMain(Datum main_arg)
 							   fps->parallel_master_pid))
 		return;
 
-	/*
-	 * Load libraries that were loaded by original backend.  We want to do
-	 * this before restoring GUCs, because the libraries might define custom
-	 * variables.
-	 */
-	libraryspace = shm_toc_lookup(toc, PARALLEL_KEY_LIBRARY, false);
-	RestoreLibraryState(libraryspace);
-
 	/*
 	 * Identify the entry point to be called.  In theory this could result in
 	 * loading an additional library, though most likely the entry point is in
@@ -1147,9 +1139,17 @@ ParallelWorkerMain(Datum main_arg)
 	 */
 	SetClientEncoding(GetDatabaseEncoding());
 
+	/*
+	 * Load libraries that were loaded by original backend.  We want to do
+	 * this before restoring GUCs, because the libraries might define custom
+	 * variables.
+	 */
+	libraryspace = shm_toc_lookup(toc, PARALLEL_KEY_LIBRARY, false);
+	StartTransactionCommand();
+	RestoreLibraryState(libraryspace);
+
 	/* Restore GUC values from launching backend. */
 	gucspace = shm_toc_lookup(toc, PARALLEL_KEY_GUC, false);
-	StartTransactionCommand();
 	RestoreGUCState(gucspace);
 	CommitTransactionCommand();