Revert "Allow parallel workers to cope with a newly-created session user ID."

This reverts commit 216201027d90e99a0a2b2d2efba85dc0aac94c62.

Some buildfarm animals are failing with "cannot change
"client_encoding" during a parallel operation".  It looks like
assign_client_encoding is unhappy at being asked to roll back a
client_encoding setting after a parallel worker encounters a
failure.  There must be more to it though: why didn't I see this
during local testing?  In any case, it's clear that moving the
RestoreGUCState() call is not as side-effect-free as I thought.
Given that the bug f5f30c22e intended to fix has gone unreported
for years, it's not something that's urgent to fix; I'm not
willing to risk messing with it further with only days to our
next release wrap.

src/backend/access/transam/parallel.c

@@ -1401,6 +1401,10 @@ ParallelWorkerMain(Datum main_arg)
 	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);
+	RestoreGUCState(gucspace);
 	CommitTransactionCommand();
 
 	/* Crank up a transaction state appropriate to a parallel worker. */
@@ -1442,14 +1446,6 @@ ParallelWorkerMain(Datum main_arg)
 	 */
 	InvalidateSystemCaches();
 
-	/*
-	 * Restore GUC values from launching backend.  We can't do this earlier,
-	 * because GUC check hooks that do catalog lookups need to see the same
-	 * database state as the leader.
-	 */
-	gucspace = shm_toc_lookup(toc, PARALLEL_KEY_GUC, false);
-	RestoreGUCState(gucspace);
-
 	/*
 	 * Restore current role id.  Skip verifying whether session user is
 	 * allowed to become this role and blindly restore the leader's state for

src/backend/commands/variable.c

@@ -519,16 +519,14 @@ check_transaction_read_only(bool *newval, void **extra, GucSource source)
  * We allow idempotent changes at any time, but otherwise this can only be
  * changed in a toplevel transaction that has not yet taken a snapshot.
  *
- * As in check_transaction_read_only, allow it if not inside a transaction,
+ * As in check_transaction_read_only, allow it if not inside a transaction.
- * or if restoring state in a parallel worker.
  */
 bool
 check_XactIsoLevel(int *newval, void **extra, GucSource source)
 {
 	int			newXactIsoLevel = *newval;
 
-	if (newXactIsoLevel != XactIsoLevel &&
+	if (newXactIsoLevel != XactIsoLevel && IsTransactionState())
-		IsTransactionState() && !InitializingParallelWorker)
 	{
 		if (FirstSnapshotSet)
 		{
@@ -563,10 +561,6 @@ check_XactIsoLevel(int *newval, void **extra, GucSource source)
 bool
 check_transaction_deferrable(bool *newval, void **extra, GucSource source)
 {
-	/* Just accept the value when restoring state in a parallel worker */
-	if (InitializingParallelWorker)
-		return true;
-
 	if (IsSubTransaction())
 	{
 		GUC_check_errcode(ERRCODE_ACTIVE_SQL_TRANSACTION);

src/test/regress/expected/select_parallel.out

@@ -1221,21 +1221,3 @@ SELECT 1 FROM tenk1_vw_sec
 (9 rows)
 
 rollback;
--- test that a newly-created session role propagates to workers.
-begin;
-create role regress_parallel_worker;
-set session authorization regress_parallel_worker;
-select current_setting('session_authorization');
-     current_setting     
--------------------------
- regress_parallel_worker
-(1 row)
-
-set force_parallel_mode = 1;
-select current_setting('session_authorization');
-     current_setting     
--------------------------
- regress_parallel_worker
-(1 row)
-
-rollback;

src/test/regress/sql/select_parallel.sql

@@ -464,12 +464,3 @@ SELECT 1 FROM tenk1_vw_sec
   WHERE (SELECT sum(f1) FROM int4_tbl WHERE f1 < unique1) < 100;
 
 rollback;
-
--- test that a newly-created session role propagates to workers.
-begin;
-create role regress_parallel_worker;
-set session authorization regress_parallel_worker;
-select current_setting('session_authorization');
-set force_parallel_mode = 1;
-select current_setting('session_authorization');
-rollback;