Fix rd_firstRelfilenodeSubid for nailed relations, in parallel workers.

Move applicable code out of RelationBuildDesc(), which nailed relations
bypass.  Non-assert builds experienced no known problems.  Back-patch to
v13, where commit c6b92041d3 introduced
rd_firstRelfilenodeSubid.

Kyotaro Horiguchi.  Reported by Justin Pryzby.

Discussion: https://postgr.es/m/20200907023737.GA7158@telsasoft.com

src/backend/utils/cache/relcache.c

@@ -1243,14 +1243,6 @@ RelationBuildDesc(Oid targetRelId, bool insertIt)
 	if (insertIt)
 		RelationCacheInsert(relation, true);
 
-	/*
-	 * For RelationNeedsWAL() to answer correctly on parallel workers, restore
-	 * rd_firstRelfilenodeSubid.  No subtransactions start or end while in
-	 * parallel mode, so the specific SubTransactionId does not matter.
-	 */
-	if (IsParallelWorker() && RelFileNodeSkippingWAL(relation->rd_node))
-		relation->rd_firstRelfilenodeSubid = TopSubTransactionId;
-
 	/* It's fully valid */
 	relation->rd_isvalid = true;
 
@@ -1273,6 +1265,8 @@ RelationBuildDesc(Oid targetRelId, bool insertIt)
 static void
 RelationInitPhysicalAddr(Relation relation)
 {
+	Oid			oldnode = relation->rd_node.relNode;
+
 	/* these relations kinds never have storage */
 	if (!RELKIND_HAS_STORAGE(relation->rd_rel->relkind))
 		return;
@@ -1330,6 +1324,19 @@ RelationInitPhysicalAddr(Relation relation)
 			elog(ERROR, "could not find relation mapping for relation \"%s\", OID %u",
 				 RelationGetRelationName(relation), relation->rd_id);
 	}
+
+	/*
+	 * For RelationNeedsWAL() to answer correctly on parallel workers, restore
+	 * rd_firstRelfilenodeSubid.  No subtransactions start or end while in
+	 * parallel mode, so the specific SubTransactionId does not matter.
+	 */
+	if (IsParallelWorker() && oldnode != relation->rd_node.relNode)
+	{
+		if (RelFileNodeSkippingWAL(relation->rd_node))
+			relation->rd_firstRelfilenodeSubid = TopSubTransactionId;
+		else
+			relation->rd_firstRelfilenodeSubid = InvalidSubTransactionId;
+	}
 }
 
 /*

src/test/regress/expected/reindex_catalog.out

@@ -36,3 +36,13 @@ REINDEX INDEX pg_index_indexrelid_index; -- non-mapped, non-shared, critical
 REINDEX INDEX pg_index_indrelid_index; -- non-mapped, non-shared, non-critical
 REINDEX INDEX pg_database_oid_index; -- mapped, shared, critical
 REINDEX INDEX pg_shdescription_o_c_index; -- mapped, shared, non-critical
+-- Check the same REINDEX INDEX statements under parallelism.
+BEGIN;
+SET min_parallel_table_scan_size = 0;
+REINDEX INDEX pg_class_oid_index; -- mapped, non-shared, critical
+REINDEX INDEX pg_class_relname_nsp_index; -- mapped, non-shared, non-critical
+REINDEX INDEX pg_index_indexrelid_index; -- non-mapped, non-shared, critical
+REINDEX INDEX pg_index_indrelid_index; -- non-mapped, non-shared, non-critical
+REINDEX INDEX pg_database_oid_index; -- mapped, shared, critical
+REINDEX INDEX pg_shdescription_o_c_index; -- mapped, shared, non-critical
+ROLLBACK;

src/test/regress/sql/reindex_catalog.sql

@@ -39,3 +39,14 @@ REINDEX INDEX pg_index_indexrelid_index; -- non-mapped, non-shared, critical
 REINDEX INDEX pg_index_indrelid_index; -- non-mapped, non-shared, non-critical
 REINDEX INDEX pg_database_oid_index; -- mapped, shared, critical
 REINDEX INDEX pg_shdescription_o_c_index; -- mapped, shared, non-critical
+
+-- Check the same REINDEX INDEX statements under parallelism.
+BEGIN;
+SET min_parallel_table_scan_size = 0;
+REINDEX INDEX pg_class_oid_index; -- mapped, non-shared, critical
+REINDEX INDEX pg_class_relname_nsp_index; -- mapped, non-shared, non-critical
+REINDEX INDEX pg_index_indexrelid_index; -- non-mapped, non-shared, critical
+REINDEX INDEX pg_index_indrelid_index; -- non-mapped, non-shared, non-critical
+REINDEX INDEX pg_database_oid_index; -- mapped, shared, critical
+REINDEX INDEX pg_shdescription_o_c_index; -- mapped, shared, non-critical
+ROLLBACK;